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KStars

KStars

KStars is a planetarium program that runs under GNU/Linux and other Unix like operating systems. It is part of KDE. It provides an accurate graphical representation of the night sky, from any location on Earth, at any date and time. The display includes up to 126,000 stars, 13,000 deep sky objects, all 88 constellations, all 8 planets, the Sun and Moon, and thousands of comets and asteroids. It has features to appeal to users of all levels, from informative hypertext articles about astronomy, to robust control of telescopes and CCD cameras. KStars is also a San Francisco Running Club: http://www.kstars.org

External links

- [http://edu.kde.org/kstars/ KStars project webpage] category:KDE

Planetarium

A planetarium is a theater built primarily for presenting educational and entertaining shows about astronomy and the night sky, or for training in celestial navigation. The plural of planetarium is planetariums or planetaria. The term "planetarium" is sometimes used generically to describe other devices which illustrate the solar system, such as a computer simulation or an orrery. orrery orrery] orrery orrery

Overview

The most striking feature of most planetaria is their large dome shaped projection screens onto which scenes of stars, planets and other celestial objects can be made to appear and move realistically to simulate the complex 'motions of the heavens'. These domes can be anything from 3 to 30 m in diameter, accommodating from 1 to 500 people. Traditionally, planetaria domes were mounted horizontally, matching the natural horizon of the real night sky. However, because that configuration requires highly inclined chairs for comfortable viewing "straight up", increasingly domes are being built tilted from the horizontal by between 5 and 30 degrees to provide greater comfort. Tilted domes tend to create a favoured 'sweet spot' for optimum viewing, centrally about a third of the way up the dome from the lowest point. For this reason, tilted domes generally have seating arranged 'stadium-style' in rows as opposed to the traditional epicentric/circular arrangement of seating common in horizontal domes. The celestial scenes on the dome can be created using a wide variety of technologies, ranging from precision-engineered 'star balls' that combine optical and electro-mechanical technology, through slide projector, video and digital projector systems to lasers. Whatever technologies are used, the objective is normally to link them together to provide an accurate relative motion of the sky. Typical systems can be can be set to display the sky at any point in time, past or present, and often to show the night sky as it would appear from any point of latitude on Earth. Since the early 1990s, fully featured 3-D digital planetaria have added an extra degree of freedom to a presenter giving a show because they allow simulation of the view from any point in space, not just the earth-bound view with which we are most familiar and to which traditional 'star-ball' planetarium technology is limited. This new virtual reality-capability to travel through the universe provides important educational benefits because it conveys the fact that space has depth vividly, helping audiences to leave behind the ancient misconception that the stars are stuck on the inside of a giant celestial sphere and instead to understand the true layout of the solar system and beyond. For example, a planetarium can now 'fly' the audience in the direction of one of the familiar constellations such as Orion, revealing that, in fact, the stars which appear to make up a co-ordinated shape from our earth-bound viewpoint are actually at vastly different distances from Earth and so not really connected at all, except in human imagination and mythology. For audiences with learning styles that are visual or kinesthetic, this can be a particularly memorable demonstration that delivers a learning outcome that would otherwise be hard to achieve.

History

Archimedes is attributed with possessing a primitive planetarium device that could predict the movements of the Sun, the Moon and the planets. The discovery of the Antikythera mechanism proved that such devices already existed during antiquity. The first modern planetarium projectors were designed and built by Carl Zeiss in 1924 Germany, and have grown more complex. Smaller projectors include a set of fixed stars, Sun, Moon, and planets, and various nebulae. Larger machines also include comets and a far greater selection of stars. Additional projectors can be added to show twilight around the outside of the screen (complete with city or country scenes) as well as the Milky Way. Still others add coordinate lines and constellations, photographic slides, laser displays, and other images. The OmniMax movie system (now known as IMAX Dome) was originally designed to operate on planetarium screens. In recent years, planetariums — or dome theaters — have broadened their offerings to include wide-screen or "wraparound" films, all-sky video, and laser shows that combine music with laser-drawn patterns. The newest generation of planetariums such as Evans & Sutherland's Digistar 3 or Sky-Skan's DigitalSky, offer a fully digital projection system, in which a single large projector with a fish eye lens, or a system of digital video or laser video projectors around the edge of the dome, are used to create any scene provided to it from a computer. This gives the operator tremendous flexibility in showing not only the modern night sky as visible from Earth, but any other image they wish (including the night sky as visible from points far distant in space and time). A portable class of planetariums can be set up for programs at schools, for example, on a temporary basis. Easily transported and quickly erected inflatable structures have been used for this purpose.

Notable planetariums

- Adler Planetarium, Chicago, Illinois
- Artis Planetarium, Amsterdam [http://www.artis.nl/international/cultural/4.html]
- Cernan Earth and Space Center, Triton College, River Grove, Illinois
- Clark Planetarium, Salt Lake City, Utah
- Davis Planetarium [http://www.mdsci.org/shows/davis/index.cfm] at the Maryland Science Center [http://www.mdsci.org] 601 Light Street, Baltimore, Maryland 21230
- Eise Eisinga Planetarium, Franeker, 1774
- Ehime Prefectural Science Museum,Ehime,Japan has one of largest dome in the world (30m in diameter)
- Fels Planetarium at the Franklin Institute (Philadelphia, Pennsylvania)
- Griffith Observatory, Los Angeles, California
- Charles Hayden Planetarium at the Museum of Science, (Boston, Massachusetts)
- Hayden Planetarium, at the Rose Center for Earth and Space, American Museum of Natural History, New York, NY, James Stewart Polshek, architect, 2000.
- London Planetarium, Marylebone Road, London (part of Madame Tussaud's)
- Minneapolis Planetarium, Minneapolis Public Library, Minneapolis, Minnesota
- Manitoba Museum, Winnipeg, Manitoba, Canada
- Montreal Planetarium, Montreal, Quebec, Canada
- H.R. MacMillan Space Centre, Vancouver, Canada
- Särkänniemi Planetarium, Tampere, Finland
- Hamburg Planetarium [http://www.planetarium-hamburg.de], Hamburg, Germany

Planetarium computer software

- Aladin Sky Atlas (Java)
- Asynx Planetarium[http://www.free-planetarium.com] (Windows)
- Celestia (Linux, Windows, Mac OS X; successor of 3DPlanetarium, OpenUniverse)
- Digistar 3 (proprietary hardware, Windows XP Pro, Evans and Sutherland : Digital Theater Division
- KStars (Linux)
- StarStrider[http://www.starstrider.com] (Windows)
- Starry Night (Windows, Mac OS X)
- Stellarium (Linux, Windows, Mac OS X)
- Winstars (Windows)
- XEphem[http://www.clearskyinstitute.com/xephem/] (Linux, FreeBSD, Mac OS X, Solaris, AIX, HP-UX , Windows with Cygwin)

External links

- [http://www.seds.org/billa/astrosoftware.html List of Planetarium Software]
- [http://www.ips-planetarium.org International Planetarium Society]
- [http://lochness.com/lpco/lpco.html List of planetariums worldwide]
- [http://www.vastbeyond.com/clickmap.htm List of American planetariums, by state] Category:Theatre Category:Observation ja:プラネタリウム

KDE

KDE (K Desktop Environment) is a free desktop environment and development platform built with Trolltech's Qt toolkit. It runs on most Unix and Unix-like systems, such as Linux, BSD, AIX and Solaris. There are also ports to Mac OS X using its X11 layer and Microsoft Windows using Cygwin. Currently, a large portion of the primary KDE libraries and a few other applications can work natively on Microsoft Windows, thanks to the [http://wiki.kde.org/tiki-index.php?page=KDElibs+for+win32 KDElibs/win32 Project]. Ports of other KDE applications are being discussed. KDE is developed in conjunction with KDevelop, a software development suite, and KOffice, an office suite. The "K" originally stood for "Kool" (as the "C" as in "cool" was already used in the acronym for the Common Desktop Environment), but was changed soon after to stand simply for "K", which is "the first letter before 'L' (which stands for Linux) in the Latin alphabet." The project's mascot is a green dragon named Konqi. Konqi can be found in various applications, including when the user logs out and in the "About KDE" screen.

