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Celestia is a free 3D astronomy program for Windows, Mac OS X, and GNU/Linux. Created by Chris Laurel, it is licensed under the GNU General Public License.
The program, based on the Hipparcos Catalogue, allows user to display objects ranging in scale from artificial satellites to entire galaxies in three dimensions using OpenGL. Unlike most planetarium software, the user is free to travel about the Universe.
NASA and ESA have used Celestia, but it should not be confused with ESA's similar program, [http://www.rssd.esa.int/SA-general/Projects/Hipparcos/CELESTIA/celestia-pr.html Celestia 2000].
Functions
* Tour Guide
* Hipparcos Catalogue (120,000 Stars)
* Solar and lunar eclipse finder to the year 9999.
* It is possible to record the program as video file up to a resolution of 720x576 pixels.
* The time can be set as far in the future or past as you like (though planet orbits are only accurate within a few millennia of the present day, and date arithmetic overflows at the year 5874774)
* Constellations and orbits of planets (including extrasolar planets around stars that have one or more), moons, asteroids, comets and satellites can be indicated.
* The names of all objects in space can be indicated: galaxies, clusters, stars, planets, moons, asteroids, comets and satellites.
* The names of cities, craters, observatories, valleys, landing sites, continents, mountains, seas and other surface features can be shown.
* The radius, the distance, length of day and average temperature of the planets are indicated.
* The distance, luminosity relative to the sun, spectral class, surface temperature and radius of stars are indicated.
* The number of visible stars can be varied.
* The field of view is infinitely variable.
* The apparent luminosity of the stars can be adjusted in three stages.
* The picture can be divided, in order to observe several objects at the same time.
* Light time delay (due to the finite speed of light) can be included or ignored.
* Gamepad and joystick support
Limitations
There are some limitations in Celestia's modelling:
* The default setting for Celestia's Earth is a perfect spheroid, which means that low Earth orbit satellites are not modeled accurately. However, this setting can be adjusted.
* Many, if not most, binary stars are not yet properly simulated.
* Only objects which form part of a planetary system move; galaxies are stationary. Stars rotate about their axes and orbit each other in multiple star systems, but stellar proper motion is not simulated.
* The only orbit you can be in is a synchronous orbit.
Add-ons
More than 10 GB of extensions are available in addition to the base program. This enormous quantity is the result of the activities of a small, but very productive, user community. The add-ons address the following subjects:
* There are many 3-D graphics and textures available as add-ons. Users can download fictional and educational add-ons, to show what a protostar looks like from the back in detail, or 3-D stars and solar systems. There is a specialised version of Celestia for educational users. The standard package can be used for fictional and theoretical 3-D texturing, scripts and animation.
* 3-D textures and models are also available for planet Earth, such as Stonehenge and some cities, or objects and landmarks on other planets, including fictional ones.
* There are several databases with objects in orders of magnitude from satellites and moons up to stars and galaxies. The extended star database possesses two million stars and the revised NGC/IC galaxy database in excess of 10,000 galaxies with accurate locations, orientations, sizes and generic colors, whereby galaxy clusters are also visible. The latter is part of the regular distribution. The Earth city database is expandable to 96,000 cities.
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