On a prograde planet like the Earth
, the sidereal day
is shorter than the solar day
. At time 1, the Sun
and a certain distant star
are both overhead. At time 2, the planet has rotated 360° and the distant star is overhead again (1→2 = one sidereal day). But it is not until a little later, at time 3, that the Sun is overhead again (1→3 = one solar day). Or more simply, 1-2 is a complete rotation of the Earth. However because the Earth moving around the Sun affects the angle
the Sun hits a position on the Earth, 1-3 is how long it takes noon
to happen again.
Solar time is a calculation of the passage of time based on the position of the Sun in the sky. The fundamental unit of solar time is the day. Two types of solar time are apparent solar time (sundial time) and mean solar time (clock time).
Images for kids
Sun and Moon, Nuremberg Chronicle, 1493