Early history

KDE was founded in 1996 by Matthias Ettrich, who was then a student at the University of Tübingen. He found a number of things wrong with the UNIX desktop at that time. Among his qualms, outlined in [http://groups.google.com/groups?selm=53tkvv%24b4j%40newsserv.zdv.uni-tuebingen.de a now-famous newsgroup post], were that none of the applications looked, felt, or worked alike. He proposed the formation of not only a set of applications, but rather a desktop environment, in which users could expect things to look, feel, and work consistently. He also wanted to make this desktop easy to use. One of his complaints with desktop applications of the time was that his girlfriend could not use them. That post spurred a lot of interest, and the KDE project was born. Matthias chose to use the Qt toolkit as the toolkit of choice of the KDE project. Other programmers quickly started developing KDE/Qt applications, and by early 1997, large and complex applications were being released. In mid-1997, the GNU project had concerns about the licensing of Qt, leading to their founding the GNOME Desktop project and Harmony, a now-abandoned project to clone Qt. Qt was later relicensed to provide the GNU General Public License as an option, which has eliminated the concerns of the GNU project. There is still considerable disagreement over the use of the full GPL for a library like Qt, and the restrictions this imposes on code linking to it. In particular, in order to develop proprietary software with KDE and Qt, it is necessary to purchase a commercial license from Trolltech. To prevent the codebase from being lost should Trolltech fail commercially, ownership of the code is held in a trust to be released under a BSD license should Trolltech cease to exist or stop updating the code. Both KDE and GNOME now participate in Freedesktop.org, an effort to standardise Unix desktop interoperability, although there is still some friendly competition between them.

Organization of the KDE project

Like many open source/free software projects, KDE is primarily a volunteer effort, although various companies, such as Novell (in the form of SUSE), Trolltech, and Mandriva employ developers to work on the project. Since a large number of individuals contribute to KDE in various ways (e.g. code, translation, artwork), organization of such a project is complex. Most problems are discussed on a number of different mailing lists. Important decisions, such as release dates and inclusion of new applications, are made on the kde-core-devel list by the so-called core developers. These are developers who have made significant contributions to KDE over a long period of time. Decisions are not made by a formal voting process, but by discussion on the mailing lists. In most cases this seems to work well, and major discussions (such as the question of whether the KDE 2 API should be broken in favour of KDE 3) are rare. While developers and users are now located all over the world, the project retains a strong base in Germany. The web servers are located at the universities of Tübingen and Kaiserslautern, a German non-profit organization (KDE e.V.) owns the trademark on "KDE", and KDE conferences often take place in Germany.

Release cycle and version numbers

As the project history below shows, the KDE team releases new versions on a frequent basis. It is rare that a release is delayed for more than one or two weeks. An exception was KDE 3.1, which was delayed for more than a month because of a number of security issues in the code base. There are two main types of releases:

Major release

There have been 11 major releases: 1.0, 1.1, 2.0, 2.1, 2.2, 3.0, 3.1, 3.2, 3.3, 3.4 and 3.5. trademark.]] A major KDE release has two version numbers, e.g. KDE 1.1. All KDE releases in the same major version (e.g. KDE1, KDE2 and KDE3) are both binary and source-compatible. This means for instance that software developed against KDE 3.0.x will work with all KDE3 releases. Only a major KDE release will incorporate new features. Changes requiring recompilation or porting never occur except during major version changes; this maintains a stable API for KDE application developers. The changes between KDE 1 and KDE 2 series were large and many, while the API changes between KDE 2 and KDE 3 were comparatively minor, meaning that applications could be easily ported to the new architecture. Up to now the KDE major version numbers follow the Qt release cycle. As soon as a major release is ready and announced, work on the next major release starts. A major release needs several months to be finished and many bugs that are fixed during this time are "backported" to the stable branch, meaning that these fixes are incorporated into the last stable release. The current major release is 3.5, which arrived on November 29, 2005. KDE 4 will succeed 3.5 sometime in 2006, and will be based on Qt 4.0 encompassing some major changes to the desktop.

Minor release

A minor KDE release has three version numbers, e.g. KDE 1.1.1, and the developers focus on fixing bugs, minor glitches and small usability improvements, as opposed to adding new features. For minor releases, a shortened release schedule is used. A minor release is based on a Subversion branch of a previous release and does not affect the "HEAD branch", the branch where the current development of the next major release takes place.

                     new features, 
                       bug fixes
 KDE 3.2 released -------------------->  KDE 3.3 (also called HEAD branch)
 (new development
  started)          bug fixes only
                  -------------------->  KDE 3.2 BRANCH (becoming a minor release)

The somewhat unusual name "3.0.5a" was used because of a lack of version numbers. Work on KDE 3.1 had already started and, up to that day, the release coordinator used version numbers such as 3.0.5, 3.0.6 internally in the main Subversion repository to mark snapshots of the upcoming 3.1. Then after 3.0.3, a number of important and unexpected bug fixes suddenly became necessary, leading to a conflict, because 3.0.6 was at this time already in use. More recent KDE release cycles have tagged pre-release snapshots with large revision numbers, such as 3.1.95, to avoid such conflicts. While development on KDE 2.x in general has stopped, important security fixes are backported to KDE 2.x, since many people still use it.

Criticism

The KDE interface is sometimes criticised for being too complex and including too many configurable options, as opposed to, say, Gnome, that actually reduces the configurable options available to the user (and is actually criticised for this [http://mail.gnome.org/archives/usability/2005-December/msg00021.html]). Although the subject is controversial, some areas of the user interface have been identified as being too cluttered and work has gone into reducing visual complexity for versions 3.4 and 3.5. One of the major goals of KDE 4.0 is to identify further areas that are lacking from a usability perspective and address these concerns. Other criticisms include the heavy integration of Konqueror into the desktop, which may not be the user's preferred browser and the way auto-detected removable media and samba shares must be mounted using konqueror. This type of behavior can make it seem like KDE is attempting to take over the operating systems. Despite the relative abundance of KDE-themed "K" applications some Linux users still prefer GNOME or desktop independant applications. Users may not like the style of such applications or the fact that these require the large KDE libraries to run when used outside the native KDE environment. Some of these applications suffer from long start up times when used with other window managers, however work is being done to correct this behavior. The default KDE looks very similar to Microsoft Windows and replicates some of the more controversial Windows features, such as integrating a web browser into a file manager and the removable media prompt dialogue recently added in KDE 3.5. While some say this might be a good thing to encourage Windows users to migrate to Linux, such "newbie" features are sometimes unappreciated by the hard-core Linux community. KDE is sometimes associated with distributions designed for less-experienced Linux users, though this is really not the case. Many other criticisms of KDE are addressed at the [http://kdemyths.urbanlizard.com/ KDE Myths] web site. However, the bottom line is that KDE is as functional as other environments (such as Gnome), and that criticisms usually originate from users of these other environments. Most KDE users are KDE users by choice, and are quite happy with the way their chosen environment is developing.

Architecture

- aRts - soundserver
- DCOP - system for communication between processes
- KHTML - HTML engine
- KIO - extensible network-transparent file access for KDE applications
- Kiosk - disable features within KDE to create a more controlled environment
- KParts - lightweight in-process graphical component framework
- Kwin - window manager
- KConfigXT - takes an XML file and produces source code to manage configuration options, including classes to glue the resulting code to configuration dialogs.
- Qt - cross platform graphical widget toolkit
- XMLGUI - allows defining UI elements such as menus and toolbars via XML files

Packaging

Due to the size of KDE, it is divided into several package categories to simplify installation. This is a reference scheme, packagers are free to use their own packages for KDE.
- aRts - KDE sound server.
- kdelibs - Primary libraries, containing most pieces of KDE architecture.
- kdebase - The base desktop and applications. Requires kdelibs.
- kdeaccessibility - Accessibility software.
- kdeaddons - Add-on software.
- kdeadmin - Administrative tools, intended for administering UNIX machines.
- kdeartwork - Additional artwork (widget style, screensavers, wallpapers, etc...)
- kdeedu - Educational software.
- kdegames - Games.
- kdegraphics - Tools for manipulating graphics.
- kde-i18n - Internationalization for KDE.
- kdemultimedia - Multimedia software.
- kdenetwork - Network tools and software.
- kdepim - Personal information management and E-mail software.
- kdesdk - Developer tools.
- kdetoys - Desktop Toys and Amusements.
- kdeutils - Utilities.
- kdewebdev - Web Development.
- koffice - Office suite. There is also a Subversion module, kdeextragear-(libs-)
- , which is used by applications which are part of the KDE project but don't depend on the release cycle of the main codebase; K3b and amaroK are part of this module. More info can be found on the [http://extragear.kde.org/ homepage].

