leap year

A leap year is a year with one extra day added, keeping a solar calendar synchronised with Earth's actual orbit around the Sun.

• Earth's orbit takes nearly a quarter-day more than 365 days, and those extra quarter-days add up to roughly one full day every four years — which is exactly the gap a leap year corrects.
• In the Gregorian calendar, any year divisible by four is a leap year, giving February 29 days instead of the usual 28.
• This simple four-year rule slightly overcorrects over long stretches of time, so century years are normally skipped as leap years (like 1700, 1800, 1900) — except when the year is also divisible by 400 (like 1600 and 2000), when the leap year is added back.
• Without leap years at all, a solar calendar would slowly drift out of step with the actual seasons — eventually, a date like a national holiday could shift into a completely different season over enough years.