sthscan Like the names imply, they are two different things. Jnow are RA/Dec coordinates based on the current position of the Earth's axis, J2000 coordinates are based on where the North Pole was 25 years ago. Both are useful, Jnow for precisely pointing to a target right now, J2000 as a common reference for images of the same object taken at different times. There is a difference of about 8 arc minutes between the two right now.