Major KDE applications

For a full list, see list of KDE applications. Applications for KDE include:
- amaroK - Media player
- K3b - CD and DVD burning application
- Kate - Text editor
- KDevelop - Integrated Development Environment (IDE)
- KMail - Email client
- Konsole - Terminal emulator
- Kopete - Instant messaging
- Konqueror - File manager and web browser using KHTML
- KPresenter - Presentation application
- KSpread - Spreadsheet
- KWord - Word processor
- KWrite - Light weight text editor with syntax highlights and other features

Timeline

- 14 October 1996: Project was announced by Matthias Ettrich. [http://groups.google.com/groups?selm=53tkvv%24b4j%40newsserv.zdv.uni-tuebingen.de]
- 12 July 1998: [http://www.kde.org/announcements/announce-1.0.php KDE 1.0] released
- 6 February 1999: KDE 1.1 released
- 3 May 1999: [http://www.kde.org/announcements/announce-BW-1.1.1.php KDE 1.1.1] released
- 13 September 1999: [http://www.kde.org/announcements/announce-1.1.2.php KDE 1.1.2] released (KDE 1.2 was planned, but never released)
- 15 December 1999: [http://www.kde.org/announcements/announce-1.89.php KDE 1.89], aka Krash (unstable developers' release)
- 23 October 2000: KDE 2.0 released
- 5 December 2000: KDE 2.0.1 released
- 26 February 2001: KDE 2.1 released
- 27 March 2001: KDE 2.1.1 released
- 30 April 2001: KDE 2.1.2 released
- 15 August 2001: KDE 2.2 released
- 19 September 2001: KDE 2.2.1 released
- 21 November 2001: KDE 2.2.2 released
- 3 April 2002: KDE 3.0 released
- 22 May 2002: KDE 3.0.1 released
- 2 July 2002: KDE 3.0.2 released
- 19 August 2002: KDE 3.0.3 released
- 9 October 2002: KDE 3.0.4 released
- 18 November 2002: KDE 3.0.5 released
- 21 December 2002: KDE 3.0.5a released
- 28 January 2003: KDE 3.1 released
- 20 March 2003: KDE 3.1.1 released
- 9 April 2003: KDE 3.1.1a released
- 19 May 2003: KDE 3.1.2 released
- 29 July 2003: KDE 3.1.3 released
- 16 September 2003: KDE 3.1.4 released
- 14 January 2004: KDE 3.1.5 released
- 3 February 2004: KDE 3.2 released
- 9 March 2004: KDE 3.2.1 released
- 19 April 2004: KDE 3.2.2 released
- 9 June 2004: KDE 3.2.3 released
- 19 August 2004: KDE 3.3 released
- 12 October 2004: KDE 3.3.1 released
- 8 December 2004: KDE 3.3.2 released
- 16 March 2005: KDE 3.4 released
- 31 May 2005: KDE 3.4.1 released
- 27 July 2005: KDE 3.4.2 released
- 13 October 2005: KDE 3.4.3 released
- 29 November 2005: KDE 3.5 released

Naming convention

Most KDE applications have a K in the name, mostly as an initial letter and capitalized. However, there are notable exceptions like kynaptic, whose K is not capitalized, amaroK, which has its K in the end and Gwenview, which doesn't have a K in the name at all. Many KDE applications get their K by misspelling a word which originally begins with C or Q, for example Konsole and Kuickshow. Also, some just append a commonly used word to a K, an instance being KMix. It should be noted that some application names (such as Konsole) are correctly spelled German words.

External links

- [http://www.kde.org The KDE website]
- [http://wiki.kde.org KDE Wiki]
- [http://lists.kde.org KDE mailinglists]
- [http://dot.kde.org KDE News Site]
- [http://groups.google.com/groups?selm=53tkvv%24b4j%40newsserv.zdv.uni-tuebingen.de The original project announcement (from Google Groups)]
- [http://developer.kde.org/development-versions/ KDE release schedules]
- [http://www.lynucs.org/?kde KDE screenshots]
- [http://kde-cygwin.sourceforge.net/ KDE on Windows using Cygwin]
- [http://events.kde.org/ KDE Events]
- [http://developer.kde.org/ KDE Developer's Corner]: a directory of everything to do with KDE development.
- [http://quality.kde.org/ KDE Quality Team]: opportunity to learn and to contribute to the KDE project.
- [http://planetkde.org/ PlanetKDE]: Aggregation of public weblogs written by contributors of KDE
- [http://kdedevelopers.org/ KDE Developer Journals]
- [http://www.kde-forum.org/ KDE-Forums.org]
- [http://plasma.kde.org/ Plasma]: KDE4 Desktop Shell
- [http://accessibility.kde.org/ KDE Accessibility Project]
- [http://i18n.kde.org/ KDE Internationalization]
- [http://bugs.kde.org/ KDE Bug Tracking System]
- [http://kde-look.org/ KDE-Look.org]: Download unofficial KDE artwork and themes
- [http://kde-apps.org/ KDE-Apps.org]: Download unofficial KDE applications
- [http://kde-files.org/ KDE-Files.org]: Download unofficial KDE documents and templates
- [http://www.kde-artists.org/ KDE-Artists.org]: the place to "kollaborate" on artwork for KDE
- [http://kdemyths.urbanlizard.com/ KDE-Myths]
- [http://www.ofb.biz/modules.php?name=News&file=article&sid=364&mode=&order=0&thold=0 Debate Without End]: Article on ongoing licencing controversy Category:Desktop environments Category:KDE Category:X Window System Category:Projects using Subversion ja:KDE simple:KDE

Deep sky object

Deep sky object (DSO) is a term used often in amateur astronomy to denote objects in the night sky other than solar system objects (such as planets, comets and asteroids), single stars and multiple star systems. With a few exceptions such as the Andromeda galaxy, these objects are not visible with the naked eye. The brighter ones can be seen with a small telescope or with a good pair of binoculars, and many DSOs can be photographed through small telescopes with extended exposure times. For visual observation in good clarity a larger telescope is required. binoculars Types of DSO's:
- Star clusters
- Open clusters
- Globular clusters
- Nebulae
- Bright nebulae
- Emission nebulae
- Reflection nebulae
- Dark nebulae
- Planetary nebulae
- Galaxies
- Quasars These are classified by the Messier catalogue of 110 objects and the much more comprehensive New General Catalogue which contains nearly 8000 objects. Many sets of these and other objects from more specialised catalogues such as the UGC are used by amateurs as a test of their observing skills and their equipment. The so called Messier marathons occur only at a specific time of year when observers try to spot all 110 objects in one night. A much more demanding test known as Herschell's 400 is designed to tax larger telescopes. Category:Astronomical objects

Sun

:: For the astrological significance of the Sun, see Solar system in astrology. ::"Solar" redirects here; for the superhero by that name, see Solar (comics). The Sun (or Sol) is the star at the center of our Solar system. Earth orbits the Sun, as do many other bodies, including other planets, asteroids, meteoroids, comets and dust. Its heat and light support almost all life on Earth. The Sun is a ball of plasma with a mass of about 2 kg, which is somewhat higher than that of an average star. About 74% of its mass is hydrogen, with 25% helium and the rest made up of trace quantities of heavier elements. It is thought that the Sun is about 5 billion years old, and is about halfway through its main sequence evolution, during which nuclear fusion reactions in its core fuse hydrogen into helium. In about 5 billion years time the Sun will become a white dwarf. Although it is the nearest star to Earth and has been intensively studied by scientists, many questions about the Sun remain unanswered, such as why its outer atmosphere has a temperature of over 10⁶ K when its visible surface (the photosphere) has a temperature of just 6,000 K. Looking directly at the Sun can damage the retina and one's eyesight. See below for details.

General information

See below The Sun is classified as a main sequence star, which means it is in a state of "hydrostatic balance", neither contracting nor expanding, and is generating its energy through nuclear fusion of hydrogen nuclei into helium. The Sun has a spectral class of G2V, with the G2 meaning that its color is yellow and its spectrum contains spectral lines of ionized and neutral metals as well as very weak hydrogen lines [http://www.astro.uiuc.edu/~kaler/sow/spectra.html#classes], and the V signifying that it, like most stars, is a "dwarf" star on the main sequence[http://www.physics.uq.edu.au/people/ross/phys2080/spec/analyz.htm]. The Sun has a predicted main sequence lifetime of about 10 billion years. Its current age is thought to be about 4.5 billion years, a figure which is determined using computer models of stellar evolution, and nucleocosmochronology . The Sun orbits the center of the Milky Way galaxy at a distance of about 25,000 to 28,000 light-years from the galactic centre, completing one revolution in about 226 million years. The orbital speed is 217 km/s, equivalent to one light year every 1400 years, and one AU every 8 days. The astronomical symbol for the Sun is a circle with a point at its centre (Image:Sol.gif).

