Why Do the Positions of the Stars Change Each Month?

The stars rotate 360 degrees in a sidereal day.
••• Stockbyte/Stockbyte/Getty Images

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.

Sidereal Day

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.

Solar Day

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.

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.

Related Articles

What Is Solar Altitude?
Calendar Year vs. Earth Orbit
How Much Time Is One Day on Mars?
What Causes the Day/Night Cycle on Earth?
Constellations That Can Be Seen Year Round
Why Are Days Longer and Shorter?
How to Calculate the Sun's Declination
Sun Intensity vs. Angle
Chances of a Solar Eclipse
Characteristics of a Star
Why Do Objects Appear to Move Across the Sky at Night?
The Difference Between Solar & Lunar Years
What Is Mercury's Rotation Period?
Does an Equinox Happen All Over the Earth at the Same...
How is Parallax Used to Measure the Distances to Stars?
Which Planet Moves the Slowest Along Its Orbital Path?
How to Calculate the Winter Solstice Sun Angle
How to Calculate Longitude from Right Ascension
How Many Hours of Daylight in Summer?