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If I go to google sky - http://www.google.com/sky/

Ihere is a longitude and latitude measure at the bottom of the page. Iow are these coordinates determined and who sets it?

Do those coordinates move at all over time?

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up vote 2 down vote accepted

These coordinates are really called right ascension and declination

http://en.wikipedia.org/wiki/Right_ascension
http://en.wikipedia.org/wiki/Declination

and they're the most widespread pair of coordinates to describe the celestial sphere. They're analogous to longitude and latitude on Earth but they're not quite the same thing.

See also other coordinates sometimes used in astronomy

http://en.wikipedia.org/wiki/Celestial_coordinates#Coordinate_systems

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is there an absolute coordinate system for the stars? like something that star trek people use? how would one convert between one and the other? –  zcaudate Sep 25 '12 at 13:29
    
The sky is a sphere, so there are many ways to parameterize the sphere. This pair of coordinates is an equatorial coordinate system, based on the celestial equator – which is the region on the skies in the same plane as the Earth's own equator (given by the Earth's spin). So the declination (the "latitude") is zero on this celestial equator. The right ascension is like the longitude and the convention puts 0 (counterpart of Greenwich) at the vernal (spring) equinox, i.e. where the Sun is located at the special moment around March 20th. –  Luboš Motl Sep 25 '12 at 13:33
    
Note that both the Earth's spin direction as well as the tilt of the axis relatively to the ecliptic (the plane of Earth orbit around the Sun) is needed to fix the conventions for these coordinates. So aliens attached to another planet would quite certainly use different coordinates. –  Luboš Motl Sep 25 '12 at 13:35
    
If I understand correctly about the wiki posts say if I wanted to contact an alien civilization in a galaxy far far away... we would be then coordinating via this? en.wikipedia.org/wiki/Supergalactic_coordinate_system. –  zcaudate Sep 25 '12 at 13:46
    
OK, I think that all of our iPhones can translate one coordinate system into another within a microsecond so I guess we would allow the aliens to use whatever conventions they like. I find it rather unlikely for them to use the supergalactic coordinates - moreover, they're not really too accurately defined. The supergalactic coordinates still rely on some random features of our local neighborhood - less sharply defined one than the earth's equator and equinox which are really special and linked to a small planet but much more accurate than some perceived clusters. –  Luboš Motl Sep 25 '12 at 13:51
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