Structure

Image:Sol.gif The Sun is a near-perfect sphere, with an oblateness estimated at about 9 millionths, which means the polar diameter differs from the equatorial by about 10 km. This is because the centrifugal effect of the Sun's slow rotation is 18 million times weaker than its surface gravity (at the equator). Tidal effects from the planets do not significantly affect the shape of the Sun, although the Sun itself orbits the center of mass of the solar system, which is offset from the Sun's center mostly because of the large mass of Jupiter. The mass of the Sun is so comparatively great that the center of mass of the solar system is generally within the bounds of the Sun itself. The Sun does not have a definite boundary as rocky planets do, as the density of its gases drops off following an approximately exponential relationship with distance from the centre of the Sun. Nevertheless, the Sun has well defined interior structure, described below. The Sun's radius is measured from centre to the edges of the photosphere. The solar interior is not directly observable and the Sun itself is opaque to electromagnetic radiation. However, just as the study of the waves generated by earthquakes (seismology) can be used to study the interior structure of the Earth, helioseismology, the study of sound waves that travel through the Sun's interior, has also contributed greatly to our understanding of the Sun's structure . Computer modeling of the Sun is also used as a theoretical tool to investigate its deep layers.

Core

At the center of the Sun, where its density reaches up to 150,000 kg/m³ (150 times the density of water on Earth), thermonuclear reactions (nuclear fusion) convert hydrogen into helium, producing the energy that keeps the Sun in a state of equilibrium. About 8.9 protons (hydrogen nuclei) are converted to helium nuclei every second, releasing energy at the matter-energy conversion rate of 4.26 million tonnes per second or 383 yottawatts (9.15 tons of TNT per second). The core extends from the center of the Sun to about 0.2 solar radii, and is the only part of the Sun where an appreciable amount of heat is produced by fusion: the rest of the star is heated by energy that is transferred outward. All of the energy of the interior fusion must travel through the successive layers to the solar photosphere, before it escapes to space. The high-energy photons (gamma and X rays) released in fusion reactions take a long time to reach the Sun's surface, slowed down by the indirect path taken, as well as constant absorption and re-emission at lower energies in the solar mantle (see below). Estimates of the "photon travel time" range from as much as 50 million years (Richard S. Lewis, The Illustrated Encyclopedia of the Universe, Harmony Books, New York, 1983, p. 65) to as little as 17,000 years [http://www.badastronomy.com/bitesize/solar_system/sun.html]. Upon reaching the surface after a final trip through the convective outer layer, the photons escape as visible light. Neutrinos are also released in the fusion reactions in the core, but unlike photons they very rarely interact with matter, and so almost all are able to escape the Sun immediately.

Radiation zone

From about 0.2 to about 0.7 solar radii, the material is hot and dense enough that thermal radiation is sufficient to transfer the intense heat of the core outward. In this zone, there is no thermal convection: while the material grows cooler with altitude, this temperature gradient is slower than the adiabatic lapse rate and hence cannot drive convection. Heat is transferred by ions of hydrogen and helium emitting photons, which travel a brief distance before being re-absorbed by other ions. Because of this, it can take a photon nearly 1,000,000 years to reach the photosphere.

Convection zone

photosphere From about 0.7 solar radii to 1.0 solar radii, the material in the Sun is not dense enough or hot enough to transfer the heat energy of the interior outward via radiation. As a result, thermal convection occurs as thermal columns carry hot material to the surface (photosphere) of the Sun. Once the material cools off at the surface, it plunges back downward to the base of the convection zone, to receive more heat from the top of the radiative zone. Convective overshoot is thought to occur at the base of the convection zone, carrying turbulent downflows into the outer layers of the radiative zone. The thermal columns in the convection zone form an imprint on the surface of the Sun, in the form of the solar granulation and supergranulation. The turbulent convection of this outer part of the solar interior gives rise to a 'small-scale' dynamo that produces magnetic north and south poles all over the surface of the Sun.

Photosphere

The visible surface of the Sun, the photosphere, is the layer below which the Sun becomes opaque to visible light. Above the photosphere, sunlight is free to propagate into space and its energy escapes the Sun entirely. Sunlight has approximately a black-body spectrum that indicates its temperature is about 6,000 K, interspersed with atomic absorption lines from the tenuous layers above the photosphere. The photosphere has a particle density of about 10²³/m³ (this is about 1% of the particle density of Earth's atmosphere at sea level). The parts of the Sun above the photosphere are referred to collectively as the solar atmosphere. They can be viewed with telescopes operating across the electromagnetic spectrum, from radio through visible light to gamma rays.

Temperature minimum

The coolest layer of the Sun is the temperature minimum region about 500 km above the photosphere. It is about 4,000 K. It is the only part of the Sun cool enough to support simple molecules such as carbon monoxide and water; all other parts of the Sun are hot enough to break chemical bonds.

Chromosphere

Above the visible surface of the Sun is a thin layer, about 2,000 km thick, that is dominated by a spectrum of emission and absorption lines. It is called the chromosphere from the Greek root chromos, meaning color, because the chromosphere is visible as a colored flash at the beginning and end of total eclipses of the Sun.

Corona

The corona is the extended outer atmosphere of the Sun, which is much larger in volume than the Sun itself. The corona merges smoothly with the solar wind that fills the solar system and heliosphere. The low corona, which is very near the surface of the Sun, has a particle density of 10¹¹/m³ (Earth's atmosphere near sea level has a particle density of about 2x10²⁵/m³). The temperature of the corona is several megakelvins.

Theoretical problems

Solar neutrino problem

megakelvin For some time it was thought that the number of neutrinos produced by the nuclear reactions in the Sun was only a third of the number predicted by theory, a result that was termed the solar neutrino problem. Several neutrino observatories were constructed, including the Sudbury Neutrino Observatory and Kamiokande to try to measure the solar neutrino flux. It has recently been found that neutrinos have rest mass, and can therefore transform into harder-to-detect varieties of neutrinos while en route from the Sun to Earth in a process known as neutrino oscillation . Thus, measurement and theory have been reconciled.

Coronal heating problem

The optical surface of the Sun (the photosphere) is known to have a temperature of about 6,000 K. Above it lies the solar corona with a temperature of one million kelvins. The high temperature of the corona suggests that it is heated by something other than the photosphere. It is thought that the energy necessary to heat the corona is provided by turbulent motion in the convection zone below the photosphere. Two main mechanisms have been proposed to explain coronal heating: Wave heating, in which sound, gravitational and magnetohydrodynamic waves are produced by turbulence in the convection zone. These waves travel upward and dissipate in the corona, depositing their energy in the ambient gas in the form of heat. The other proposed mechanism is flare heating, in which magnetic energy is continuously built up by photospheric motion and released through magnetic reconnection in the form of solar flares and waves. , , , . Currently, it is unclear whether waves are an efficient heating mechanism. All waves except Alfven waves have been found to dissipate or refract before reaching the corona (, ). In addition, Alfven waves do not easily dissipate in the corona . Current research focus has therefore shifted towards flare heating mechanisms. One possible candidate to explain coronal heating is continuous flaring at small scales , but this is still an open topic of investigation.

Faint young sun problem

Theoretical models of the sun's development suggest that 3.8 to 2.5 billion years ago, during the Archean period, the Sun was only about 75 percent as bright as it is today. Such a weak star would not have been able to sustain liquid water on the Earth's surface, and thus life should not have been able to develop. However, the geologic record shows that the Earth has remained at a fairly constant temperature throughout its history. In fact, the young Earth was actually warmer than it is today. Some scientists have suggested that the young Earth's atmosphere contained much larger quantities of greenhouse gases such as carbon dioxide and/or ammonia than are present today . Others suggest that cosmic rays might strongly influence the Earth's climate, and that their flux was much higher in the early history of the solar system .

