Celestia Forums

And one more question...

Go hither and take a look at the list of the closest 100 stars.

I'm not following column 6 (Proper Motion). The first number is obviously arcseconds per year... but the second number is an angle where (at the bottom of the page) it claims that 0 is "north" and 90 is "east". Barnards star, I know, is moving "up" in the sky, toward the north pole, and its angle is stated at 355.6 degrees, so that kinda makes sense.

But what does "east" mean? Just that it's moving "right" in the sky (if viewed from the northern hemisphere)? I guess the problem is that the proper motion numbers seem rather two-dimensional - shouldn't there be some component telling us whether the star is moving toward or away from us too?

Oh Evil One,

Proper motion is indeed two dimensional. It's motion "in the plane of the sky". It's measured by watching stars for years and measuring their positions, usually relative to stars that aren't moving. Well, traditionally, by measuring their relative positions on photographic plates taken years or decades apart.

Motion toward or away from us is called "radial motion." It's usually measured by looking at the doppler shifts in spectrographic lines, although changes in parallax can be used for nearby stars.

East is east If North is up, East is toward your left on the sky. It's toward your right if you're looking down at the ground. It's toward increasing Right Ascension at the same Declination. Of course, this doesn't hold true for something exactly at one of the poles, but how likely is that to happen?

I just noticed that the Hipparcos Catalogue has proper motion in terms of mu-alpha* and mu-delta (Fields H12 and H13).

I can translate that (via trigonometric wizardry) into motion in a direction that the star is moving, can't I? Or is that again only the direction it's moving in the sky as seen from Earth?

As I mentioned above, those are in the plane of the sky. You also need the star's radial motion (toward or away from the Sun) in order to be able to generate 3D vectors.