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Exokernel

Exokernel is an operating system kernel developed by the MIT Parallel and Distributed Operating Systems group, and also a class of similar operating systems.

Operating systems generally present hardware resources to applications through high-level abstractions such as (virtual) file systems. The idea behind exokernels is to force as few abstractions as possible on application developers, enabling them to make as many decisions as possible about hardware abstractions. Exokernels are tiny, since functionality is limited to ensuring protection and multiplexing of resources, which is considerably simpler than conventional microkernels' implementation of message passing and monolithic kernels' implementation of high-level abstractions.

Implemented abstractions are called library operating systems; they may request specific memory addresses, disk blocks, etc. The kernel only ensures that the requested resource is free, and the application is allowed to access it. This low-level hardware access allows the programmer to implement custom abstractions, and omit unnecessary ones, most commonly to improve a program's performance. It also allows programmers to choose what level of abstraction they want, high, or low.

Exokernels can be seen as an application of the end-to-end principle to operating systems, in that they do not force an application program to layer its abstractions on top of other abstractions that were designed with different requirements in mind. For example, in the MIT Exokernel project, the Cheetah web server stores preformatted Internet Protocol packets on the disk, the kernel provides safe access to the disk by preventing unauthorized reading and writing, but how the disk is abstracted is up to the application or the libraries the application uses.

Motivation
Traditionally, kernel designers have sought to make individual hardware resources invisible to application programs by requiring the programs to interact with the hardware via some abstraction model. These models include file systems for disk storage, virtual address spaces for memory, schedulers for task management, and sockets for network communication. These abstractions of the hardware make it easier to write programs in general, but limit performance and stifle experimentation in new abstractions. A security-oriented application might need a file system that does not leave old data on the disk, while a reliability-oriented application might need a file system that keeps such data for failure recovery.

One option is to remove the kernel completely and program directly to the hardware, but then the entire machine would be dedicated to the application being written (and, conversely, the entire application codebase would be dedicated to that machine). The exokernel concept is a compromise: let the kernel allocate the basic physical resources of the machine (e.g. disk blocks, memory pages, and processor time) to multiple application programs, and let each program decide what to do with these resources. The program can then link to a support library that implements the abstractions it needs (or it can implement its own).

MIT exokernels
MIT developed two exokernel-based operating systems, using two kernels: Aegis, a proof of concept with limited support for storage, and XOK, which applied the exokernel concept more thoroughly.

An essential idea of the MIT exokernel system is that the operating system should act as an executive for small programs provided by the application software, which are constrained only by the requirement that the exokernel must be able to guarantee that they use the hardware safely.

Design
The MIT exokernel manages hardware resources as follows:

 * Processor
 * The kernel represents the processor resources as a timeline from which programs can allocate intervals of time. A program can yield the rest of its time slice to another designated program. The kernel notifies programs of processor events, such as interrupts, hardware exceptions, and the beginning or end of a time slice. If a program takes a long time to handle an event, the kernel will penalize it on subsequent time slice allocations; in extreme cases the kernel can abort the program.

 * Memory
 * The kernel allocates physical memory pages to programs and controls the translation lookaside buffer. A program can share a page with another program by sending it a capability to access that page. The kernel ensures that programs access only pages for which they have a capability.

 * Disk storage
 * The kernel identifies disk blocks to the application program by their physical block address, allowing the application to optimize data placement. When the program initializes its use of the disk, it provides the kernel with a function that the kernel can use to determine which blocks the program controls. The kernel uses this callback to verify that when it allocates a new block, the program claims only the block that was allocated in addition to those it already controlled.

 * Networking
 * The kernel implements a programmable packet filter, which executes programs in a bytecode language designed for easy security-checking by the kernel.

Applications
The available library operating systems for Exokernel include the custom ExOS system and an emulator for BSD. In addition to these, the exokernel team created the Cheetah web server, which uses the kernel directly.

History
The exokernel concept has been around since at least 1994, but exokernels are still a research effort and have not been used in any major commercial operating systems.

A concept operating exokernel system is Nemesis, written by University of Cambridge, University of Glasgow, Citrix Systems, and the Swedish Institute of Computer Science. MIT has also built several exokernel-based systems, including ExOS.

title: Exokernel


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Atua

Atua are the gods and spirits of the Polynesian peoples such as the Māori or the Hawaiians (see also Kupua); the Polynesian word literally means "power" or "strength" and so the concept is similar to that of mana. Today, it is also used for the monotheistic conception of God. Especially powerful atua include:
 * Rongo-mā-Tāne – god of agriculture and peace;
 * Tāne Mahuta – creator of all living things such as animals, birds and trees;
 * Tangaroa – god of the sea;
 * Tūmatauenga – a god of war; and
 * Whiro – god of darkness and evil.

In Samoa, where atua means "god" in the Samoan language, traditional tattooing was based on the doctrine of tutelary spirits. There is also a district on the island of Upolu in Samoa called Atua.

Atua or gods are also at the centre of Māori mythology. In traditional Māori belief, there is no specific word for "religion" because the natural and supernatural world are seen as one.

In other Austronesian cultures, cognates of atua include the Polynesian aitu, Micronesian aniti, Bunun hanitu, Filipino and Tao anito, and Malaysian and Indonesian hantu or antu.

In popular culture, Atua is the name that is used to refer to the deity which the character Angie Yonaga worships in the English dub of Danganronpa V3: Killing Harmony. The term "Atua" is often associated with her character.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Atua


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Nuku-mai-tore

In Māori mythology the Nuku-mai-tore are forest-dwelling spirits. Tura joins Whiro's canoe party, but when it enters a whirlpool he catches the overhanging boughs of a tree and lives among the Nuku-mai-tore, to whom he teaches the use of fire, the art of cooking, and the natural way of childbirth together with the ceremonies attending to the birth of a child.

title: Nuku-mai-tore


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Aitu

In Polynesian languages the word aitu refers to ghosts or spirits, often malevolent. The word is common to many languages of Western and Eastern Polynesia. In the mythology of Tonga, for example, aitu or eitu are lesser gods, many being patrons of specific villages and families. They often take the form of plants or animals, and are often more cruel than other gods. These trouble-making gods are regarded as having come from Samoa. The Tongan word tangi lauaitu means to cry from grief, to lament.

In Māori mythology, the word aitu refers to sickness, calamity, or demons; the related word aituā means misfortune, accident, disaster. In Tahitian, aitu (syn. atua/raitu) can mean 'god' or 'spirit'; in other languages, including Rarotongan, Samoan, Sikaiana, Kapingamarangi, Takuu, Tuamotuan, and Niuean, aitu are ghosts or spirits.

In Cook Islands Aitu is also the name of ancient tribes who came from the east.

"According to tradition, some of the Aitu tribes settled on the islands of Aitutaki, Atiu and Mangaia. At Aitutaki (Aitu-taki) they were eventually destroyed or driven away. At Mangaia they were from time to time slaughtered in order to provide sacrifice to the gods. There still exists at Mangaia the remains of a great oven named te umu Aitu where large numbers of these people were cooked after being slain."

In the Samoa Islands, aitu also means ghost. In other Austronesian cultures, cognates of aitu include the Micronesian aniti, Bunun hanitu, Filipino and Tao anito, and Malaysian and Indonesian hantu or antu.

title: Aitu


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Amai-te-rangi

According to the mythology of the Cook Islands, Amai-te-rangi was a cannibal and demon who attempted to entrap Ngaru. Ngaru, however, ascended and successfully defeated Amai-te-rangi with the help of his grandfather, Mokoroa.

title: Amai-te-rangi


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L4 microkernel family

L4 is a family of second-generation microkernels, used to implement a variety of types of operating systems (OS), though mostly for Unix-like, Portable Operating System Interface (POSIX) compliant types.

L4, like its predecessor microkernel L3, was created by German computer scientist Jochen Liedtke as a response to the poor performance of earlier microkernel-based OSes. Liedtke felt that a system designed from the start for high performance, rather than other goals, could produce a microkernel of practical use. His original implementation in hand-coded Intel i386-specific assembly language code in 1993 created attention by being 20 times faster than Mach. The follow-up publication two years later was considered so influential that it won the 2015 ACM SIGOPS Hall of Fame Award. Since its introduction, L4 has been developed to be cross-platform and to improve security, isolation, and robustness.

There have been various re-implementations of the original L4 kernel application binary interface (ABI) and its successors, including L4Ka::Pistachio (implemented by Liedtke and his students at Karlsruhe Institute of Technology), L4/MIPS (University of New South Wales (UNSW)), Fiasco (Dresden University of Technology (TU Dresden)). For this reason, the name L4 has been generalized and no longer refers to only Liedtke's original implementation. It now applies to the whole microkernel family including the L4 kernel interface and its different versions.

L4 is widely deployed. One variant, OKL4 from Open Kernel Labs, shipped in billions of mobile devices.

Design paradigm
Specifying the general idea of a microkernel, Liedtke states:

"A concept is tolerated inside the microkernel only if moving it outside the kernel, i.e., permitting competing implementations, would prevent the implementation of the system's required functionality."

In this spirit, the L4 microkernel provides few basic mechanisms: address spaces (abstracting page tables and providing memory protection), threads and scheduling (abstracting execution and providing temporal protection), and inter-process communication (for controlled communication across isolation boundaries).

An operating system based on a microkernel like L4 provides services as servers in user space that monolithic kernels like Linux or older generation microkernels include internally. For example, to implement a secure Unix-like system, servers must provide the rights management that Mach included inside the kernel.

History
The poor performance of first-generation microkernels, such as Mach, led a number of developers to re-examine the entire microkernel concept in the mid-1990s. The asynchronous in-kernel-buffering process communication concept used in Mach turned out to be one of the main reasons for its poor performance. This induced developers of Mach-based operating systems to move some time-critical components, like file systems or drivers, back inside the kernel. While this somewhat ameliorated the performance issues, it plainly violates the minimality concept of a true microkernel (and squanders their major advantages).

Detailed analysis of the Mach bottleneck indicated that, among other things, its working set is too large: the IPC code expresses poor spatial locality; that is, it results in too many cache misses, of which most are in-kernel. This analysis gave rise to the principle that an efficient microkernel should be small enough that the majority of performance-critical code fits into the (first-level) cache (preferably a small fraction of said cache).