Magnetic field

cosmic ray's rotating magnetic field on the plasma in the interplanetary medium (Solar Wind) [http://quake.stanford.edu/~wso/gifs/HCS.html]. (click to enlarge)]] All matter in the Sun is in the form of gas and plasma due to its high temperatures. This makes it possible for the Sun to rotate faster at its equator (about 25 days) than it does at higher latitudes (28 days near its poles). The differential rotation of the Sun's latitudes causes its magnetic field lines to become twisted together over time, causing magnetic field loops to erupt from the Sun's surface and trigger the formation of the Sun's dramatic sunspots and solar prominences. (See magnetic reconnection.) The solar activity cycle includes old magnetic fields being stripped off the Sun's surface starting from one pole and ending at the other. The magnetic field of the sun reverses once for each 11-year sunspot cycle. The influence of the Sun's rotating magnetic field on the plasma in the interplanetary medium creates the largest structure in the Solar System, the Heliospheric current sheet. The plasma in the interplanetary medium is also responsible for the strength of the Sun's magnetic field at the orbit of the Earth being over 100 times greater than originally anticipated. If space were a vacuum, then the Sun's 10^-4 tesla magnetic dipole field would reduce with the cube of the distance to about 10^-11 tesla. But satellite observations show that it is about 100 times greater at around 10^-9 tesla. Magnetohydrodynamic (MHD) theory predicts that the motion of a conducting fluid (e.g. the interplanetary medium) in a magnetic field, induces electric currents which in turn generates magnetic fields, and in this respect it behaves like an MHD dynamo.

Position of the Sun through the year

The path of the Sun across the sky varies throughout the year. The shape described by the Sun's position, considered at the same time each day for a complete year, is called the analemma, and resembles a figure 8, aligned along the North/South direction. The most obvious variation in the Sun's apparent position through the year is a North/South swing over 47 degrees of angle, due to the 23.5 degree tilt of the Earth, but there is an East/West component as well. The North/South swing in apparent angle is the main source of seasons on Earth.

Solar space missions

seasons using UV light from the He⁺ emission line at 30.4 nm. (Animation (980 kB MPEG))]] To obtain an uninterrupted view of the Sun, the European Space Agency and NASA cooperatively launched the Solar and Heliospheric Observatory (SOHO) on December 2, 1995. Originally a two-year mission, SOHO is now over ten years old (as of late 2005). It has proved so useful that a follow-on mission, the Solar Dynamics Observatory, is planned for launch in 2008. Elemental abundances in the photosphere are well known from spectroscopic studies, but the composition of the interior of the Sun is much less well known. A solar wind sample return mission, Genesis, was designed to allow astronomers to directly measure the composition of solar material. It returned to Earth in 2004 and is undergoing analysis, but it was damaged by crash-landing when its parachute failed to deploy on reentry to Earth's atmosphere.

History and future of the Sun

The Sun is thought to be a second-generation star, whose formation may have been triggered by shockwaves from a nearby supernova. This is suggested by a high abundance of heavy elements such as iron, gold and uranium in the solar system: the most plausible ways that these elements could be produced are by endothermic nuclear reactions during a supernova or by transmutation via neutron absorption inside a massive first generation star. Our Sun does not have enough mass to explode as a supernova, and its mass is below the Chandrasekhar limit. Instead, in 4-5 billion years it will enter its red giant phase, its outer layers expanding as the hydrogen fuel in the core is consumed and the core contracts and heats up. Helium fusion will begin when the core temperature reaches about 3 K. While it is likely that the expansion of the outer layers of the Sun will reach the current position of Earth's orbit, recent research suggests that mass lost from the Sun earlier in its red giant phase will cause the Earth's orbit to move further out, preventing it from being engulfed. Following the red giant phase, giant thermal pulsations will cause the Sun to throw off its outer layers forming a planetary nebula. The Sun will then evolve into a white dwarf, slowly cooling over eons. This stellar evolution scenario is typical of low to medium mass stars.

Human understanding of the Sun

:see also sun worship sun worship mythology]] Mankind's most fundamental understanding of the Sun is as the luminous disk in the heavens whose presence above the horizon creates day, and whose absence causes night. In many prehistoric and ancient cultures, the Sun was thought to be a deity or other supernatural phenomenon. One of the first people in the Western world to offer a scientific explanation for the sun was the Greek philosopher Anaxagoras, who reasoned that it was a giant flaming ball of metal even larger than the Peleponessus, and not the chariot of Helios. For teaching this heresy he was imprisoned by the authorities and sentenced to death (though later released through the intervention of Pericles). With respect to the fixed stars, the Sun appears from Earth to revolve once a year along the ecliptic through the zodiac. Thus, the Sun was considered by Greek astronomers to be one of the seven planets (Greek planetes "wanderer"), after which the seven days of the week are named in some languages.

The Sun as a power source

Sunlight — that is, light radiated from the surface of the Sun — is thought to be the main source of energy near the surface of Earth. The solar constant is the amount of power that the Sun deposits per unit area that is directly exposed to sunlight. It is about 1370 watts per square meter of area. Sunlight on the surface of Earth is attenuated by the Earth's atmosphere, so that less power arrives at the surface — closer to 1000 watts per directly exposed square meter in clear conditions. This energy can be harnessed through several natural and synthetic processes. Photosynthesis by plants captures the energy of sunlight and converts it to chemical form (oxygen and reduced carbon compounds), while direct heating or electrical conversion by solar cells are used by solar power equipment to generate electricity or do other useful work. The energy stored in petroleum is thought to have been converted from sunlight by photosynthesis in the distant past.

Sun and eye damage

Sunlight is very bright, and looking directly at the Sun is painful to the eyes. Looking directly at the Sun when it is high in the sky causes temporary bleaching of the photosensitive pigments in the retina, which makes phosphene visual artifacts and may cause temporary partial blindness. Direct viewing of the Sun with the naked eye delivers about 4 milliwatts of sunlight to the retina that is in the solar image, heating it up and potentially (though not normally) damaging it. Brief viewing of the full direct Sun with the naked eye is unpleasant but generally safe. Viewing the Sun through light-concentrating optics such as binoculars is hazardous without an attenuating (ND) filter to dim the sunlight. Suitable filters are available at welding supply shops and camera stores. Using a proper filter is very important as some improvised filters reduce visible light while passing either infrared or ultraviolet rays that can still damage the eye. Viewing the Sun through unfiltered 7x50 mm binoculars can deliver as much as 2.5 watts of sunlight into each eye, over 300 times more power than naked eye viewing. Even brief glances at the midday Sun through unfiltered binoculars can cause permanent blindness. During partial eclipses of the Sun, another hazardous condition exists because of the way the eye responds to bright light. The pupil is controlled by the total amount of light in the visual field, not by the brightest object in the field. During partial eclipses, most sunlight is blocked by the Moon passing directly in front of the Sun, but the uncovered parts of the photosphere have the same surface brightness as during a normal day. In the dim overall light, the pupil tends to dilate from about 2 mm to perhaps 6 mm diameter, increasing the eye's collecting area by a factor of nearly 10. Each retinal cell that is exposed to the partially-eclipsed solar image thus receives about ten times as much light as it would looking at the normal, non-eclipsed Sun. Viewing the partially eclipsed Sun with the naked eye can cause permanent localized damage to the retina, resulting in small, permanent blind spots for the viewer. This is an especially insidious hazard for inexperienced observers and for children, because there is no immediate perception of pain and it is tempting to stare at the spectacle of the eclipsing Sun, compounding any damage. During sunrise and sunset, sunlight is attenuated by a particularly long passage through Earth's atmosphere, and the direct Sun is sometimes faint enough to be viewed directly without discomfort or safely with binoculars. Hazy conditions, atmospheric dust, and high humidity contribute to this atmospheric attenuation.

