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Sidereal Day

Also called: stellar day

Definition: A sidereal day is one rotation of the Earth relative to the stars: about 86,164.09 seconds, or 23 h 56 min 4.09 s, roughly 3 min 56 s shorter than a 24-hour solar day.

Updated November 4, 2026 · How we check facts

The sidereal day measures Earth's rotation against distant stars rather than against the Sun. NIST lists it as 8.616409 × 10⁴ s, which is not an exact value. The clock day of 86,400 s is the longer solar day, because Earth also moves along its orbit between one noon and the next.

Formula

86,164.09 s ÷ 3,600 = 23.9345 h = 23 h 56 min 4.09 s

Example

The difference from the clock day is 86,400 − 86,164.09 = 235.91 s, about 3 min 56 s. See how many seconds in a day for the exact 86,400 s day and the effect of the leap second.

Calculate it

Free toolDate Duration CalculatorDays between two dates, or a date plus N days.

Guides that explain it

Related terms

Leap SecondA leap second is a one-second adjustment added to Coordinated Universal Time (UTC) so it stays within 0.9 s of Earth-rotation time, giving a day of 86,401 seconds.Second (s)The second is the SI base unit of time, defined from the caesium-133 transition frequency of 9,192,631,770 Hz; there are exactly 60 seconds in a minute and 86,400 in a day.Tropical YearThe tropical year is the time between successive spring equinoxes, about 365.2422 days (about 31,556,930 seconds). It is the natural year the Gregorian calendar approximates.Gregorian CalendarThe Gregorian calendar is the civil calendar in worldwide use. Its 400-year cycle has 146,097 days, an average year of 365.2425 days, kept by the leap-year rule.Hour (h)The hour is a unit of time equal to exactly 60 minutes or 3,600 seconds, and one 24th of a day. It is accepted for use with the SI but is not an SI unit itself.Leap YearA leap year has 366 days instead of 365. In the Gregorian calendar a year is a leap year if divisible by 4, except centurial years that are not divisible by 400.Minute (min)The minute is a unit of time equal to exactly 60 seconds, one 60th of an hour. It is accepted for use with the SI, and a day has 1,440 of them.

Sources

  1. NIST Guide to the SI (SP 811), Appendix B.8 — Factors for Units Listed Alphabetically