The monthly positions of the stars change because of the interaction between the rotation of the earth around its axis and the orbit of the earth around the sun. The stars rotate around the north and south celestial poles; hence the stars are always moving relative to a point on the earth's surface. Additionally, the earth is always moving around the sun. However, the stars "move" in the sky slightly faster than the sun.
The position of the stars in the sky changes by 360 degrees every 23 hours, 56 minutes and 4 seconds. This period of time is called a sidereal day. For example, if you locate a particular constellation at precisely midnight on one night of the year, it will be in that exact same part of the sky at 11:56:04 the following night.
The position of the sun in the sky changes by 360 degrees every 24 hours. This period of time is called a solar day. The sun is in the exact same spot in the sky every 24 hours of apparent solar time. Apparent solar time is the kind of time told by sundials. However, most other clocks keep track of mean solar time: this is an average of the deviations caused by the Earth's tilt and its elliptical orbit.
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Solar Days versus Sidereal Days
The amount of time it takes for the sun to make a complete trip across the sky is different from that of the stars. The difference between sidereal days and solar days causes the positions of the stars to change each month relative to solar time. The stars make it across the sky quicker than the sun; therefore, they appear to move west slightly over a solar day. Alternatively, the sun appears to lag eastward behind the stars.
Changes in Position Per Month
With the exception of the North Star, the position of the stars in the sky changes by nearly one degree every 24 hours of solar time. For example, if you locate the bright star Sirius in the night sky, it will appear to have moved westward by one degree 24 hours later. Therefore, over the course of a month, the position of the stars at a given time will shift by roughly 30 degrees. Over 12 months, the position of the stars will shift by 360 degrees. Hence, we see the same group of stars at the same time each year.