L3
Jochen Liedtke set out to prove that a well-designed thinner inter-process communication (IPC) layer, with careful attention to performance and machine-specific (in contrast to cross-platform software) design could yield large real-world performance improvements. Instead of Mach's complex IPC system, his L3 microkernel simply passed the message with no added overhead. Defining and implementing the required security policies were considered to be duties of the user space servers. The role of the kernel was only to provide the needed mechanism to enable the user-level servers to enforce the policies. L3, developed in 1988, proved itself a safe and robust operating system, used for many years for example by Technischer Überwachungsverein (Technical Inspection Association).

L4
After some experience using L3, Liedtke came to the conclusion that several other Mach concepts were also misplaced. By simplifying the microkernel concepts even further he developed the first L4 kernel which was primarily designed for high performance. To maximise performance, the whole kernel was written in assembly language, and its IPC was 20 times faster than Mach's. Such dramatic performance increases are a rare event in operating systems, and Liedtke's work triggered new L4 implementations and work on L4-based systems at a number of universities and research institutes, including IBM, where Liedtke started to work in 1996, TU Dresden and UNSW. At IBM's Thomas J. Watson Research Center Liedtke and his colleagues continued research on L4 and microkernel based systems in general, especially the Sawmill OS.

L4Ka::Hazelnut
In 1999, Liedtke took over the Systems Architecture Group at the University of Karlsruhe, where he continued the research into microkernel systems. As a proof of concept that a high performance microkernel could also be constructed in a higher level language, the group developed L4Ka::Hazelnut, a C++ version of the kernel that ran on IA-32- and ARM-based machines. The effort was a success, performance was still acceptable, and with its release, the pure assembly language versions of the kernels were effectively discontinued.

L4/Fiasco
In parallel to the development of L4Ka::Hazelnut, in 1998 the Operating Systems Group TUD:OS of the TU Dresden started to develop their own C++ implementation of the L4 kernel interface, named L4/Fiasco. In contrast to L4Ka::Hazelnut, which allows no concurrency in the kernel, and its successor L4Ka::Pistachio, which allows interrupts in the kernel only at specific preemption points, L4/Fiasco was fully preemptible (with the exception of extremely short atomic operations) to achieve a low interrupt latency. This was considered necessary because L4/Fiasco is used as the basis of DROPS, a hard real-time computing capable operating system, also developed at the TU Dresden. However, the complexities of a fully preemptible design prompted later versions of Fiasco to return to the traditional L4 approach of running the kernel with interrupts disabled, except for a limited number of preemption points.

L4Ka::Pistachio
Up until the release of L4Ka::Pistachio and newer versions of Fiasco, all L4 microkernels had been inherently tied close to the underlying CPU architecture. The next big shift in L4 development was the development of a cross-platform (platform-independent) application programming interface (API) that still retained the high performance characteristics despite its higher level of portability. Although the underlying concepts of the kernel were the same, the new API provided many significant changes relative to prior L4 versions, including better support for multi-processor systems, looser ties between threads and address spaces, and the introduction of user-level thread control blocks (UTCBs) and virtual registers. After releasing the new L4 API (version X.2 a.k.a. version 4) in early 2001, the System Architecture Group at the University of Karlsruhe implemented a new kernel, L4Ka::Pistachio, completely from scratch, now with focus on both high performance and portability. It was released under the two-clause BSD license.

Newer Fiasco versions
The L4/Fiasco microkernel has also been extensively improved over the years. It now supports several hardware platforms ranging from x86 through AMD64 to several ARM platforms. Notably, a version of Fiasco (Fiasco-UX) can run as a user-level application on Linux.

L4/Fiasco implements several extensions to the L4v2 API. Exception IPC enables the kernel to send CPU exceptions to user-level handler applications. With the help of alien threads, it is possible to perform fine-grained control over system calls. X.2-style UTCBs have been added. Also, Fiasco contains mechanisms for controlling communication rights and kernel-level resource use. On Fiasco, a collection of basic user level services are developed (named L4Env) that among others are used to para-virtualise the current Linux version (4.19 ) (named L4Linux).

University of New South Wales and NICTA
Development also occurred at the University of New South Wales (UNSW), where developers implemented L4 on several 64-bit platforms. Their work resulted in L4/MIPS and L4/Alpha, resulting in Liedtke's original version being retrospectively named L4/x86. Like Liedtke's original kernels, the UNSW kernels (written in a mix of assembly and C) were unportable and each implemented from scratch. With the release of the highly portable L4Ka::Pistachio, the UNSW group abandoned their own kernels in favor of producing highly tuned ports of L4Ka::Pistachio, including the fastest-ever reported implementation of message passing (36 cycles on the Itanium architecture). The group has also demonstrated that device drivers can perform equally well at user-level as in-kernel, and developed Wombat, a highly portable version of Linux on L4 that runs on x86, ARM, and MIPS processors. On XScale processors, Wombat context-switching costs are up to 50 times lower than in native Linux.

Later the UNSW group, at their new home at NICTA (formerly National ICT Australia, Ltd.), forked L4Ka::Pistachio into a new L4 version named NICTA::L4-embedded. As the name implies, it was for use in commercial embedded systems, and consequently the implementation trade-offs favored small memory size and reduced complexity. The API was modified to keep almost all system calls short enough that they need no preemption points to ensure high real-time responsiveness.

Commercial deployment
In November 2005, NICTA announced that Qualcomm was deploying NICTA's L4 version on their Mobile Station Modem chipsets. This led to the use of L4 in mobile phone handsets on sale from late 2006. In August 2006, ERTOS leader and UNSW professor Gernot Heiser spun out a company named Open Kernel Labs (OK Labs) to support commercial L4 users and further develop L4 for commercial use under the brand name OKL4, in close collaboration with NICTA. OKL4 μKernel Version 2.1, released in April 2008, was the first generally available version of L4 which featured capability-based security. OKL4 μKernel 3.0, released in October 2008, was the last open-source version of OKL4 μKernel. More recent versions are closed source and based on a rewrite to support a native hypervisor variant named the OKL4 Microvisor. OK Labs also distributed a paravirtualized Linux named OK:Linux, a descendant of Wombat, and paravirtualized versions of SymbianOS and Android. OK Labs also acquired the rights to seL4 from NICTA.

OKL4 shipments exceeded 1.5 billion in early 2012, mostly on Qualcomm wireless modem chips. Other deployments include automotive infotainment systems.

Apple A series processors beginning with the A7 contain a Secure Enclave coprocessor running an L4 operating system called sepOS (Secure Enclave Processor OS) based on the L4-embedded kernel developed at NICTA in 2006. As a result, L4 ships on all modern Apple devices including Macs with Apple silicon. In 2015 alone, total shipments of iPhone was estimated at 310 million.

High assurance: seL4
In 2006, the NICTA group commenced a from-scratch design of a third-generation microkernel, named seL4, with the aim of providing a basis for highly secure and reliable systems, suitable for satisfying security requirements such as those of Common Criteria and beyond. From the beginning, development aimed for formal verification of the kernel. To ease meeting the sometimes conflicting requirements of performance and verification, the team used a middle-out software process starting from an executable specification written in the language Haskell. seL4 uses capability-based security access control to enable formal reasoning about object accessibility.

A formal proof of functional correctness was completed in 2009. The proof provides a guarantee that the kernel's implementation is correct against its specification, and implies that it is free of implementation bugs such as deadlocks, livelocks, buffer overflows, arithmetic exceptions or use of uninitialised variables. seL4 is claimed to be the first-ever general-purpose operating-system kernel that has been verified. The work on seL4 won the 2019 ACM SIGOPS Hall of Fame Award.

seL4 takes a novel approach to kernel resource management, exporting the management of kernel resources to user level and subjects them to the same capability-based access control as user resources. This model, which was also adopted by Barrelfish, simplifies reasoning about isolation properties, and was an enabler for later proofs that seL4 enforces the core security properties of integrity and confidentiality. The NICTA team also proved correctness of the translation from the programming language C to executable machine code, taking the compiler out of the trusted computing base of seL4. This implies that the high-level security proofs hold for the kernel executable. seL4 is also the first published protected-mode OS kernel with a complete and sound worst-case execution time (WCET) analysis, a prerequisite for its use in hard real-time computing.

On 29 July 2014, NICTA and General Dynamics C4 Systems announced that seL4, with end to end proofs, was now released under open-source licenses. The kernel source code and proofs are licensed under GNU General Public License version 2 (GPLv2), and most libraries and tools are under the BSD 2-clause. In April 2020, it was announced that the seL4 Foundation was created under the umbrella of the Linux Foundation to accelerate development and deployment of seL4.

The researchers state that the cost of formal software verification is lower than the cost of engineering traditional "high-assurance" software despite providing much more reliable results. Specifically, the cost of one line of code during the development of seL4 was estimated at around US$400, compared to US$1000 for traditional high-assurance systems.

Under the Defense Advanced Research Projects Agency (DARPA) High-Assurance Cyber Military Systems (HACMS) program, NICTA together with project partners Rockwell Collins, Galois Inc, the University of Minnesota and Boeing developed a high-assurance drone using seL4, along with other assurance tools and software, with planned technology transfer onto the optionally piloted autonomous Boeing AH-6 Unmanned Little Bird helicopter being developed by Boeing. Final demonstration of the HACMS technology took place in Sterling, VA in April 2017. DARPA also funded several Small Business Innovative Research (SBIR) contracts related to seL4 under a program started by Dr. John Launchbury. Small businesses receiving an seL4-related SBIR included: DornerWorks, Techshot, Wearable Inc, Real Time Innovations, and Critical Technologies.

In October 2023, Nio Inc. announced that their seL4-based SkyOS operating systems will be in mass-produced electric cars from 2024.

In 2023, seL4 won the ACM Software System Award.

Other research and development
Osker, an OS written in Haskell, targeted the L4 specification; although this project focused mainly on the use of a functional programming language for OS development, not on microkernel research.

