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RA and DEC Notation

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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. :)

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Look for "precession of the equinox".


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.

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