On moonless nights, when the sky is clear, standing in an open area, you will see stars twinkling against the black canvas of the night.
If you observe closely, you will notice that the stars rise in the East, slowly glide across the sky, and gradually set in the West, just as we see the Sun rise in the East and set in the West each day. In fact, this movement is caused by the Earth rotating from west to east.
The constellations gradually move westward because the Earth is also revolving around the Sun. (Illustrative image).
In addition to the daily sight of stars moving from East to West, each star rises four minutes earlier each subsequent day. Consequently, during the same moments of each night throughout the year, the stars we observe are not the same; the positions of the constellations shift gradually westward. For example, the Orion constellation, which we are familiar with, rises in the East at sunset at the beginning of December, and after three months, it sets in the West at sunset. This constellation travels across the southern sky. However, by the end of spring, it sets at the same time as the Sun during sunset.
With the changing seasons, the constellations gradually move westward because the Earth is also revolving around the Sun. If we could see stars during the day, we would observe the Sun shifting eastward among the constellations. Each day, the Sun moves one degree eastward, which is equivalent to a distance of twice the diameter of the Sun. Thus, over the course of a year, the stars have “a circular motion in a year” across the sky.
In summary, stars exhibit two types of motion: one type is caused by the Earth’s rotation, resulting in daily motion where stars rise in the East and set in the West.
The second phenomenon, caused by the Earth revolving around the Sun, leads to annual motion, causing constellations to appear and change according to the seasons. We should not confuse these two phenomena as one.
