File:Sidereal day (prograde).png
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.
A solar day is a unit of time. It is a cycle of day and night.