External links

- [http://sohowww.nascom.nasa.gov/data/realtime-images.html Current SOHO snapshots]
- [http://soi.stanford.edu/data/farside/index.html Far-Side Helioseismic Holography] from [http://www.stanford.edu Stanford]
- [http://sunearth.gsfc.nasa.gov/eclipse/eclipse.html NASA Eclipse homepage]
- [http://sohowww.nascom.nasa.gov/ Nasa SOHO (Solar & Heliospheric Observatory) satellite] [http://sohowww.nascom.nasa.gov/explore/faq/sun.html FAQ]
- [http://soi.stanford.edu/results/sounds.html Solar Sounds] from [http://www.stanford.edu Stanford]
- [http://www.spaceweather.com Spaceweather.com]
- [http://scienceworld.wolfram.com/astronomy/Sun.html Eric Weisstein's World of Astronomy - Sun]
- [http://www.astro.uu.nl/~strous/AA/en/antwoorden/zonpositie.html The Position of the Sun]
- [http://www.lmsal.com/YPOP/FilmFestival/index.html A collection of solar movies]
- [http://www.solarphysics.kva.se/ The Institute for Solar Physics- Movies of Sunspots and spicules]
- [http://science.msfc.nasa.gov/ssl/pad/solar/default.htm NASA/Marshall Solar Physics website]
- [http://rredc.nrel.gov/solar/codesandalgorithms/spa Solar Position Algorithm] and [http://www.nrel.gov/docs/fy04osti/34302.pdf documentation] from the [http://www.nrel.gov National Renewable Energy Laboratory]
- [http://libnova.sourceforge.net/index.html libnova] - a celestial mechanics and astronomical calculation library

References

# Alfven, H., 1947, Monthly Notices of the Royal Astronomical Society., 107, 211 # # Biermann, L., 1946, Naturwissenschaffen, 33, 118 # Bonanno, A., Schlattl, H., Paternò, L. (2002), The age of the Sun and the relativistic corrections in the EOS, Astronomy and Astrophysics, v.390, p.1115-1118 # Carslaw, K.S., Harrison, R.G., Kirkby, J., 2002, Cosmic Rays, Clouds, and Climate, Science, 298, 1732-1737 # Kasting, J.F., Ackerman, T.P., 1986, Climatic Consequences of Very High Carbon Dioxide Levels in the Earth’s Early Atmosphere, Science, v. 234, p. 1383-1385 # Parker, E.N., 1958, Astrophysical Journal, 128, 644 # Parker, E.N., 1988, Astrophysical Journal, 330, 474 # Priest, E.R., 1982, Solar Magnetohydrodynamics (Dordrecht: Reidel), pp. 206-245 # Schlattl, H. (2001), Three-flavor oscillation solutions for the solar neutrino problem, Physical Review D, vol. 64, Issue 1 # Sturrock, P.A., & Uchida, Y., 1981, Astrophysical Journal., 246, 331 # Thompson, M.J. (2004), Solar interior: Helioseismology and the Sun's interior, Astronomy & Geophysics, v. 45, p. 4.21-4.25 Category:Yellow dwarfs Category:Space plasmas Category:Plasma physics als:Sonne zh-min-nan:Ji̍t-thâu ko:태양 ms:Matahari ja:太陽 simple:Sun th:ดวงอาทิตย์

Moon

:For other moons in the solar system see natural satellite. For the astrological meaning of the Moon, see Solar system in astrology. For other uses see Moon (disambiguation). The Moon is the planet Earth's only natural satellite. It has no formal name other than "The Moon", although it is occasionally called Luna (Latin for moon), or Selene, to distinguish it from the generic "moon" (natural satellites of other planets are also called moons). Its symbol is a crescent (Unicode: ☾). The terms lunar, selene/seleno-, and cynthion (from the Lunar deities Selene and Cynthia) refer to the Moon (aposelene, selenocentric, pericynthion, etc.). The average distance from the Moon to the Earth is 384,403 kilometers (238,857 miles). The Moon's diameter is 3,476 kilometers (2,160 miles). The first manmade object to land on the Moon was Luna 2 in 1959, the first photographs of the otherwise occluded far side of the Moon were made by Luna 3 that same year, and the first people to land on the Moon came aboard Apollo 11 in 1969.

The two sides

The far side is sometimes called the "dark side". In this case "dark" means "unknown and hidden" and not "lacking light" as percieved by the name; in fact the far side receives (on average) as much sunlight as the near side, but at opposite times. Spacecraft are cut off from direct radio communication with the Earth when on the far side of the Moon. One distinguishing feature of the far side is its almost complete lack of maria (singular: mare), which are the dark albedo features.

Orbit

The Moon makes a complete orbit about once every 28 days. Each hour the Moon moves relative to the stars by an amount roughly equal to its angular diameter, or by about 0.5°. The Moon differs from most satellites of other planets in that its orbit is close to the plane of the ecliptic and not in the Earth's equatorial plane. Several ways to consider a complete orbit are detailed in the table below, but the two most familiar are: the sidereal month being the time it takes to make a complete orbit with respect to the stars, about 27.3 days; and the synodic month being the time it takes to reach the same phase, about 29.5 days. These differ because in the meantime the Earth and Moon have both orbited some distance around the Sun. The gravitational attraction that the Moon exerts on Earth is the cause of tides in the sea. The tidal flow period, but not the phase, is synchronized to the Moon's orbit around Earth. The tidal bulges on Earth, caused by the Moon's gravity, are carried ahead of the apparent position of the Moon by the Earth's rotation, in part because of the friction of the water as it slides over the ocean bottom and into or out of bays and estuaries. As a result, some of the Earth's rotational momentum is gradually being transferred to the Moon's orbital momentum, resulting in the Moon slowly receding from Earth at the rate of approximately 38 mm per year. At the same time the Earth's rotation is gradually slowing, the Earth's day thus lengthens by about 15 µs every year. A more detailed discussion follows in the section titled Earth & Moon. The Moon is in synchronous rotation, meaning that it keeps the same face turned to the Earth at all times. This synchronous rotation is only true on average because the Moon's orbit has definite eccentricity. When the Moon is at its perigee, its rotation is slower than its orbital motion, and this allows us to see up to an extra eight degrees of longitude of its East (right) side. Conversely, when the Moon reaches its apogee, its rotation is faster than its orbital motion and reveals another eight degrees of longitude of its West (left) side. This is called longitudinal libration. Because the lunar orbit is also inclined to the Earth's equator, the Moon seems to oscillate up and down (as a person's head does when nodding) as it moves in celestial latitude (declination). This is called latitudinal libration and reveals the Moon's polar zones over about seven degrees of latitude. Finally, because the Moon is only at about 60 Earth radii distance, an observer at the equator who observes the Moon throughout the night moves by an Earth diameter sideways. This is diurnal libration and reveals about one degree's worth of lunar longitude. Earth and Moon orbit about their barycenter, or common center of mass, which lies about 4700 km from Earth's center (about 3/4 of the way to the surface). Since the barycenter is located below the Earth's surface, Earth's motion is more commonly described as a "wobble". When viewed from Earth's North pole, Earth and Moon rotate counter-clockwise about their axes; the Moon orbits Earth counter-clockwise and Earth orbits the Sun counter-clockwise. It may seem curious that the inclination of the lunar orbit and the tilt of the Moon's axis of rotation are listed as varying considerably. One must be reminded here that the orbital inclination is measured with respect to the primary's equatorial plane (in this case the Earth's), and that the axis of rotation's tilt is measured with respect to the normal to the satellite's orbital plane (the Moon's). For most planetary satellites, but not for the Moon, these conventions model physical reality and the values are therefore stable. The plane of the lunar orbit maintains an inclination of 5.145 396° with respect to the ecliptic (the orbital plane of the Earth), and the lunar axis of rotation maintains an inclination of 1.5424° with respect to the normal to that same plane. The lunar orbital plane precesses quickly (i.e. its intersection with the ecliptic rotates clockwise), in 6793.5 days (18.5996 years), mostly because of the gravitational perturbation induced by the Sun. During that period, the lunar orbital plane thus sees its inclination with respect to the Earth's equator (itself inclined 23.45° to the ecliptic) vary between 23.45° + 5.15° = 28.60° and 23.45° - 5.15° = 18.30°. Simultaneously, the axis of lunar rotation sees its tilt with respect to the Moon's orbital plane vary between 5.15° + 1.54° = 6.69° and 5.15° - 1.54° = 3.60°. Note that the Earth's tilt reacts to this process and itself varies by 0.002 56° on either side of its mean value; this is called nutation. The points where the Moon's orbit crosses the ecliptic are called the "lunar nodes": the North (or ascending) node is where the Moon crosses to the North of the ecliptic; the South (or descending) node where it crosses to the South. Solar eclipses occur when a node coincides with the new Moon; lunar eclipses when a node coincides with the full Moon.

