Sidereal Day (d (sid))

AstronomicalCurrent unitApproximate value

About 86,164.09 seconds (23 h 56 min 4 s), one rotation of the Earth relative to the stars.

1 sidereal day = 86,164.0905 seconds (approximately).

Approximate value. Standards for the sidereal day varied by place and period, so this converter uses the most commonly cited value. Treat results as estimates, not exact figures.

About the sidereal day

Sidereal comes from the Latin word for star, and the unit measures how long Earth takes to turn once relative to the distant stars rather than to the Sun. The figure used here is about 86,164.09 seconds, or 23 hours 56 minutes 4.09 seconds. NASA's Earth fact sheet gives the same period as 23.9345 hours.

The sidereal day is about four minutes shorter than the mean solar day because Earth also moves roughly one degree along its orbit during each rotation, so it must turn slightly further before the Sun is back on the meridian. Over a year this adds up to one extra turn, giving about 366.24 sidereal days against 365.24 solar days.

For centuries astronomers measured it by timing stars as they crossed the meridian and checking observatory clocks against star catalogues. From the 1970s, radio methods such as very long baseline interferometry and pulsar timing proved more precise, and since 2003 the Earth Rotation Angle has replaced the older sidereal time conventions in formal use.

Sources: Wikipedia: Sidereal time; NASA Earth Fact Sheet

Sidereal Day conversion chart

1 sidereal day expressed in every other time unit:

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