Time, includingsidereal time, can be measured in many ways. Sidereal day is the time elapsed since the precedding transit of Sayana first point of Aries to the next transit of the meridian of a place. In other words, one sidereal day is the time taken by the Earth in completing one rotation with respect to a fixed star which is equal to 23 hours 56 minutes and a few seconds.
This sidereal day is expressed in sidereal hours and minutes. One sidereal day is equal to 24 sidereal hours. One such hour comprises of 60 minutes etc.
It can be observed that a fixed star which is rising along the Sun will rise about 4 minutes earlier than the sunrise next day i.e. the Sun has moved 1° in the zodiac.
If an observer continues to observe the sky for one month, he will notice that the Sun has risen 1 rashi after the fixed star. After one year, he will notice that the same star is rising again with the Sun.
As the Earth is moving round the Sun and the Sun is fixed, the earth completes one revolution around the Sun in one year. The Earth rotates around its axis once in a day. The same part of the Earth appears approximately 365 + 1 = 366 times in front  of  the same fixed star in a year (appro-ximately 365  solar days) or the 1st point of Aries has transited the meridian 366 times in a year of 365 days and 367 times in a year of 366 days i.e. a leap year. Therefore, a sidereal day is shorter than the solar day by 24 hrs/ 365. 25 = 24 x 60/365.25 = 3 minutes 56 sec   approximately

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