Earth & Moon

The tides on Earth are generated by the Moon's gravitation (see tide and tidal force for a more detailed discussion). There are two tidal bulges, one in the direction of the Moon, and one in the opposite direction (figure 1). The buildup of these bulges and their movement around the earth causes an energy loss due to friction. The energy loss decreases the rotational energy of the Earth. Since the Earth spins faster than the Moon moves around it, the tidal bulges are dragged along with the Earth's surface faster than the Moon moves, and move "in front of the Moon" (figure 2). Because of this, the Earth's gravitational pull on the Moon has a component in the Moon's "forward" direction with respect to its orbit. This component of the gravitational forces between the two bodies acts like a torque on the Earth's rotation, and transfers angular momentum and rotational energy from the Earth's spin to the Moon's orbital movement. angular momentum Because the Moon is accelerated in forward direction, it moves to a higher orbit. As a result, the distance between the Earth and Moon increases, and the Earth's spin slows down (figure 3). Measurements reveal that the Moon's distance to the Earth increases by 38 mm per year (lunar laser ranging experiments with laser reflectors are used to determine this). Atomic clocks also show that the Earth's day lengthens by about 15 µs every year. However, the formation of tidal bulges on Earth is irregular and not directly related to the frictional energy loss which accompanies the tides. For example, continents on Earth may cause an increase in frictional energy losses and hamper the buildup of tidal bulges (figure 4). The energy loss of the Earth's spin (loss of rotational energy of the Earth) is related to both the energy transfer to the Moon, which depends on the geometry of the mass distributions on Earth (causing a gravity component which pulls the Moon forward), and also to frictional losses, which depends on the properties of the material moving around within tides. The transfer of angular momentum to the Moon's orbit, in contrast, depends only on the geometry of the mass distribution. In general, the angular momentum transferred to the Moon will not correspond to an equivalent energy transfer. There will be a surplus or a deficit in the transfer of angular momentum to the Moon, compared to the energy transfer (figure 5). Since both angular momentum and energy are conserved, there must be a mechanism on earth to store a surplus or a deficit of angular momentum. Candidates for this mechanism are the Earth's magnetic field and internal material currents of the Earth (figure 6). The lunar surface is also subjected to tides from earth, and rises and falls by around 10 cm over 27 days. The lunar tides comprise a mobile component, due to the Sun, and a selenographically fixed one, due to Earth (the Moon keeps the same face turned to the Earth, but not to the Sun). The vertical motion of the Earth-induced component comes entirely from the Moon's orbital eccentricity; if the Moon's orbit were perfectly circular, there would be solar tides only. The magnitude of the Moon's tides corresponds to a Love number of 0.0266, and supports the idea of a partially melted zone around its core. Moonquake waves lose energy below 1000 km depth, and this may also show that the deep material is at least partially melted. The Earth’s Love number is 0.3, corresponding to a movement of 0.5 metres per day; for Venus the Love number is also 0.3. (Source: Patrick Moore, The Data Book of Astronomy - June 2003 Updates)

Origin and history

magnetic field The inclination of the Moon's orbit makes it implausible that the Moon formed along with the Earth or was captured later; its origin is the subject of some scientific debate. Early speculation proposed that the Moon broke off from the Earth's crust due to centrifugal force, leaving an ocean basin (presumed to be the Pacific) behind as a scar. This concept requires too great an initial spin of the Earth. Others speculated the Moon formed elsewhere and was captured into its orbit. Two of the other theories include the coformation or condensation theory and the impact theory, which speculates that the Moon formed from the debris that resulted from a collision between the early Earth and a planetesimal. The Coformation or Condensation hypothesis posits that the Earth and the Moon formed together at about the same time from the primordial accretion disk, the Moon forming from material surrounding the coalescing proto-Earth, similar to the way the planets formed around the Sun. Some suggest that this hypothesis fails to adequately explain the depletion of iron in the Moon. Recently, the Giant Impact theory has been considered a more viable scientific theory for the moon's origin than the coformation or condensation theory. The Giant Impact theory holds that the Moon formed from the ejecta resulting from a collision between a semi-molten Earth and a planet-like object the size of Mars, which has been referred to as Theia. The geological epochs of the Moon are defined based on the dating of various significant impact events in the Moon's history. Analysis of craters and Moon rocks show that there was a late heavy bombardment by asteroids around the period 4000 to 3800 million years ago. Tidal forces deformed the once molten Moon into an ellipsoid, with the major axis pointed towards Earth.

Physical characteristics

Composition

More than 4.5 billion years ago, the surface of the Moon was a liquid magma ocean. Scientists think that one component of lunar rocks, KREEP (K-potassium, Rare Earth Elements, and P-phosphorus), represents the last chemical remnant of that magma ocean. KREEP is actually a composite of what scientists term "incompatible elements": those which cannot fit into a crystal structure and thus were left behind, floating to the surface of the magma. For researchers, KREEP is a convenient tracer, useful for reporting the story of the volcanic history of the lunar crust and chronicling the frequency of impacts by comets and other celestial bodies. The lunar crust is composed of a variety of primary elements, including uranium, thorium, potassium, oxygen, silicon, magnesium, iron, titanium, calcium, aluminium and hydrogen. When bombarded by cosmic rays, each element bounces back into space its own radiation, in the form of gamma rays. Some elements, such as uranium, thorium and potassium, are radioactive and emit gamma rays on their own. However, regardless of what causes them, gamma rays for each element are all different from one another — each produces a unique spectral "signature", detectable by a spectrometer. A complete global mapping of the Moon for the abundance of these elements has never been performed. However, some spacecraft have done so for portions of the Moon; Galileo did so when it flew by the Moon in 1992. [http://photojournal.jpl.nasa.gov/catalog/PIA00131] The overall composition of the Moon is believed to be similar to that of the Earth other than a depletion of volatile elements and of iron.

Selenography

1992 photo.]] When observed with earth based telescopes, the moon can be seen to have some 30,000 craters having a diameter of at least 1 kilometers, but close up observation from lunar orbit reveals a multitude of ever smaller craters. Most are hundreds of millions or billions of years old; the lack of atmosphere or weather or recent geological processes ensures that most of them remain permanently preserved. In the lunar terrae, it is indeed impossible to add a crater of any size without obliterating another; this is termed saturation. The largest crater on the Moon, and indeed the largest known crater within the solar system, forms the South Pole-Aitken basin. This crater is located on the far side, near the south pole, and is some 2,240 km in diameter, and 13 km in depth. The dark and relatively featureless lunar plains are called maria, Latin for seas, since they were believed by ancient astronomers to be water-filled seas. They are actually vast ancient basaltic lava flows that filled the basins of large impact craters. The lighter-colored highlands are called terrae. Maria are found almost exclusively on the Lunar nearside, with the Lunar farside having only a few scattered patches. Scientists think that this asymmetry of lunar features was caused by the synchronization between the Moon's rotation and orbit about the Earth. This synchronization exposes the far side of the Moon to more asteroid and meteor impacts than the near, thereby allowing the maria on the near side to remain relatively undisturbed for many hundreds of millennia. Blanketed atop the Moon's crust is a dusty outer rock layer called regolith. Both the crust and regolith are unevenly distributed over the entire Moon. The crust ranges from 60 km (38 mi) on the near side to 100 km (63 mi) on the far side. The regolith varies from 3 to 5 m (10 to 16 ft) in the maria to 10 to 20 m (33 to 66 ft) in the highlands. In 2004, a team led by Dr. Ben Bussey of Johns Hopkins University using images taken by the Clementine mission determined that four mountainous regions on the rim of the 73 km wide Peary crater at the Moon's north pole appeared to remain illuminated for the entire Lunar day. These unnamed "mountains of eternal light" are possible due to the Moon's extremely small axial tilt, which also gives rise to permanent shadow at the bottoms of many polar craters. No similar regions of eternal light exist at the less-mountainous south pole, although the rim of Shackleton crater is illuminated for 80% of the lunar day. Clementine's images were taken during the northern Lunar hemisphere's summer season, and it remains unknown whether these four mountains are shaded at any point during their local winter season.

Presence of water

Over time, comets and meteorites continuously bombard the Moon. Many of these objects are water-rich. Energy from sunlight splits much of this water into its constituent elements hydrogen and oxygen, both of which usually fly off into space immediately. However, it has been hypothesized that significant traces of water remain on the Moon, either on the surface, or embedded within the crust. The results of the Clementine mission suggested that small, frozen pockets of water ice (remnants of water-rich comet impacts) may be embedded unmelted in the permanently shadowed regions of the lunar crust. Although the pockets are thought to be small, the overall amount of water was suggested to be quite significant — 1 km³. Some water molecules, however, may have literally hopped along the surface and gotten trapped inside craters at the lunar poles. Due to the very slight "tilt" of the Moon's axis, only 1.5°, some of these deep craters never receive any light from the Sun — they are permanently shadowed. Clementine has mapped ([http://www.lpi.usra.edu/research/clemen/clemen.html]) craters at the lunar south pole ([http://www.lpi.usra.edu/research/clemen/2polar.gif]) which are shadowed in this way. It is in such craters that scientists expect to find frozen water if it is there at all. If found, water ice could be mined and then split into hydrogen and oxygen by solar panel-equipped electric power stations or a nuclear generator. The presence of usable quantities of water on the Moon would be an important factor in rendering lunar habitation cost-effective, since transporting water (or hydrogen and oxygen) from Earth would be prohibitively expensive. Clementine twisting the shadow due to the fact that cosmic rays are charged particles.]] The equatorial Moon rock collected by Apollo astronauts contained no traces of water. Neither the Lunar Prospector nor more recent surveys, such as those of the Smithsonian Institution, have found direct evidence of lunar water, ice, or water vapor. Lunar Prospector results, however, indicate the presence of hydrogen in the permanently shadowed regions, which could be in the form of water ice.

