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Time and Calendars · entry 51 of 52
A sidereal day is slightly shorter than 24 hours because Earth also moves along its orbit.
The longer version
Measured against distant stars, one rotation takes about 23 hours 56 minutes. Against the Sun it takes 24, because the planet has advanced along its orbit and must turn a little further, roughly one extra degree, to bring the Sun back to the same spot. Both are real; they simply use different reference points.
The four-minute difference explains why stars rise about four minutes earlier each night and why constellations shift across the seasons. Observers use the star-referenced day for pointing instruments; everyone else uses the solar version, which keeps noon near the middle of the working day. Confusing the two is a common source of error for beginners.
Where this entry sits
These facts wander through clocks, calendars, time zones, and the long human effort to measure passing hours. You will find leap years, odd month lengths, ancient sundials, and the reasons modern timekeeping works the way it does. The full list sits on the Time and Calendars page, where every entry is listed in order.
More From Time and Calendars
Eight more entriesStandard time replaced thousands of local times, making railway schedules possible in the nineteenth century.
Read 1A day is roughly twenty-four hours because that is how long Earth takes to rotate once.
Read 2A year lasts about 365 days because Earth needs that long to orbit the Sun once.
Read 3Leap years add an extra day in February to keep calendars aligned with Earth's orbit.
Read 4A leap year occurs nearly every four years, except at the turn of most centuries.
Read 5February is the shortest month, with 28 days in common years and 29 in leap years.
Read 6The months of September through December take their names from Latin words for seven through ten.
Read 7January is named after Janus, the Roman god of doorways and beginnings.
Read