RedoxOS is a Rust based operating system, that is also inspired by seL4, and uses a micro kernel design.

CodeZero is an L4 microkernel for embedded systems with a focus on virtualization and implementation of native OS services. There is a GPL-licensed version, and a version that was relicensed by B Labs Ltd., acquired by Nvidia, as closed source and forked in 2010.

F9 microkernel, a BSD-licensed L4 implementation, is dedicated to ARM Cortex-M processors for deeply embedded devices with memory protection.

The NOVA OS Virtualization Architecture is a research project with focus on constructing a secure and efficient virtualization environment with a small trusted computing base. NOVA consists of a microhypervisor, a user level hypervisor (virtual machine monitor), and an unprivileged componentised multi-server user environment running on it named NUL. NOVA runs on ARMv8-A and x86-based multi-core systems.

WrmOS is a real-time operating system based on L4 microkernel. It has own implementations of kernel, standard libraries, and network stack, supporting ARM, SPARC, x86, and x86-64 architectures. There is the paravirtualized Linux kernel (w4linux ) working on WrmOS.

Helios is a microkernel inspired by seL4. It is part of the Ares operating system, supports x86-64 and aarch64 and is still under active development as of February 2023.

title: L4 microkernel family


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Ao (Māori mythology)

Ao (daylight) is one of the primal deities who are the unborn forces of nature in Māori mythology. Ao is the personification of light, clouds, and the ordinary world, as opposed to darkness (Pō) and the underworld.

He is spoken of under many forms or manifestations, including Aotūroa (enduring day, this world) and Aotahi (bright day, world of light and life). With his companions Ata (morning) and Whaitua (space) Ao resists the forces of darkness.

The term Te Ao Māori, loosely translated as "The Māori world", is used to refer to Māori tradition and culture.

Genealogy
Ao appears evolving through the forms Aonui, Aoroa, Aowheneke, Ao-whetara out of the darkness as part of the great cosmological genealogies in Te Arawa's traditions, as part of the creation of the universe.

Aonui, Aoroa, Aopouri, Aopotango, Aowhetuma, Aowhekere, Aokahiwahiwa, Aokanapanapa, Aopakakina, Aopakarea, and Aotakawe were also the names of the atua who were the storm clouds, the children, of Tāwhirimātea, which were sent to punish his brothers after the separation of his parents, Rangi and Papa.

In a version recorded from Hūkiki Te Ahukaramū, a Ngāti Raukawa chief, Te Ao was born out of Te Ata, which itself came from the darkness. Ao's last form in Hūkiki's version is Te Ao Mārama. A version given by the Kāi Tahu of Moeraki is similar.

Pan-Polynesian
In Tahiti, Aonui was Tāne's residence in the sky, and Aoaomaraia was the discoverer of fire; a similar role is taken on by Māui in other parts of Polynesia.

title: Ao (Māori mythology)


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Apakura

In Māori mythology, Apakura is the wife of Tūhuruhuru, the son of Tinirau. She had several children, among whom are Tūwhakararo, Mairatea, Reimatua, and Whakatau. In another legend, Apakura is said to be the wife of Tūwhakararo, who was the son of Rātā and father of Whakatau. Whakatau was born in a miraculous manner, from the girdle or apron which Apakura threw into the ocean which was made into a child by a sea deity (Tregear 1891:15).

title: Apakura


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Apukohai

Apukohai is a marine monster in the mythology of the island of Kauai, Hawaii. Kawelo, a giant of enormous strength, departs for Oahu and encounters Apukohai. After a fierce struggle, Kawelo kills Apukohai, having invoked the assistance of the owl god and the fish Ulu-makaikai (Beckwith 1970: 409–411).

title: Apukohai


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Aremata-Popoa and Aremata-Rorua

In the Tahitian story of Rata 'Aremata-Popoto ("tidal wave") and 'Aremata-Roroa ("long wave") are two ocean-dwelling monsters that Rata must overcome.

title: Aremata-Popoa and Aremata-Rorua


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Ārohirohi

In Māori mythology, Ārohirohi is the goddess of mirages and shimmering heat, and is the wife of Tama-nui-te-rā (the Sun).

She created Mārikoriko (Twilight), the first woman, from a mirage and then asked Paoro (Echo) to give her a voice.

title: Ārohirohi


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Atanua

Atanua (or Atanea) in Polynesian mythology (specifically: the Marquesas Islands) is the goddess of the dawn and wife of Atea (Atea and Atanua emerged from Tanaoa, Atea first, who then made space for Atanua).

Their son is the first man, Tu-Mea. She created the seas after having a miscarriage and filling the oceans with her amniotic fluid.

title: Atanua


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Atarapa

In Polynesian mythology, Atarapa ("daybreak") is the goddess of the dawn and a daughter of Haronga.

title: Atarapa


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Atea

Atea is a deity in several Polynesian cultures, including the Marquesas and Tuamotu Islands, and New Zealand.

Marquesas Islands
In the mythology of the Marquesas Islands, Atea is the giver of light. In one legend Atea and Tāne are brothers, the sons of Toho. Another tradition relates that Atea (as light) evolved himself, and then brought forth Ono. Joining forces, they broke up the boundless darkness of the underworld (Po), where Tanaoa, lord of darkness, and Mutu-hei (silence) had lived for eternity. Atea and Ono made war on Tanaoa and Mutu-hei, and defeated them. They confined the gods of night within set boundaries. Out of the struggle came forth Atanua, the dawn. Atea then married Atanua, and their children include the lesser gods and humankind (Tregear 1891:29).

Tuamotu Islands
In the mythology of the Tuamotu islands, Atea is killed by Tāne, his second son (Meletinsky 2000:421). Their first son, Tahu, dies of starvation and the two gods switch sexes. Later, Atea tries to kidnap Tane, but Tane escapes to earth and eventually becomes so hungry that he eats a man, thus becoming the first cannibal. Tane declares war on Atea and kills him with the lightning bolts of Fatu-tiri, his ancestor).

New Zealand
Friedrich Ratzel in The History of Mankind (1896) related the Māori belief that creation commenced with the night then, after untold periods, desire awoke, then longing, then feeling. Thought followed upon the first pulse of life, or the first breath drawn; and upon thought, mental activity. Then sprang up the wish, directed to the sacred mystery or great riddle of life. Later, from the material procreative power of love develops the clinging to existence, permeated by a joyous sense of pleasure. Lastly, Atea, the universe, floated in space, divided by the difference of sex into Rangi and Papa, Heaven and Earth; and individual creations then began.

title: Atea


text:
Rain gauge

A rain gauge (also known as udometer, pluviometer, ombrometer, and hyetometer) is an instrument used by meteorologists and hydrologists to gather and measure the amount of liquid precipitation over a predefined area, over a period of time. It is used to determine the depth of precipitation (usually in mm) that occurs over a unit area and measure rainfall amount.

History
The first known rainfall records were kept by the Ancient Greeks, at around 500 BCE.

People living in India began to record rainfall in 400 BCE The readings were correlated against expected growth. In the Arthashastra, used for example in Magadha, precise standards were set as to grain production. Each state storehouse was equipped with a rain gauge to classify land for taxation purposes. Rainfall measurement was also mentioned in the Jewish text in Palestine. In 1247, the Song Chinese mathematician and inventor Qin Jiushao invented Tianchi basin rain and snow gauges to reference rain, and snowfall measurements, as well as other forms of meteorological data.

In 1441, the Cheugugi was invented during the reign of Sejong the Great of the Joseon Dynasty of Korea as the first standardized rain gauge. In 1662, Christopher Wren created the first tipping-bucket rain gauge in Britain in collaboration with Robert Hooke. Hooke also designed a manual gauge with a funnel that made measurements throughout 1695. Richard Towneley was the first to make systematic rainfall measurements over a period of 15 years from 1677 to 1694, publishing his records in the Philosophical Transactions of the Royal Society. Towneley called for more measurements elsewhere in the country to compare the rainfall in different regions, although only William Derham appears to have taken up Towneley's challenge. They jointly published the rainfall measurements for Towneley Park and Upminster in Essex for the years 1697 to 1704.

The naturalist Gilbert White took measurements to determine the mean rainfall from 1779 to 1786, although it was his brother-in-law, Thomas Barker, who made regular and meticulous measurements for 59 years, recording temperature, wind, barometric pressure, rainfall and clouds. His meteorological records are a valuable resource for knowledge of the 18th-century British climate. He was able to demonstrate that the average rainfall varied greatly from year to year with little discernible pattern.

National coverage and modern gauges

The meteorologist George James Symons published the first annual volume of British Rainfall in 1860. This pioneering work contained rainfall records from 168 land stations in England and Wales. He was elected to the council of the British Meteorological Society in 1863 and made it his life's work to investigate rainfall within the British Isles. He set up a voluntary network of observers, who collected data which were returned to him for analysis. So successful was he in this endeavour that by 1866 he was able to show results that gave a fair representation of the distribution of rainfall and the number of recorders gradually increased until the last volume of British Rainfall which he lived to edit, for 1899, contained figures from 3,528 stations — 2,894 in England and Wales, 446 in Scotland, and 188 in Ireland. He also collected old rainfall records going back over a hundred years. In 1870 he produced an account of rainfall in the British Isles starting in 1725.

Due to the ever-increasing numbers of observers, standardisation of the gauges became necessary. Symons began experimenting with new gauges in his own garden. He tried different models with variations in size, shape, and height. In 1863 he began a collaboration with Michael Foster Ward from Calne, Wiltshire, who undertook more extensive investigations. By including Ward and various others around Britain, the investigations continued until 1890. The experiments were remarkable for their planning, execution, and drawing of conclusions. The results of these experiments led to the progressive adoption of the well-known standard gauge, still used by the UK Meteorological Office today, namely, one made of "... copper, with a five-inch funnel having its brass rim one foot above the ground ..."

Most modern rain gauges generally measure the precipitation in millimetres in height collected during a certain period, equivalent to litres per square metre. Previously rain was recorded as inches or points, where one point is equal to 0.254 mm or 0.01 of an inch.

