Institute of Astronomy

J1950 and J2000 Epochs

Published on 23/08/2011 

What are the epochs J1950 and J2000 when looking at objects in the sky? And what do I have to put in my telescope program?

Astronomers use different epochs to give coordinates of objects in the sky due to changes in motion due to primarily the precession of the Earth on its rotation axis. Much like a spinning top, as the Earth rotates, it's rotation axis gradually rotates as well although much slower than the daily rotation we see. Because of this precession, the positions of the stars change over time with a small motion every day. On small timescales this motion isn't noticeable however over decades it is. For this reason, astronomers update their coordinates every 50 years to make it simpler when finding objects.

Although for an object you can find coordinates in either J1950 or J2000 (the two most recent epochs), this won't actually be the correct location in the sky today. However, what your telescope program will do will take the coordinates from those epochs and then calculate what they should be today i.e. J2011 and then move your telescope there. This saves people doing things by hand and so means you can find objects with the standard coordinates quickly and easily.

As such, all you should need to do is find the coordinates of the object in either J2000 or J1950 and can then put those into your program. The program will then do the rest.

Page last updated: 23 August 2011 at 10:27