Sidereal time measures time relative to distant stars, with a sidereal day lasting 23 hours, 56 minutes and four seconds, about four minutes shorter than a solar day. Astronomers use sidereal time to track the movement of stars across the sky and determine their location. The difference between solar and sidereal time adds up over time, causing the stars to appear to move westward.
Sidereal time refers to the length of days and other measurements of time relative to stars other than our sun. While a solar day lasts 24 hours, a sidereal day is about four minutes shorter. Solar time is always used in everyday life, but astronomers often find it helpful to use sidereal time when expressing the apparent movement of the stars across the sky, as observed from the earth. The word “sidereal” comes from the Latin word sider, which means “star”.
Since the earth orbits the sun as it rotates, the earth must rotate slightly more than one full rotation on its axis to complete a solar day. In one day, the earth moves a small part of its orbit around the sun, slightly changing the apparent angle at which the sun looks at the earth. This small addition of about one degree to the sun’s angle means that the earth must rotate for another four minutes to “catch up” to where the sun shone exactly one rotation ago.
However, since stars other than our sun are mostly very distant, it only takes one full 360-degree rotation before a distant star returns to its starting point above the earth. This 360-degree rotation is called a sidereal day and lasts 23 hours, 56 minutes and four seconds. On a solar day, the earth rotates approximately 361 degrees on its axis, taking a full 24 hours.
Astronomers often use sidereal time because they are concerned with how long it takes for the earth to rotate, relative to distant stars, rather than the sun. Sidereal time offers us a way of measuring time that does not depend on the sun. This removes a slight mathematical complication and focuses solely on how long it takes the earth to rotate 360 degrees relative to the stars.
To an astronomer, the sky is like a map, divided into 24 hours. Sidereal time helps these scientists determine where a given star, or set of stars, is at any given moment. The difference between solar and sidereal time is not much more than an hour or a day, but it adds up to a very large difference over a period of months and years. This can be observed even without a telescope. If a person looked at or photographed the night sky at the same time each night, they would notice that the stars as a whole move westward over time.
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