Sidereal day

From Wikipedia, the free encyclopedia

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 but the Sun is not (1→2 = one sidereal day). It is not until a little later, at time 3, that the Sun is overhead again (1→3 = one solar day).
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 but the Sun is not (1→2 = one sidereal day). It is not until a little later, at time 3, that the Sun is overhead again (1→3 = one solar day).


An apparent sidereal day is the time it takes for the Earth to turn 360 degrees in its rotation; more precisely, is the time it takes a typical star to make two successive upper meridian transits. This is slightly shorter than a solar day. Earth's orbit contains 366.25636042 sidereal days in a sidereal year appearing as 365.25636042 mean solar days. There are 366.2422 sidereal days in a tropical year appearing as 365.2422 mean solar days, resulting in a sidereal day of 86,164.091 seconds (or: 23 hours, 56 minutes, 4.091 seconds).

The Earth rotates in the same direction around its axis, as it does around the Sun: seen from the northern sky, counter-clockwise. So the Earth's orbit around the Sun offsets by one sidereal day, giving observers on Earth 365 1/4 days, even though the planet itself rotated 366 1/4 times. This is the reason there is one fewer "normal" solar day in a year than there are sidereal days.

Midnight, in sidereal time, is when the First Point of Aries crosses the upper meridian.

A mean sidereal day is reckoned, not from the actual transit, but from the transit of the mean vernal equinox (see: mean sun).

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