RA and DEC Notation

[Mikea]

Sometimes when an object's location is specified, there are two sets of RA and DEC quoted: one of them will say J2000. The values are quite similar - but not the same. Could someone explain please? Sorry if it's been asked before, I did Google it but found nothing.

[Merlin66]

Look for "precession of the equinox".

http://en.wikipedia.org/wiki/Precession_(astronomy)

Basically the RA DEc frame of reference moves relative to the "fixed" stars due to a "wobble" in the Earths' axis ie a bit like the spinning top slowing down.

You need then to specify the Epoch ( date) ie J2000 to co-ordinate the stars position in RA an DEc

Hope this helps.

[Mikea]

Yes it makes perfect sense. Thanks very much!