A spare day every few years
A calendar year has 365 days. But the Earth's trip takes about a quarter day more. Where does the spare time go?
Step 1 of 3
- 1900 divides by 4. Leap, so far.
- 1900 also divides by 100. Ordinary, so far.
- 1900 does not divide by 400. So it is ordinary.
New word
- Leap year — a year with an extra day, 29 February.
The rule
The spare quarter days are kept. They make one extra day, added in most fourth years. A century year is a year ending in 00, like 1900.
Earth's trip round the Sun takes a bit less than 365¼ days. Four quarter days make about one extra day. So a year divisible by 4 is usually a leap year. A century year must divide by 400.
A leap day every four years adds a little too much. Skipping most century years corrects that. 1900 ÷ 400 = 4.75, not a whole number. So 1900 was not a leap year. 2000 ÷ 400 = 5, a whole number. So 2000 was.
Pick a year. Run the tests.
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