# Short term changes in celestial coordinates of stars

The right ascension and declination of stars change on a day to day basis. Some of the changes are quite large. The right ascension of Polaris, for example, changes by more than a degree over the course of some years. The excursions are irregular. Their time and extent differ from year to year. I know that precession will change coordinates of all the stars over tens of thousands of years, but I can't find an explanation for these short term changes.

Polaris started 2017 with a sidereal hour angle of 316 deg 23.9 min. The SHA increased monotonically to 317 3.6 by the end of April, then decreased in small steps to 315 58.3 by November 9, then increased again, ending the year at 316 6.2.

• Could you quantify "quite large"? Every star has its proper motion, e.e. we see them as they move through 3D space relative to us, this causes coordinates to change with time. Proper motion values of stars are known. So if you would quantify what you've seen, one could compare those values. Jun 1, 2019 at 15:41
• I found the answer. Explained very well in the Wikipedia article on Nutation. I was led to "nutation" by following links I found searching on "proper motion." The question should be closed. Thanks, @atmosphericprisonescape Jun 1, 2019 at 18:50
• Polaris is close to the celestial pole so relatively small changes in position can correspond to a large change in right ascension - this is an effect of the coordinate system. An equivalent effect is that a degree of longitude is a longer distance at the Equator than in the vicinity of the North or South poles.
– user24157
Jun 1, 2019 at 21:13
• @stretch It is always okay to answer your own question in Stack Exchange. If you feel you've figured out what you want to know, feel free to post an answer!
– uhoh
Jun 1, 2019 at 23:19
• To reinforce @uhoh's comment, I would be genuinely interested in reading (and upvoting) an answer - and by providing it yourself, you would be in the running for a relatively rare badge: Self-learner! Jun 1, 2019 at 23:30