What is a sidereal day?
A sidereal day is defined as approximately 23 hours, 56 minutes and 4.091 seconds. This is the time it takes for the Earth to complete one rotation relative to the vernal equinox. The solar day is equivalent to the time taken by the Earth so that the Sun is at its highest point after each day. This takes an average 24 hours. In the case of stars however, as they are so much farther away than the Sun, they return to the same point after every sidereal day. The sidereal day is also known as stellar day.
To keep track of which way to point telescopes while viewing the stars, the sidereal time is used. For geostationary satellites also, the orbit is chosen such that they complete one revolution around the Earth in one sidereal day.