Rain gauge amounts are read either manually or by automatic weather station (AWS). The frequency of readings will depend on the requirements of the collection agency. Some countries will supplement the paid weather observer with a network of volunteers to obtain precipitation data (and other types of weather) for sparsely populated areas.

In most cases the precipitation is not retained, but some stations do submit rainfall and snowfall for testing, which is done to obtain levels of pollutants.

Rain gauges have their limitations. Attempting to collect rain data in a tropical cyclone can be nearly impossible and unreliable (even if the equipment survives) due to wind extremes. Also, rain gauges only indicate rainfall in a localized area. For virtually any gauge, drops will stick to the sides or funnel of the collecting device, such that amounts are very slightly underestimated, and those of .01 inches or .25 mm may be recorded as a "trace".

Another problem encountered is when the temperature is close to or below freezing. Rain may fall on the funnel and ice or snow may collect in the gauge, blocking subsequent rain. To alleviate this, a gauge may be equipped with an automatic electric heater to keep its moisture-collecting surfaces and sensor slightly above freezing.

Rain gauges should be placed in an open area where there are no buildings, trees, or other obstacles to block the rain. This is also to prevent the water collected on the roofs of buildings or the leaves of trees from dripping into the rain gauge after a rain, resulting in inaccurate readings.

Types
Types of rain gauges include graduated cylinders, weighing gauges, tipping bucket gauges, and simply buried pit collectors. Each type has its advantages and disadvantages while collecting rain data.

U.S. standard rain gauge
The standard United States National Weather Service rain gauge, developed at the start of the 20th century, consists of an 8 in funnel emptying into a graduated cylinder, 2.525 in in diameter, which fits inside a larger container that is 8 in in diameter and 20 in tall. If the rainwater overflows the graduated inner cylinder, then the larger outer container will catch it. When measurements are taken, then the height of the water in the small graduated cylinder is measured, and the excess overflow in the large container is carefully poured into another graduated cylinder and measured to give the total rainfall. A cone meter is sometimes used to prevent leakage that can result in alteration of the data. In locations using the metric system, the cylinder is usually marked in mm and will measure up to 250 mm of rainfall. Each horizontal line on the cylinder is 0.5 mm. In areas using Imperial units, each horizontal line represents 0.01 in inches.

Pluviometer of intensities
The pluviometer of intensities (or Jardi's pluviometer) is a tool that measures the average intensity of rainfall in a certain interval of time. It was initially designed to record the rainfall regime in Catalonia but eventually spread throughout the world. "It employs the principle of feedback ... the incoming water pushes the buoy upwards, making the lower 'adjusting conic needle' to let pass the same amount of water that enters into the container, this way ... the needle records on the drum the amount of water flowing through it at every moment—in mm of rainfall per square meter. " It consists of a rotating drum that rotates at constant speed, this drum drags a graduated sheet of cardboard, which has the time at the abscissa while the y-axis indicates the height of rainfall in mm of rain. This height is recorded with a pen that moves vertically, driven by a buoy, marking on the paper the rainfall over time. Each cardboard sheet is usually used for one day.

As the rain falls, the water collected by the funnel falls into the container and raises the buoy that makes the pen arm rise in the vertical axis, marking the cardboard accordingly. If the rainfall does not vary, the water level in the container remains constant, and while the drum rotates, the pen's mark is more or less a horizontal line, proportional to the amount of water that has fallen. When the pen reaches the top edge of the recording paper, it means that the buoy is "up high in the tank" leaving the tip of the conical needle in a way that uncovers the regulating hole, i.e., the maximum flow that the apparatus is able to record. If the rain suddenly decreases, making the container (as it empties) quickly lower the buoy, that movement corresponds to a steep slope line that can reach the bottom of the recorded cardboard if it stops raining.

The rain gauge of intensities allowed precipitation to be recorded over many years, particularly in Barcelona (95 years), apart from many other places around the world, such as Hong Kong.

Weighing precipitation gauge
A weighing-type precipitation gauge consists of a storage bin, which is weighed to record the mass. Certain models measure the mass using a pen on a rotating drum, or by using a vibrating wire attached to a data logger. The advantages of this type of gauge over tipping buckets are that it does not underestimate intense rain, and it can measure other forms of precipitation, including rain, hail, and snow. These gauges are, however, more expensive and require more maintenance than tipping bucket gauges.

The weighing-type recording gauge may also contain a device to measure the number of chemicals contained in the location's atmosphere. This is extremely helpful for scientists studying the effects of greenhouse gases released into the atmosphere and their effects on the levels of the acid rain. Some Automated Surface Observing System (ASOS) units use an automated weighing gauge called the AWPAG (All Weather Precipitation Accumulation Gauge).

Tipping bucket rain gauge

The tipping bucket rain gauge consists of a funnel that collects and channels the precipitation into a small seesaw-like container. After a pre-set amount of precipitation falls, the lever tips, dumping the collected water and sending an electrical signal. An old-style recording device may consist of a pen mounted on an arm attached to a geared wheel that moves once with each signal sent from the collector. In this design, as the wheel turns the pen arm moves either up or down leaving a trace on the graph and at the same time making a loud "click".

The tipping bucket rain gauge is not as accurate as the standard rain gauge, because the rainfall may stop before the lever has tipped. When the next period of rain begins it may take no more than one or two drops to tip the lever. This would then indicate that a pre-set amount has fallen when only a fraction of that amount has actually fallen. Tipping buckets also tend to underestimate the amount of rainfall, particularly in snowfall and heavy rainfall events. The advantage of the tipping bucket rain gauge is that the character of the rain (light, medium, or heavy) may be easily obtained. Rainfall character is decided by the total amount of rain that has fallen in a set period (usually 1 hour) by counting the number of pulses during that period. Algorithms may be applied to the data as a method of correcting the data for high-intensity rainfall.

Modern tipping rain gauges consist of a plastic collector balanced over a pivot. When it tips, it actuates a switch (such as a reed switch) which is then electronically recorded or transmitted to a remote collection station.

Tipping gauges can also incorporate elements of weighing gauges whereby a strain gauge is fixed to the collection bucket so that the exact rainfall can be read at any moment. Each time the collector tips, the strain gauge (weight sensor) is re-zeroed to null out any drift.

To measure the water equivalent of frozen precipitation, a tipping bucket may be heated to melt any ice and snow that is caught in its funnel. Without a heating mechanism, the funnel often becomes clogged during a frozen precipitation event, and thus no precipitation can be measured. Many Automated Surface Observing System (ASOS) units use heated tipping buckets to measure precipitation.

Optical rain gauge
This type of gauge has a row of collection funnels. In an enclosed space below each is a laser diode and a photo transistor detector. When enough water is collected to make a single drop, it drops from the bottom, falling into the laser beam path. The sensor is set at right angles to the laser so that enough light is scattered to be detected as a sudden flash of lights. The flashes from these photodetectors are then read and transmitted or recorded. Different type of optical range gauges have been used throughout the decades. The technology has also improved.

Acoustic rain gauge
Acoustic disdrometers, also referred to as hydrophones, are able to sense the sound signatures for each drop size as rain strikes a water surface within the gauge. Since each sound signature is unique, it is possible to invert the underwater sound field to estimate the drop-size distribution within the rain. Selected moments of the drop-size distribution yield rainfall rate, rainfall accumulation, and other rainfall properties.

title: Rain gauge


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List of figures in the Hawaiian religion

Hawaiian narrative or mythology, tells stories of nature and life. It is considered a variant of a more general Polynesian narrative, developing its own unique character for several centuries before about 1800. It is associated with the Hawaiian religion. The religion was officially suppressed in the 19th century, but kept alive by some practitioners to the modern day.

Prominent figures and terms in Hawaiian mythology

 * Aumakua - spirit of an ancestor or family god
 * ʻElepaio - monarch flycatcher
 * Kanehekili - god of thunder
 * Haumea - goddess of birth
 * Hiʻiaka - sister of Pele, daughter of Haumea & Kāne
 * Hina - goddess of Moon
 * Kahōālii - see Kamohoalii
 * Kalanipoo - bird goddess Queen
 * Kamapuaʻa - warlike god of wild boars, husband of Pele
 * Kāmohoalii - bird god King (Kaiiwialii)
 * Kanaloa – God of the ocean, working in concert with Kāne
 * Kāne – God of male procreation, fishponds, agriculture, sorcery; created world with help from Lono and Kū
 * Kānehekili – Thunder god
 * Kapo
 * Kapu - the code of conduct of laws and regulations
 * Kapua
 * Kaulu - killer of Haumea
 * Kihawahine - lizard woman
 * Kinilau
 * Kū – God of war, forests, canoe-building, deep-sea fishing
 * Laka – Goddess described as both the daughter and sister of Pele
 * Lohiʻau - chief of Kaua'i
 * Lono – God of irrigated agriculture, peace, and fertility
 * Mana - impersonal force
 * Māui - ancient hero and chief, demigod, shape shifter.
 * Menehune
 * Nāmaka - sea goddess and sister of Pele.
 * Nanaue - demigod, son of Kamohoalii the shark god, and Kalei, a mortal women
 * Nuakea - goddess of milk
 * Nightmarchers
 * Nuu - Hawaiian Noah
 * Papa - Goddess of Nature
 * Paʻao
 * Pakaʻa - a god of the wind, gatekeeper of underworld, wife of Kaiwa, Mother of Kaha’i
 * Paliuli
 * Papahānaumoku
 * Paupueo
 * Pele – Goddess of volcanoes, frequently described as an aumakua
 * Poliʻahu - goddesses of snow
 * Tuna
 * Ukupanipo - another shark god
 * Wahieloa
 * Waka - lizard goddess
 * Wākea - Sky father father of islands


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Samoan mythology

Samoan culture tells stories of many different deities. There were deities of the forest, the seas, rain, harvest, villages, and war. There were two types of deities, atua, who had non-human origins, and aitu, who were of human origin.

Tagaloa was a supreme god who made the islands and the people. Mafuiʻe was the god of earthquakes. There were also a number of war deities. Nafanua, Samoa's warrior goddess hails from the village of Falealupo at the western end of Savai'i island, which is also the site of the entry into Pulotu, the spirit world. She also is regarded as a peace bringer, having brought peace to Savai'i through winning the wars between the two regions of the island. Tilafaiga is the mother of Nafanua. Nafanua's father, Saveasi'uleo, was the god of Pulotu. Another well-known legend tells of two sisters, Tilafaiga, the mother of Nafanua, and Taema, bringing the art of tattooing to Samoa from Fiji.

