Cosmology
Text only © 1998 - 2001
Paul J. Marquard.
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Stellar motion comes in three types.
The first is the actual movement of the stars through the backdrop of other stars. This motion takes a very long time to notice. The study of this motion is called astrometry. (You Star Trek fans may have heard of the astrometrics lab.) This motion is sometimes due to binary motion. That is the motion of two stars as they orbit a common center. We'll study this more later. Another is the actual motion of stars as they orbit the center of the galaxy at different speeds. This can cause changes in stellar positions and constellation shapes. However, these motions are too slow to see without careful measurements over a long time. So we'll leave their discussion for other classes.
The second motion is the motion of all the constellations and stars as they pass over the evening sky. This is due to the spinning of the Earth's axis and is called diurnal motion. If you look to the north for an extended time, you will notice the stars moving in circles around the north celestial pole.
Actually, the above image is from the southern hemisphere, but its the same around the south celestial pole as it is the north celestial pole.
If you look to the east or west the rising and setting stars will show arcs. In the below picture, however, the arcs are short enough in duration and large enough in curvature that they look straight. It doesn't help that the picture is two dimensional either.
The third might not count as stellar motion, but changes in the daily (nightly) stellar motion from day to day. Because the sun is changing position in the sky, the stars that are visible from night to night will change. The alteration is very noticeable over long periods of time. The constellations visible tonight are different than those visible near the end of the semester, or next semester. The exception are circumpolar stars. They are always in the sky, but are in different positions around the pole, depending on the time of night AND the time of year.
Ancient people knew of this motion, and would plan migrations or planting seasons based upon when stars first became visible in the early morning sky.
This page was last updated on 06/06/01.