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Two clocks that do not match

Why Does a Year Have 365 Days?

A year is one orbit. A day is one spin. Neither divides into the other.

A day One spin of EarthA year One trip around the Sun
Two unrelated motions, forced into one calendar.

  1. Step 1 of 5

    A day and a year measure different things.

    1 Earth spins —one day2 Earth orbits —one year3?How many spinsper orbit?

    A day is Earth turning once on its axis. A year is Earth going once around the Sun. Nothing links the two.

  2. Step 2 of 5

    The answer is not a whole number.

    Actual year365.2422 daysCalendar year365 daysThe gapabout 6 hours ayear

    Earth spins about 365.2422 times per orbit. That awkward quarter is the source of every calendar complication in history.

  3. Step 3 of 5

    Ignoring the quarter day drifts the seasons.

    1 Lose 6 hours ayear2+One day every 4years3 Seasons driftout of place

    Six hours lost each year becomes a full day every four years. In a few centuries, midsummer would drift into what the calendar calls winter.

  4. Step 4 of 5

    Leap years put the quarter back.

    Normal year 365 daysLeap year+366 days — the quartersrepaid

    Adding a day every fourth year restores the balance. That is why February gets a 29th day in leap years.

  5. Step 5 of 5

    Even that overcorrects slightly.

    Divisible by 4 — leap year2024,2028Divisible by 100 — skip it1900,2100Divisible by 400 — leapafter all1600,2000

    365.25 is a touch too long, so century years skip their leap day unless divisible by 400. That is why 1900 was not a leap year but 2000 was.

    When Britain switched calendars in 1752, eleven days were deleted. People went to bed on the 2nd and woke on the 14th.

The short version

A year is 365 days because Earth spins that many times per orbit — and the leftover quarter is why we have leap years.

Try it yourself

Check whether 2100 is a leap year. It is divisible by 4, but it is a century year not divisible by 400 — so no.

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