The Sidereal Time is used to determine the ascendant or the first house. The rule for determining Sidereal Time is as follows:

Earth takes 24 hours to complete one rotation on its axis. But, the fact is when the earth is measured with any particular planet, then we come to know that Earth takes 24 hours 3 minutes 56 seconds to complete one rotation on its axis. This time is different for every place. Normally, an Ephimeris is used to find out Sidereal Time.