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To ancient Man, the night sky may have seemed like the only thing that was, for the most part, unchanging and eternal. Of course, as early as the dawn of human intelligence came the realization that some of the points of light were “wandering.” We now know these “wandering stars” are planets. Once in a while, comets dazzle the humans on Earth. And once in a great while, a visible nova or supernova causes a bright new point of light appear where none could be seen before. So even ancient Man knew that the night sky changes, in some ways. The truth is, the night sky changes in every way, when you consider many thousands of years of time.

Stellar drift causes the stars to move noticably over the course of thousands of years, hence distorting the constellations at first and eventually making the old constellations unrecognizable. Proper motion is the apparent speed that a star is shifting across Earth’s sky. A star with a very high velocity relative to the Earth would nevertheless have a proper motion of zero if this star were headed directly towards (or away from) the Earth. With a proper motion of zero, it would not shift apparent position in Earth’s sky. (This is loosely speaking, because the Earth is in orbit around the Sun, so the proper motions of all stars shift slightly as the Earth orbits.)

In the image at top, you can see that a nearby star with unusually high proper motion can be seen to shift relative to other stars in less than 50 years time, from 1952 to 2000. The star with the highest proper motion of all is Barnard’s Star, which will appear to move about the width of the moon in a space of 180 years. Most stars have much less extreme proper motions, so it usually takes thousands of years to make much of a difference. (By the way, the spikes in the image at top are explained in the article “Diffraction Spikes“.)

Here is how Pisces will change from now to 18,000 years in the future:

(If the above movie fails, then try this quicktime movie.)

Aries is there too, on the left, but apparently there is a mistake in the data for the constellation lines there, since the constellation lines don’t seem to be tracking one of the stars in Aries. A program called Starry Night Pro 5 was used to create this movie, but if you want to play around with proper motion and time travel for free, then try the free program moovastar.

For a brief background on the subject, see the Proper Motion page at utk.edu.

Resources

• Starry Night Pro 5
• moovastar
• 2MASS Sky Data at Caltech.edu
• Proper Motion at utk.edu

This article's tags are: ancient man, aries, constellations, pisces, proper motion, Revelations, stars in the night sky, stellar drift

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