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. There are 366.2422 sidereal days in a tropical year, but 365.2422 solar days, resulting in a sidereal day of 86,164.091 seconds (or: 23 hours, 56 minutes, 4.091 seconds).

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

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).

See also

• sidereal month
• sidereal time
• time

External link

• Web based Sidereal time calculator

Units of time

Звезден ден | Siderischer Tag | Jour sidéral | Siderički dan | Siderische dag | 恒星日 | Siderisk døgn | Doba gwiazdowa | Rotação da Terra | Stjärndygn | 恒星日