The solar day is the time between two successive meridian passages of
the sun. Because the Earth is orbiting the sun in the same sense as
it is rotating about its North/South axis, the Earth has to do slightly more
than one complete revolution in order to achieve two successive meridian
passages of the sun. Sidereal time is based on a fixed point in space
(referred to as the first point of Aries) which may be considered at an
infinite distance from the Earth which means that the movement of the Earth
around the orbit does not affect the time between two successive meridian
passages of the first point of Aries. Therefore the sidereal day is a constant
period and slightly less than the solar day; there are 366¼ sidereal days in a
year.