Magnetic field

Compared to that of Earth, the Moon has a very weak magnetic field. While some of the Moon's magnetism is thought to be intrinsic (such as a strip of the lunar crust called the Rima Sirsalis), collision with other celestial bodies might have imparted some of the Moon's magnetic properties. Indeed, a long-standing question in planetary science is whether an airless solar system body, such as the Moon, can obtain magnetism from impact processes such as comets and asteroids. Magnetic measurements can also supply information about the size and electrical conductivity of the lunar core — evidence that will help scientists better understand the Moon's origins. For instance, if the core contains more magnetic elements (such as iron) than Earth, then the impact theory loses some credibility (although there are alternate explanations for why the lunar core might contain less iron).

Atmosphere

The Moon has a relatively insignificant and tenuous atmosphere. One source of this atmosphere is outgassing — the release of gases, for instance radon, which originate deep within the Moon's interior. Another important source of gases is the solar wind, which is briefly captured by the Moon's gravity.

Eclipses

The angular diameters of the Moon and the Sun as seen from Earth overlap in their variation, so that both total and annular solar eclipses are possible. In a total eclipse, the Moon completely covers the disc of the Sun and the solar corona becomes visible to the naked eye. Since the distance between the Moon and the Earth is very slightly increasing over time, the angular diameter of the Moon is decreasing. This means that several million years ago the Moon always completely covered the Sun on solar eclipses so that no annular eclipses occurred. Likewise, in several million years the Moon will no longer cover the Sun completely and no total eclipses will occur. Eclipses happen only if Sun, Earth and Moon are lined up. Solar eclipses can only occur at new moon; lunar eclipses can only occur at full moon. See also Solar eclipse and Lunar Eclipse.

Observation of the Moon

Lunar Eclipse During the brightest full moons, the Moon can have an apparent magnitude of about −12.6. For comparison, the Sun has an apparent magnitude of −26.8. The Moon appears larger when close to the horizon. This is a purely psychological effect (see Moon illusion). The angular diameter of the Moon from Earth is about one half of one degree. Various lighter and darker colored areas (primarily maria) create the patterns seen by different cultures as the Man in the Moon, the rabbit and the buffalo, amongst others. Craters and mountain chains are also prominent lunar features. From any location on Earth, the highest altitude of the Moon on a day varies between the same limits as the Sun, and depends on season and lunar phase. For example, in winter the Moon is highest in the sky when it is full, and the full moon is highest in winter. The orientation of the Moon's crescent side also depends on the latitude of the observing site. Close to the equator an observer can see a boat Moon. [http://curious.astro.cornell.edu/question.php?number=393] Like the Sun, the Moon can also give rise to an optical effect known as a halo. For more information on how the Moon appears in Earth's sky, see Lunar phase.

Exploration of the Moon

Lunar phase prepares to descend towards the surface of the Moon. NASA photo.]] NASA standing next to boulder at Taurus-Littrow during third EVA (extravehicular activity). NASA photo.]] The first leap in Lunar observation was caused by the invention of the telescope. Especially Galileo Galilei made good use of this new instrument and observed mountains and craters on the Moon's surface. The Cold War-inspired space race between the Soviet Union and the United States of America led to an acceleration. What was the next big step is politically laden. In the US (and the West in general) the landing of the first humans on the moon in 1969 is seen as a culmination, indeed of the space race in general. But from a scientific point of view the first photographs of the until then unseen far side of the moon in 1959 constituted the second leap in Lunar observation. 1959 and Luna missions]] The first man-made object to reach the Moon was the unmanned Soviet probe Luna 2, which made a hard landing on September 14, 1959, at 21:02:24 Z. The far side of the Moon was first photographed on October 7, 1959 by the Soviet probe Luna 3. Luna 9 was the first probe to soft land on the Moon and transmit pictures from the Lunar surface on February 3, 1966. It was proven that a lunar lander would not sink into a thick layer of dust, as had been feared. The first artificial satellite of the Moon was the Soviet probe Luna 10 (launched March 31, 1966). The first robot lunar rover to land on the Moon was the Soviet vessel Lunokhod 1 on November 17 1970 as part of the Lunokhod program. On December 24, 1968 the crew of Apollo 8, Frank Borman, James Lovell, and William Anders became the first human beings to see the far side of the Moon with their own eyes (as opposed to seeing it on a photograph). Humans first landed on the Moon on July 20, 1969. The first man to walk on the lunar surface was Neil Armstrong, commander of the American mission Apollo 11. The last man to stand on the Moon was Eugene Cernan, who as part of the mission Apollo 17 walked on the Moon in December 1972. See also: A full list of lunar astronauts. Moon samples have been brought back to Earth by three Luna missions (nrs. 16, 20, and 24) and the Apollo missions 11 through 17 (minus Apollo 13, which almost ended in a disaster). On January 14 2004, US President George W. Bush called for a plan to return manned missions to the Moon by 2020. NASA's [http://www.nasa.gov/missions/solarsystem/cev.html plan] to accomplish that goal was announced on March 19 2005, and was promptly dubbed Apollo 2.0 by critics. The European Space Agency has plans to launch probes to explore the Moon in the near future, too. European spacecraft Smart 1 was launched September 27, 2003 and entered lunar orbit on November 15 2004. It will survey the lunar environment and create an X-ray map of the Moon. [http://news.bbc.co.uk/2/hi/science/nature/2818551.stm] [http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=36091] The People's Republic of China has expressed ambitious plans for exploring the Moon and is investigating the prospect of lunar mining, specifically looking for the isotope Helium-3 for use as an energy source on Earth [http://space.com/missionlaunches/china_moon_030304.html]. Japan has two planned lunar missions, LUNAR-A and Selene; even a manned lunar base is planned by the Japanese Space Agency (JAXA). India will also try an unmanned orbiting satellite, called Chandrayan. From the mid-1960's to the mid-1970's there were 65 moon landings (with 10 in 1971 alone), but after Luna 24 in 1976 it suddenly stopped. The Soviet Union started focusing on Venus and space stations and the US on Mars and beyond. In 1990 Japan visited the moon with the Hiten spacecraft, becoming the third country to orbit the moon. The spacecraft released the Hagormo probe into lunar orbit, but the transmitter failed rendering the mission scientifically useless.

Human understanding of the Moon

Myth and folk culture

The Moon as muse

The Moon has been the subject of many works of art and literature and the inspiration for countless others.

Astrology

Scientific understanding

A 5,000 year old rock carving at Knowth, Ireland may represent the Moon, which would be the earliest depiction discovered. In many prehistoric and ancient cultures, the Moon was thought to be a deity or other supernatural phenomenon. Among the first in the Western world to offer a scientific explanation for the Moon was the Greek philosopher Anaxagoras, who reasoned that the Sun and Moon were both giant spherical rocks, and that the latter reflected the light of the former. His atheistic view of the heavens was one cause for his imprisonment and eventual exile. By the Middle Ages, before the invention of the telescope, more and more people began to recognize the Moon as a sphere, though they believed that it was "perfectly smooth". sphere In 1609, Galileo Galilei drew one of the first telescopic drawings of the Moon in his book Sidereus Nuncius and noted that it was not smooth but had craters. Later in the 17th century, Giovanni Battista Riccioli and Francesco Maria Grimaldi drew a map of the Moon and gave many craters the names they still have today. Francesco Maria Grimaldi. Surprisingly, the Moon is actually brighter than the Sun at gamma ray wavelengths.]] On maps, the dark parts of the Moon's surface were called maria (singular mare) or "seas", and the light parts were called terrae or continents. The possibility that the Moon could contain vegetation and be inhabited by "selenites" was seriously considered by some major astronomers even into the first decades of the 19th century. In 1835, the Great Moon Hoax fooled some people into thinking that there were exotic ani