A figure of another legend is Tui Fiti, who resides at Fagamalo village in the village district of Matautu. The village of Falelima is associated with a dreaded spirit deity called, Nifoloa. The Mata o le Alelo 'Eyes of the Demon' freshwater pool from the Polynesian legend Sina and the Eel is situated in the village of Matavai on the northern coast in the village district of Safune.

Fetu ("star") is the god of the night. His wife is Ele'ele.

Samoan mythology is a variant of a more general Polynesian mythology in the Samoa Islands.

Prominent entries on Samoan mythology

 * Afa
 * Atonga
 * Atu
 * Faʻatiu
 * Fisaga
 * Ila
 * Losi
 * Mafuiʻe
 * Moso's Footprint
 * Nafanua
 * Pulotu
 * Sava
 * Savali
 * Saveasiʻuleo
 * Sina and the Eel
 * Taema
 * Tagaloa
 * Tinilau
 * Tiʻitiʻi
 * Tui Fiti
 * Upolu

title: Samoan mythology


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Tongan narrative

Tongan narrative, Tongan mythology, or ancient Tongan religion, sometimes referred to as tala-ē-fonua (meaning, "telling of the land and its people") in Tongan, is the collation of various myths, legends, stories, traditions, characters, creatures, spirits, and gods of the Polynesian islands that now make up the island nation of Tonga.

It is a variant of other Polynesian mythologies with multiple cultural and religious overlaps, however it is considered to be distinct from other Polynesian mythologies as it can be studied more specifically to understand the history and culture of Tonga, as well it can also be studied more broadly within the fields of history, anthropology, and mythology.

Creation myth
In the beginning there was just the sea and the spirit world, Pulotu, and between them was a rock called Touiao Futuna. On the rock lived Piki and his twin sister, Kele, Atungaki and his twin sister, Maimoao Longona, Fonua'uta and his twin sister, Fonuavai, and Hemoana and his twin sister, Lupe. Piki lay with his own sister; they had two children, a son, Taufulifonua, and a daughter, Havea Lolofonua; Atungaki also lay with his sister, who bore him a daughter, Velelahi; and Fonuauta lay with his sister. They had a daughter, Velesii.

When Taufulifonua grew to manhood, his sister, Havea Lolofonua, bore him a son, Hikuleʻo, Tangaloa and Maui divided the creation between them. Hikuleo took as his portion, Pulotu, Tangaloa took the sky and Maui the underworld. Hemoana, whose form was the sea snake, and Lupe, whose form was a dove, then divided the remainder between them, Hemoana taking the sea and Lupe taking the land.

Tangaloa had several sons in the sky: Tangaloa Tamapouli Alamafoa, Tangaloa Eitumatupua, Tangaloa Atulongolongo and Tangaloa Tufunga. Old Tangaloa grew tired of looking down from the sky and seeing nothing but sea, so he sent down Tangaloa Atulongolongo in the form of a plover to see if he could find land. All Tangaloa 'Atulongolongo could find was a reef below the water, where Ata is now. So old Tangaloa told Tangaloa Tufunga to throw down into the sea the chips from the wood carving on which he was working. Tangaloa Tufunga continued to do this for a long time, and on two occasion Tangaloa Atulongolongo flew down in the form of a plover to see if anything had happened, but found nothing. On the third occasion, however, he found that the chips had formed an island. This was Eua. Later, Tangaloa Tufunga threw down more chips to form the islands of Kao and Tofua.

Tongatapu and most of the other islands were the work of Maui. One day Maui visited Manuka (Samoan: Manua) and there an old man, Tonga Fusifonua, gave him a fish-hook. Maui went fishing with this hook, but when he tried to pull in his line he found it was caught. He exerted all his strength and succeeded in hauling the line in, to find that he had dragged up Tongatapu from the bottom of the sea. Maui continued fishing with this wonderful hook and so pulled up from the deeps the rest of the islands of Tonga, and some of those of Fiji and Samoa as well.

ʻAta began as a reef below the water and slowly rose out of the sea. One day Tangaloa Atulonglongo visited Ata in the form of a plover and dropped a seed from the beak upon the island. The next time he visited 'Ata he found that the seed had grown into a creeper that covered the island. He pecked at the root of this creeper until it split in two. Then he returned to the sky. A few days later he returned to find that the root had rotted and a fat, juicy worm was curled up in it. He pecked the worm in two. From the top section a man was formed called Kohai. The bottom section also turned into a man called Koau. Then the plover felt a morsel left on his beak; he shook it off and it turned into a man called Momo. Kohai, Koau and Momo were the first men in Tonga. Maui brought them wives from Pulotu and they became the ancestors of the Tonga people.

Other prominent entries on Tongan narrative

 * ʻAhoʻeitu
 * Aitu
 * ʻEua
 * Hikule'o
 * 'Ilaheva
 * Kae and Longopoa
 * Kohai, Koau, mo Momo
 * Laufakana'a
 * Maui
 * Niuatoputapu
 * Sangone
 * Seketo'a
 * Tafahi
 * Tamapo'uli'alamafoa
 * Tangaloa
 * Taufa
 * Tu'itatui
 * ʻAta
 * Limu

title: Tongan narrative


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Ati

Ati or ATI may refer to:
 * Ati people, a Negrito ethnic group in the Philippines
 * Ati language (Philippines), the language spoken by this people group
 * Ati-Atihan festival, an annual celebration held in the Philippines
 * Ati language (China), an unclassified Loloish language of Huaning County, Yunnan, China
 * Ati, a queen of the fabled Land of Punt in Africa
 * Ati, Chad, a town in Chad
 * Ati, Iran, a village in Khuzestan Province, Iran
 * Maha Ati, one of the nine subdivisions of Buddhist Tantra in the Nyingma school

The acronym ATI may refer to:

Academic institutions

 * Auckland Technical Institute, now the Auckland University of Technology, Auckland, New Zealand
 * Ohio State University Agricultural Technical Institute, known as Ohio State ATI
 * An associate of the Tax Institute of Australia

Electronics and information technology

 * Acronis True Image, cloning and backup software
 * Allied Telesis, a telecommunications company, formerly Allied Telesyn
 * Tunisian Internet Agency, also known as ATI (short for Agence tunisienne d'Internet), telecommunications company in Tunisia

Companies

 * ATI Inc., an American specialty metals company headquartered in Dallas, Texas
 * ATI Technologies, a former Canadian semiconductor company manufacturing GPUs and chipsets

Transportation

 * Aero Trasporti Italiani, a defunct Italian airline (ICAO designator)
 * Air Transport International, a charter cargo airline
 * Artigas Airport, IATA designation

Other uses

 * Above threshold ionization, a photoelectrochemical process ionizing an atom by an excessive number of photons
 * Access to Insight, a Theravada Buddhist website
 * Associated Television International, a production and distribution company
 * Astatine monoiodide, a radioactive molecule
 * Amylase/trypsin inhibitor, a substance that inhibits the enzymes amylase or trypsin (see Non-celiac gluten sensitivity)

title: Ati


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Atonga

In Polynesian mythology (specifically: Samoa), Atonga is a culture hero, half-mortal and half-spirit. He built the first canoe on Savaii with his brothers Olokeu and Olo-i-nano, then had it transported to Upolu by birds.

title: Atonga


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Fijian mythology

Fijian mythology refers to the set of beliefs practiced by the indigenous people of the island of Fiji.

Their indigenous religion, like many others around the world, is based on cyclic existence where their ancestors and the environment exist in a dynamic cycle through experience, history and one with nature. Like a plant that bears seeds to exist, similar to humans, animals and all other life dependent of the reproductive cycle to maintain existence on earth. Fijians believe that humans exist with nature and sometimes are dominated by other species like sharks, snakes, octopuses, and more, where humans are the prey, rather than the predator.

Some primarily examples of their gods are Degei, a serpent who is the supreme god of Uluda Fiji. He is the creator of the (Fijian) world. He judges newly dead souls after they pass through one of two caves: Cibaciba or Drakulu. A few he sends to paradise Burotu or Burotukula. Most others are thrown into a lake, where they will eventually sink to the bottom (Murimuria) to be appropriately rewarded or punished.

title: Fijian mythology


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Atu

In Samoan mythology, Atu was the first man on Fiji and Tonga. (Note: This is according to the Samoans, not the Fijians or Tongans; see also Ila.)

title: Atu


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Atua I Kafika

In the mythology of Tikopia, the Atua I Kafika (also known as Sako) is a culture hero and supreme god.

title: Atua I Kafika


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Atua I Raropuka

In Polynesian mythology (specifically Tikopia), Atua I Raropuka is a creator being, often spoken of in tandem with his wife, Atua Fafine.

Raropuka is the lesser creator-god. He was invoked after the creation of the world within the sky, and was directed to give man all his knowledge.

Which foodstuffs were edible, which animals were to become man's totems and which were taboo, as these are either messengers or omens. Taking a totem animal is allowed within the clan of that animal. A man must present at least one of each one of his clan's totem animals to the heads of all the other clans to become a man (among other rites of passage). A man who accidentally takes another clan's totem animals must give it either an adult male or female member of that animal's clan. Typically, the offended clan member forgives the trespass, and will return the totem with the majority of the kill intact, save for the decorative, tool-bearing, or ritualistic portions removed. A woman of any age who finds, or fishes a totem animal may do the portioning on any totem animals within or out-clan if she intends to give the portions to that clan. If a girl wishes to marry, she will actively pursue and portion totem animals and make offerings to the boy she likes.

Raropuka is the creator of medicinal plants (either herbal or drug plants) and all fruit bearing plants. Non-edible plants and flowers bearing non-edible fruits are the work of the original creator god.

title: Atua I Raropuka


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Atutahi

In Māori mythology, Atutahi is the name of Canopus, (Alpha Carinae). Atutahi is the second brightest star in the night-time sky, second only to Sirius (Takurua). Aotahi, Autahi, and Atutahi, "First-light" or "Single-light", were equivalent names that also intimated the star's solitary or self-centered nature.

