# Fixed or Moving Stars

• Philosophaie

#### Philosophaie

Are the stars fixed according to RA and Dec? If not is there equations using Julian Date for their movement?

## Answers and Replies

A complex question. RA and Dec are an earth-based coordinate system based on an imaginary projection of the Earth's axis and equator onto the celestial sphere. However, the Earth's axis is not fixed in space, but undergoes a slow "wobble" with a period of about 26,000 years, called precession (http://en.wikipedia.org/wiki/Precession_of_the_equinoxes), So all of the stars change their RA and Dec in a regular way due to this changing of the coordinate system. This is a regular, predictable process and there are equations to compensate for it.

In addition, the stars are moving through space, and each star has its own motion, called proper motion(http://en.wikipedia.org/wiki/Proper_motion), that is determined by how fast it is moving and how far away it is. There are catalogs of the measured proper motion for each star that you can look up if you need to know it.