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Sidereal Time is the time is takes for celestial bodies to ascend and descend in the night sky. We know that celestial bodies are in reality, fixed in their positions. The reason for their dramatic movement in the night is because of the rotation of the earth. This is the same reason why the Sun and the Moon seem to rise and set. For the longest time, this motion caused many philosophers and astronomers to assume that the Earth was the center of the Universe. Fortunately later astronomers like Copernicus were able to discern the true movements of the Earth, Moon, and Sun helping to explain their movements. The time that it takes for a star, planet or other fixed celestial body to ascend and descend in the night sky is also called sidereal period. Coincidentally this time corresponds to the time it takes for the Earth to rotate one revolution which is just under 24 hours.
Sidereal time is not like solar time which is measured by the movement of the sun. Or the lunar cycles which take about 28 days. It is the relative angle of a celestial object to the prime meridian of the vernal equinox of the earth. IF these terms are confusing, here is what they mean. In cartography, the Earth is bisected by two major lines of longitude and latitude. These lines are the 0 degree points on the globe. The 0 degree point for the latitude is the Equator the point where the Earth is perfectly bisected. It cut through South America and Africa. The 0 degree point for the longitude is the prime meridian. It exact location is Greenwich, UK. The Equinoxes are essentially the times of the year when the sun rise and sets at the exact same point of the horizon at the equator. This means that these are the only times the solar day is equally divided into 12 hours of day and 12 hours of night. The hour angle for a celestial object relative to this meridian is what we call sidereal time.This angle changes with the rotation of the Earth creating a pattern of ascension and descent for celestial bodies in the Earth’s sky.
With the knowledge of sidereal time astronomers can predict the positions of stars. The values for the sidereal time of celestial objects is compile in a table or start chart called an ephemeris. With this guide to sidereal time astronomers can find a celestial object regardless of the change in their position over the year.
There are also some great resources on the net. The U.S. Naval observatory has an online clock to help you find out the sidereal time in your area. There is also a great explanation on the astronomy section of the Cornell university site.