Celestia Forums

Selden's post in the bugs forum reminded me... I never quite got my head around what the difference between all these J1991.25, J 1950, J2000 things were. All I know is that they're something to do with stars changing positions in the sky over time, is that something to do with their proper motions?

Anyone care to enlighten/correct me further?

Evil Dr.,

Actually, it's due to the precession of the earth's axis. The traditional equatorial coordinate system is defined to have its origin at one of the two points where the plane of the earth's equator crosses the plane of the ecliptic. As the earth slowly wobbles, the origin slowly moves. As a result, the coordinates of all of the stars slowly change. It's quite annoying.

(Simplistically, B1950 refers to the coordinates they had in 1950, while J2000 refers to the coordinates they had in 2000.)

So the IAU finally did something about it. Essentially the J2000 coordinates are what will be used from now on. It's slightly more subtle than that, and the new coordinate system is actually called "ICRS" -- the International Celestial Reference System. See http://www.iers.org/iers/earth/icrs/icrs.html

Ta, Selden. So presumably the idea is that this ICRS/J2000 system will not be changing over time?

Dr. Evil,

At least it won't be changing continuously as the traditional equatorial system has. Instead of being defined in terms of the earth's rotation, it's defined in terms of the positions of astronomical objects like remote quasars. The only changes will come about as their positions are measured more accurately.