Atutahi is considered to be a very tapu star, and always dwells alone. This is seen in its position outside the Milky Way.

Of all the stars known to the Māori, Atutahi had a special place, along with Rigel (Puanga), because of their intimate association with kūmara cultivation. Their appearance in the eastern sky was the sign for planting to begin. Atutahi and Puanga could be used to foretell the weather.

Atutahi has several different mythologies attached to it as well. One story tells of how Atutahi was left outside of the basket representing the Milky Way when Tane wove it. Another related myth surrounding the star says that Autahi was the first-born child of Rangi, who refused to enter the Milky Way and so turned it sideways and rose before it. The same name is used for other stars and constellations throughout Polynesia.

Other names

 * Aotahi
 * Autahi
 * Atutahi-mā-Rehua

title: Atutahi


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Conjoined twins

Conjoined twins, popularly referred to as Siamese twins, are twins joined in utero. It is a very rare phenomenon, estimated to occur in anywhere between one in 49,000 births to one in 189,000 births, with a somewhat higher incidence in Southwest Asia and Africa. Approximately half are stillborn, and an additional one-third die within 24 hours. Most live births are female, with a ratio of 3:1.

Two possible explanations of the cause of conjoined twins have been proposed. The one that is generally accepted is fission, in which the fertilized egg splits partially. The other explanation, no longer believed to be accurate, is fusion, in which the fertilized egg completely separates, but stem cells (that search for similar cells) find similar stem cells on the other twin and fuse the twins together. Conjoined twins share a single common chorion, placenta, and amniotic sac in utero, but so do some monozygotic but non-conjoined twins.

Chang and Eng Bunker (1811–1874) were brothers born in Siam (now Thailand) who traveled widely for many years and were known internationally as the Siamese Twins. Chang and Eng were joined at the torso by a band of flesh and cartilage, and by their fused livers. In modern times, they could easily have been separated. Due to the brothers' fame and the rarity of the condition, the term Siamese twins came to be associated with conjoined twins.

Causes
There are two hypotheses about the development of conjoined twins.
 * 1) a single fertilized egg does not fully split during the process of forming identical twins. If the zygote division occurs after two weeks of the development of the embryonic disc, it results in the formation of conjoined twins.
 * 2) fusion of two fertilized eggs occurs early in development.

Partial splitting of the primitive node and streak may result in the formation of conjoined twins. These twins are classified according to the nature and degree of their union. Occasionally, monozygotic twins are connected only by a common skin bridge or by a common liver bridge. The type of twins formed depends on when and to what extent abnormalities of the node and streak occurred. Misexpression of genes, such as goosecoid, may also result in conjoined twins. Goosecoid activates inhibitors of BMP4 and contributes to regulation of head development. Over- or underexpression of this gene in laboratory animals results in severe malformations of the head region, including duplications, similar to some types of conjoined twins.

Types
Conjoined twins are typically classified by the point at which their bodies are joined. The most common types of conjoined twins are:
 * Thoracopagus (28% of cases): Two bodies fused from the upper chest to the lower chest. These twins usually share a heart and may also share the liver or part of the digestive system. Survival rate is poor.
 * Thoraco-omphalopagus (18.5%): Two bodies fused from the upper chest to lower belly. The heart is always shared in these cases., twins who share a heart have not been able to both survive separation; a designated twin who is allotted the heart may survive if the other twin is sacrificed.
 * Omphalopagus (10%): Two bodies fused at the lower abdomen. Unlike thoracopagus, the heart is not shared; however, the twins often share a liver, a digestive system, a diaphragm and other organs. Survival rate is 82%.
 * Parasitic twins (10%): Twins that are asymmetrically conjoined, resulting in one twin that is small, less formed, and dependent on the larger twin's organs for survival.
 * Craniopagus (6%): Fused skulls, but separate bodies. These twins' heads may be conjoined at the back, front, or side of the head, but not on the face or at the base of the skull. Survival rate is poor.

Other, less common types of conjoined twins include:
 * Cephalopagus: Two faces on opposite sides of a single, conjoined head; the upper portion of the body is fused while the bottom portions are separate. These twins generally cannot survive due to severe malformations of the brain. This is also known as janiceps (after the two-faced Roman deity Janus).
 * Syncephalus: One head with a single face but four ears and two bodies.
 * Cephalothoracopagus: Bodies fused at the head and thorax, with two faces facing in opposite directions, or sometimes with a single face and an enlarged skull.
 * Xiphopagus: Two bodies fused in the xiphoid cartilage, which extends approximately from the navel to the lower breastbone. These twins almost never share any vital organs, with the exception of the liver. A famous example is Chang and Eng Bunker.
 * Ischiopagus: Fused lower half of the two bodies, with spines conjoined end-to-end at a 180° angle. These twins have four arms; one, two, three or four legs; and typically one set of external genitalia and one anus. Survival rate is 63%.
 * Omphalo-Ischiopagus: Fused in a similar fashion to ischiopagus twins, but facing each other, with a joined abdomen, akin to omphalopagus. These twins have four arms, and two, three, or four legs.
 * Parapagus: Fused side by side with a shared pelvis. Those that are dithoracic parapagus are fused at the abdomen and pelvis, but not at the thorax. Those that are diprosopic parapagus have one trunk and two faces. Those that are dicephalic parapagus have one trunk and two heads, and may have two (dibrachius), three (tribrachius), or four (tetrabrachius) arms. Survival rate is poor.
 * Craniopagus parasiticus: Like craniopagus, but with a second bodiless head attached to the dominant head.
 * Pygopagus or Iliopagus: Two bodies joined at the pelvis. Survival rate is 68%.
 * Rachipagus: Twins joined along the back of their bodies, with fusion of the vertebral arches and the soft tissue from the head to the buttocks. Twins of this type cannot be separated.
 * Tricephalus (conjoined triplets): Extremely rare conjoining of 3 fetuses. Very few confirmed cases, both human and animal, are known.

Separation
Surgery to separate conjoined twins may range from very easy to very difficult depending on the point of attachment and the internal parts that are shared. Most cases of separation are extremely risky and life-threatening. Though there have been a number of successful separations throughout history, in many cases, the surgery results in the death of one or both of the twins, particularly if they are joined at the head or share a vital organ. This makes the ethics of surgical separation, where the twins can survive if not separated, contentious. Alice Dreger of Northwestern University found the quality of life of twins who remain conjoined to be higher than is commonly supposed. Lori and George Schappell and Abby and Brittany Hensel are notable examples.

The first recorded separation of conjoined twins took place in the Byzantine Empire in the 900s. One of the conjoined twins had already died, so the doctors of the town attempted to separate the dead twin from the surviving twin. The result was briefly successful, as the remaining twin lived for three days after separation. The next recorded case of separating conjoined twins was several centuries later, in Germany, in 1689. The first recorded successful separation of conjoined twins was performed in 1689 by Johannes Fatio. Around this same time Dr. Böhm of Gunzenhausen separated his own children, a pair of omphalopagus or xiphopagus twins; the feebler twin died four days later, but the healthier one was still alive and well at age five, when the case was reported. In 1955, neurosurgeon Harold Voris (1902-1980) and his team at Mercy Hospital in Chicago performed the first successful operation to separate craniopagus twins (conjoined at the head), which resulted in long-term survival for both. The larger girl was reported in 1963 as developing normally, but the smaller girl was permanently impaired.

In 1957, Bertram Katz and his surgical team made international medical history performing the world's first successful separation of conjoined twins sharing a vital organ. Omphalopagus twins John Nelson and James Edward Freeman (Johnny and Jimmy) were born in Youngstown, Ohio, on April 27, 1956. The boys shared a liver but had separate hearts and were successfully separated at North Side Hospital in Youngstown, Ohio, by Bertram Katz. The operation was funded by the Ohio Crippled Children's Service Society.

Recent successful separations of conjoined twins include that of the separation of Ganga and Jamuna Shreshta in 2001, who were born in Kathmandu, Nepal, in 2000. The 97-hour surgery on the pair of craniopagus twins was a landmark one which took place in Singapore; the team was led by neurosurgeons Chumpon Chan and Keith Goh. The surgery left Ganga with brain damage and Jamuna unable to walk. Seven years later, Ganga Shrestha died at the Model Hospital in Kathmandu in July 2009, at the age of eight, three days after being admitted for treatment of a severe chest infection.

Infants Rose and Grace Attard, conjoined twins from Malta, were separated in the United Kingdom by court order Re A over the religious objections of their parents, Michaelangelo and Rina Attard. The twins were attached at the lower abdomen and spine. The surgery took place in November 2000, at St Mary's Hospital in Manchester. The operation was controversial because Rose, the weaker twin, would die as a result of the procedure as her heart and lungs were dependent upon Grace's. However, if the operation had not taken place, it was certain that both twins would die. Grace survived to enjoy a normal childhood.

In 2003, two 29-year-old women from Iran, Ladan and Laleh Bijani, who were joined at the head but had separate brains (craniopagus) were surgically separated in Singapore, despite surgeons' warnings that the operation could be fatal to one or both. Their complex case was accepted only because technologically advanced graphical imagery and modeling would allow the medical team to plan the risky surgery. However, an undetected major vein hidden from the scans was discovered during the operation. The separation was completed but both women died while still in surgery.

In 2019 Safa and Marwa Ullah were separated at Great Ormond Street Hospital in London, England. The twins, born January 2017 were joined at the top of the head with separate brains and a cylindrical shared skull with the twins each facing in opposite directions to one another. The surgery was jointly led by neurosurgeon Owase Jeelani and plastic surgeon Professor David Dunaway. The surgery presented particular difficulties due to a number of shared veins and a distortion in the shape of the girls' brains, causing them to overlap. The distortion would need to be corrected in order for the separation to go ahead. The surgery utilized a team of more than 100 including bio engineers, 3D modelers and a virtual reality designer. The separation was completed in February 2019 following a total of 52 hours of surgery over three separate operations. As of July 2019, both girls remain healthy and the family planned to return to their home in Pakistan in 2020.

History
The Moche culture of ancient Peru depicted conjoined twins in their ceramics dating back to 300 CE. Writing around 415 AD, St. Augustine of Hippo, in his book, City of God, refers to a man "double in his upper, but single in his lower half—having two heads, two chests, four hands, but one body and two feet like an ordinary man."

According to Theophanes the Confessor, a Byzantine historian of the 9th century, around 385/386 AD, he writes that "in the village of Emmaus in Palestine, a child was born perfectly normal below the navel but divided above it, so that it had two chests and two heads, each possessing the senses. One would eat and drink but the other did not eat; one would sleep but the other stayed awake. There were times when they played with each other, when both cried and hit each other. They lived for a little over two years. One died while the other lived for another four days and it, too, died."

In Arabia, the twin brothers Hashim ibn Abd Manaf and 'Abd Shams were born with Hashim's leg attached to his twin brother's head. Legend says that their father, Abd Manaf ibn Qusai, separated his conjoined sons with a sword and that some priests believed that the blood that had flowed between them signified wars between their progeny (confrontations did occur between Banu al'Abbas and Banu Ummaya ibn 'Abd Shams in the year 750 AH). The Muslim polymath Abū al-Rayhān al-Bīrūnī described conjoined twins in his book Kitab-al-Saidana.

The English twin sisters Mary and Eliza Chulkhurst, who were conjoined at the back (pygopagus), lived from 1100 to 1134 (or 1500 to 1534) and were perhaps the best-known early historical example of conjoined twins. Other early conjoined twins to attain notice were the "Scottish brothers", allegedly of the dicephalus type, essentially two heads sharing the same body (1460–1488, although the dates vary); the pygopagus Helen and Judith of Szőny, Hungary (1701–1723), who enjoyed a brief career in music before being sent to live in a convent; and Rita and Cristina of Parodi of Sardinia, born in 1829. Rita and Cristina were dicephalus tetrabrachius (one body with four arms) twins and although they died at only eight months of age, they gained much attention as a curiosity when their parents exhibited them in Paris.

Several sets of conjoined twins lived during the nineteenth century and made careers for themselves in the performing arts, though none achieved quite the same level of fame and fortune as Chang and Eng. Most notably, Millie and Christine McCoy (or McKoy), pygopagus twins, were born into slavery in North Carolina in 1851. They were sold to a showman, J.P. Smith, at birth, but were soon kidnapped by a rival showman. The kidnapper fled to England but was thwarted because England had already banned slavery. Smith traveled to England to collect the girls and brought with him their mother, Monimia, from whom they had been separated. He and his wife provided the twins with an education and taught them to speak five languages, play music, and sing. For the rest of the century, the twins enjoyed a successful career as "The Two-Headed Nightingale" and appeared with the Barnum Circus. In 1912, they died of tuberculosis, 17 hours apart.

Giacomo and Giovanni Tocci, from Locana, Italy, were immortalized in Mark Twain's short story "Those Extraordinary Twins" as fictitious twins Angelo and Luigi. The Toccis, born in 1877, were dicephalus tetrabrachius twins, having one body with two legs, two heads, and four arms. From birth they were forced by their parents to perform and never learned to walk, as each twin controlled one leg (in modern times, physical therapy allows twins like the Toccis to learn to walk on their own). They are said to have disliked show business. In 1886, after touring the United States, the twins returned to Europe with their family. They are believed to have died around this time, though some sources claim they survived until 1940, living in seclusion in Italy.

Born 19th century and earlier

 * Mary and Eliza Chulkhurst, alleged names of the Biddenden Maids (per tradition, born in the 12th century) of Kent, England. They are the earliest set of conjoined twins whose names are (purportedly) known.
 * Lazarus and Joannes Baptista Colloredo (1617 — after 1646), autosite-and-parasite pair
 * Helen and Judith of Szony (Hungary, 1701 — 1723), pygopagus.
 * Chang and Eng Bunker (1811–1874). The Bunker twins were born of Chinese origin in Siam (now Thailand), and the expression Siamese twins is derived from their case. They were joined by the areas around their xiphoid cartilages, but over time, the connective tissue stretched.
 * In 1834, a set of conjoined triplets were born in Catania. Two of the heads shared a neck while the other head had its own. The infant, a male, was described by Galvagni.
 * Millie and Christine McCoy (July 11, 1851 – October 8, 1912), (oblique pygopagus). The McCoy twins were born into slavery in Columbus County, North Carolina, United States. They went by the stage names "The Two-Headed Nightingale" and "The Eighth Wonder of the World" and had an extensive career before retiring to the farm on which they were born.
 * Giacomo and Giovanni Battista Tocci (1875? — 1912?), (dicephalus tetrabrachius dipus)
 * Josefa and Rosa Blazek (January 20, 1878 — March 30, 1922), pygopagus. The Blazek twins were born in Skrejšov, Bohemia (now the Czech Republic). They began performing in public exhibitions at the age of 13, and their act later included Rosa's son Franz. The sisters died in Chicago, Illinois.

Born 20th century

 * Daisy and Violet Hilton of Brighton, England (1908–1969), pygopagus. The Hilton twins were performers who played musical instruments, sang, and danced. At the height of their career, they had the highest paid act in vaudeville. They also appeared in the movies Freaks and Chained for Life.
 * Lucio and Simplicio Godina of Samar, Philippines (1908–1936)
 * Masha and Dasha Krivoshlyapova of Moscow, Russia (1950–2003), the rarest form of conjoined twins, one of few cases of dicephalus tetrabrachius tripus (two heads, four arms, three legs)
 * Ronnie and Donnie Galyon of Ohio (1951–2020), omphalopagus; longest-lived conjoined twins in the world at 68 years and 250 days.
 * Tjitske and Folkje de Vries of Mûnein, Netherlands (b. 1953)
 * Wariboko and Tamunotonye Davies, born July 25, 1953, in Kano, Nigeria. Separated in London by a team led by Ian Aird. Tamunotonye died postoperatively. Wariboko became a nurse.
 * Lori and George Schappell of Reading, Pennsylvania (1961–2024) American entertainers, craniopagus. At the time of their death, they were the world's oldest living conjoined twins. Guinness World Records noted that George's gender transition made him and Lori the first same-sex conjoined twins to identify as different genders.
 * Ganga and Jamuna Mondal of India, born 1969 or 1970, known professionally as The Spider Girls and The Spider Sisters. Ischiopagus.
 * Anna and Barbara Rozycki (born 1970), the first conjoined twins successfully separated in the UK.
 * Ma Nan Soe and Ma Nan San (born 1971 in Myanmar), separated in July 1971 at Yangon Pediatric Hospital. They were joined from chest to belly button. Ma Nan San died one month and seven days after operation.
 * Elisa and Lisa Hansen, Ogden, Utah (1977). Born by Caesarean section on October 18, 1977, were conjoined at the top of their head (craniopagus). They were separated 1979 after 16-hour surgery, were first to both survive surgery. Elisa lost the use of her right side after the surgery, but went on to complete school, win medals in the Special Olympics, work, and act in the theatre. Elisa and Lisa died in 2020 (age 42).
 * Ladan and Laleh Bijani of Shiraz, Iran (1974–2003); died during separation surgery in Singapore. Craniopagus.
 * Baby Girl A and Baby Girl B (born 1977 in New Jersey) shared a single six-chambered heart. Separation surgery, led by C. Everett Koop, involved the instant death of Baby Girl A; the difficult ethical and religious concerns generated significant local newspaper coverage. Baby Girl B survived for three months.
 * Viet and Duc Nguyen, born on February 25, 1981, in Kon Tum Province, Vietnam, and separated in 1988 in Ho Chi Minh City. Viet died on October 6, 2007. Ischiopagus.
 * Maria and Consolata Mwakikuti of Tanzania (1996–2018); conjoined by the abdomen; died of respiratory problems resulting from an abnormal, inoperable chest deformity.
 * Patrick and Benjamin Binder, separated in 1987 by team of doctors led by Ben Carson. Craniopagus.
 * Andrew and Alex Olson, born in 1987, separated in April 1988 at the University of Nebraska Medical Center. Omphalopagus. Alex died in 2018.
 * Katie and Eilish Holton, born August 1988 in Ireland, separated at age 3 and a half. Katie died 4 days after the separation surgery due to a weak heart which went into cardiac arrest.
 * Abigail and Brittany Hensel are dicephalic parapagus twins born on March 7, 1990, in Carver County, Minnesota. Both graduated in 2012 from Bethel University, St. Paul, hired as teachers.
 * Tiesha and Iesha Turner (born 1991 in Texas), separated in 1992 at Texas Children's Hospital in Houston, Texas. Omphalopagus.
 * Ashley and Ashil Fokeer, born on November 2, 1992, in Mauritius
 * Joseph and Luka Banda (born January 23, 1997, in Zambia), separated in 1997 in South Africa by Ben Carson (with a later intervention in 2001 to artificially close their skulls). Craniopagus.
 * José Armando and José Luis Cevallos Herrera were born in September 1999 in Milagro, Ecuador. They were accepted in 2021 to the State University of Milagro.
 * Maria del Carmen Andrade Solis and Maria Guadalupe Andrade Solis (better known as Carmen and Lupita) were born in June 2000 in Veracruz, Mexico. They later moved to the United States for healthcare with their parents.

Born 21st century

 * Carl and Clarence Aguirre, born with vertical craniopagus in Silay City, Negros Occidental, on April 21, 2002. They were successfully separated on August 4, 2004.
 * Tabea and Lea Block, from Lemgo, Germany, were born as craniopagus twins joined on the tops of their heads on August 9, 2003. The girls shared some major veins, but their brains were separate. They were separated on September 16, 2004, although Tabea died about ninety minutes later.
 * Sohna and Mohna from Amritsar, India. Born in New Delhi on June 14, 2003. They have two hearts, arms, kidneys and spinal cords while share liver, gall bladder and legs.
 * Anastasia and Tatiana Dogaru, born outside Rome in Lazio, Italy, on January 13, 2004. As craniopagus twins, the top of Tatiana's head is attached to the back of Anastasia's head.
 * Lakshmi Tatma (born 2005) was an ischiopagus conjoined twin born in Araria district in the state of Bihar, India. She had four arms and four legs, resulting from a joining at the pelvis with a headless undeveloped parasitic twin.
 * In 2005 a set of conjoined triplets was detected, characterized as tricephalus, tetrabrachius, and tetrapus parapagothoracopagus, and the pregnancy interrupted at 22 weeks.
 * Kendra and Maliyah Herrin, ischiopagus twins separated in 2006 at age 4
 * Krista and Tatiana Hogan, Canadian twins conjoined at the head. Born October 25, 2006. Share part of their brain and can pass sensory information and thoughts between each other.
 * Trishna and Krishna from Bangladesh were born in December 2006. They are craniopagus twins, joined on the tops of their skulls and sharing a small amount of brain tissue. In 2009, they were separated in Melbourne, Australia.
 * Maria and Teresa Tapia, born in the Dominican Republic on April 8, 2010. Conjoined by the liver, pancreas, and a small portion of their small intestine. Separation occurred on November 7, 2011, at Children's Hospital of Richmond at VCU.
 * Aung Myat Kyaw and Aung Khant Kyaw (born in May 2011, Mandalay, Myanmar), connected at pelvis.
 * Jesus and Emanuel de Nazaré are dicephalic parapagus twins born in Pará, Brazil on December 19, 2011.
 * Zheng Han Wei and Zheng Han Jing, born in China on August 11, 2013. Conjoined by their sternum, pericardium, and liver. In 2014, they were separated in Shanghai, China, at the Shanghai Children's Medical Center.
 * Asa and Eli Hamby were born in 2014 in Georgia but died less than two days after birth due to heart failure. The twins were dicephalic parapagus having two heads but being conjoined at the torso, arms and legs. They had separate spinal columns but one heart making postnatal operations impossible.
 * Jadon and Anias McDonald, born in September 2015. Conjoined by the head. Successfully separated at Children's Hospital of Montefiore Medical Center by James T. Goodrich in October 2016.
 * Erin and Abby Delaney, born in Philadelphia, Pennsylvania, on July 24, 2016. Conjoined by the head. They were successfully separated at Children's Hospital of Philadelphia on June 16, 2017.
 * Marieme and Ndeye Ndiaye, twin girls born in Senegal in 2017, living in Cardiff, UK in 2019.
 * Safa and Marwa Bibi, twin girls born in Hayatabad, Pakistan on January 17, 2017, conjoined by the head. Successfully separated at Great Ormond Street Hospital in February 2019.
 * Callie and Carter Torres, born January 30, 2017, in Houston Texas, from Blackfoot Idaho. They are Omphalo-Ischiopagus conjoined twins, attached by their pelvic area and sharing all organs from the belly button down with just one leg each.
 * Yiğit and Derman Evrensel, twin boys born on June 21, 2018, Antalya, Turkey. They are craniopagus twins and were separated at Great Ormond Street Hospital in 2019 by the same surgeons that separated Safa and Marwa Bibi.
 * Ervina and Prefina, born June 29, 2018, in the Central African Republic. They were separated on June 5, 2020, at the Bambino Gesù Pediatric Hospital in Rome, Italy.
 * Mercy and Goodness Ede, born August 13, 2019, conjoined by the chest and abdomen. Successfully separated at the National Hospital in Abuja, Nigeria in November 2019.
 * Marie-Cléa and Marie-Cléanne Papillon, born in Mauritius in 2019. Conjoined from neck to abdomen, but also at the heart which had seven chambers, instead of four. Marie-Cléa did not survive the surgery to separate the two.
 * Valentina and Kristina, born in 2019 in Croatia, shared a part of the digestive system and liver (xypho-omphalopagus). Several months after birth they developed twin-to-twin transfusion syndrome due to blood shunting, and they were successfully separated at University Hospital Centre Zagreb (Rebro) in an urgent procedure.
 * Susannah and Elizabeth Castle, born April 22, 2021, and separated December 10, 2021, in Philadelphia, Pennsylvania.
 * AmieLynn Rose and JamieLynn Rae Finley, born October 3, 2022, and separated January 23, 2023, in Fort Worth, Texas.

In fiction
Conjoined twins have been the focus of several works of fiction, including:

 * Irish author Sarah Crossan won the Carnegie Medal for her verse novel, One. The story follows the life and survival of conjoined twin sisters. The book also won The Bookseller 2016 prize for young adult fiction and the Irish Children's Book of the Year.
 * Lori Lansens's second novel The Girls (2005) is the life story of craniopagus twin sisters.

title: Conjoined twins


text:
Auahitūroa

Auahitūroa is a male Māori deity, personification of comets, and the origin of fire.

His consort is Mahuika, the goddess of fire.

title: Auahitūroa


text:
Mornington Crescent tube station

Mornington Crescent is a London Underground station in Somers Town in north west London, named after the nearby street. The station is on the Charing Cross branch of the Northern line, between and  stations. It is in Travelcard Zone 2.

The station was opened as part of the original route of the Charing Cross, Euston & Hampstead Railway (now the Charing Cross branch of the Northern line) on 22 June 1907. The surface building was designed by the Underground Electric Railways Company of London's (UERL's) architect Leslie Green in the Modern Style (British Art Nouveau style). Prior to the station's opening, the name of "Seymour Street" had been proposed. After opening, it was little used. For many years it was open only on weekdays, and before 1966 Edgware-bound trains passed through without stopping.

Location
The station is situated at the southern end of Camden High Street, where it meets Hampstead Road and Eversholt Street. This junction forms the north-western corner of the boundary of Somers Town, with Camden Town situated to the north and Regent's Park Estate to the south of the station.

The station's location on the Northern line is unusual due to the dual-branch nature of that line. On the branch, Mornington Crescent is between  and. The Bank branch also runs from Camden Town to Euston, but via tunnels which take an entirely different route to the Charing Cross branch and which do not pass through Mornington Crescent. Although modern-day tube maps show Mornington Crescent to the west of the Bank branch tunnels, it is actually to the east of them: the two branches cross over each other at Euston, so that between Euston and Camden Town, the Bank branch tunnels run to the west of the Charing Cross branch on which Mornington Crescent is situated. Harry Beck's 1933 tube map represented this correctly.

There is a northwards facing crossover to the north of the station to enable trains from Camden Town to terminate and head back north.

Closure and reopening
On 23 October 1992, the station was shut so that the then 85-year-old lifts could be replaced. The intention was to open it within one year. However, due to lack of funding and the state of neglect, the station remained closed for 6 years.

A concerted campaign to reopen the station was launched, with pressure from Camden Council, and assistance from the popular BBC Radio 4 panel game I'm Sorry I Haven't a Clue. The show frequently features the game Mornington Crescent, which takes its name from the station.

During the station's rebuilding, the original distinctive light blue tiling pattern was restored to the station (though taking into account modern fire safety requirements). The ticket hall was reconstructed and the original emergency stairs closed. A second lift shaft was converted (losing the unnecessary extra two lifts) into a staircase on one side and a series of station facilities on the other.

After substantial refurbishment, the station was reopened on 27 April 1998 by the regular cast of I'm Sorry I Haven't a Clue (Humphrey Lyttelton, Barry Cryer, Tim Brooke-Taylor and Graeme Garden). A memorial plaque to the late Willie Rushton, one of the longest-serving panelists, was installed at the station in 2002.

Since its 1998 reopening, the station has been open at the same times as most other stations, including weekends, in an attempt to relieve the pressure on the increasingly busy nearby station.

Services
The typical offpeak service in trains per hour (tph) is as follows:

 * 20 tph Southbound to Kennington, of which 10 continue to Battersea power station
 * 10 tph Northbound to Edgware
 * 8 tph Northbound to High Barnet
 * 2 tph Northbound to Mill Hill East

The peak services differ between the morning and evening peaks, but generally the typical peak service in trains per hour (tph) is as follows:

 * 22 tph Southbound to Kennington, of which 6 continue to Morden and 12 continue to Battersea power station
 * 12 tph Northbound to Edgware
 * 10 tph Northbound to High Barnet
 * 2 tph Northbound to Mill Hill East

In film

 * The station was used as a location for the anthology film Tube Tales (1999).
 * It was portrayed in the film Honest (2000), although the station actually used was Aldwych.
 * In Allt flyter (2008), Sara meets her mother (who has moved there from Sweden for a sportscasting job) outside the station during a Christmas trip to London.

In radio

 * Mornington Crescent is a spoof game, featured since the 1970s in the BBC Radio 4 comedy panel show I'm Sorry I Haven't a Clue, which satirises complicated strategy games. A Comic Heritage blue plaque honouring Willie Rushton, one of the show's longest-serving panelists, was installed within the station in 2002. It is located behind the ticket barrier at the top of the stairs to the platform.

In literature

 * China Miéville mentions this station and its long state of disuse during the 1990s in his novel King Rat (1998), also using it as scene of a brutal murder by dismemberment via a passing train.
 * In The Atrocity Archives (2004) by Charles Stross, the secret main entrance to the extremely secret Government establishment (the "Laundry") which the protagonist Bob Howard works for is situated in the gentlemen's toilets of Mornington Crescent tube station.
 * In Christopher Fowler's "Bryant & May" mysteries, the offices of the Peculiar Crimes Unit are above Mornington Crescent tube station.
 * Mornington Crescent is used by Robert Rankin in many of his novels as the home of the Ministry of Serendipity, a fictional agency whose main activity is to ensure the British Empire rules the globe, the top secret nature of the ministry being the main reason why the station was only open on weekdays and closed for "repairs" for much of the 1990s.

In music

 * Belle & Sebastian released a song entitled "Mornington Crescent" on their 2006 album, The Life Pursuit.
 * My Life Story's 1995 album Mornington Crescent takes its name from the station, featuring photos in its sleeve notes.
 * The promotional video for "Be There" by Unkle was filmed in this station.

Connections
London Buses routes 1, 24, 27, 29, 46, 134, 214, 253 and 274 and night routes N5, N20, N28, N29, N31, N253 and N279 serve the station.

title: Mornington Crescent tube station