Why do we have leap years?

A year is the time Earth takes to orbit the Sun, and a day is the time Earth takes to spin once. The two do not divide evenly. Earth's orbit takes about 365 days and 6 hours, or more precisely about 365.2422 days, so a calendar of 365 days falls behind. Adding one day to February roughly every four years makes up the difference. NASA describes this extra day as the way to count the leftover part of a day that would otherwise vanish from the calendar. See [spaceplace.nasa.gov](https://spaceplace.nasa.gov/leap-year/en) and [jpl.nasa.gov](https://www.jpl.nasa.gov/edu/news/doing-the-math-on-why-we-have-leap-day/). [1] [2]

Without leap days, the calendar would drift against the seasons. Dropping about six hours each year adds up to roughly 24 days of shift in a century, according to [timeanddate.com](https://www.timeanddate.com/date/leapyear.html). The Australian Bureau of Meteorology's space weather service gives a similar figure of 25 days per hundred years, and notes that summer would soon fall where winter used to. JPL says that after only a century without a leap day, summer would not start until mid-July. The Smithsonian's National Air and Space Museum estimates that over about 700 years, northern summers would arrive in December. The figures differ slightly because each source rounds the year's length a little differently. [3] [4] [2] [5]

A leap day every four years assumes the year is exactly 365.25 days. The real year is slightly shorter, so this rule overcorrects. JPL says it adds about 45 extra minutes per four-year cycle, which comes to about three days every 400 years. The Smithsonian puts the overshoot at over 44 minutes. The older Julian calendar used the simple every-four-years rule, and the Bureau of Meteorology says its seasons drift by about one day every 128 years. [2] [5] [4]

The Gregorian calendar, used today, adds an exception. A year divisible by 4 is a leap year, unless it is divisible by 100, in which case it is not, unless it is also divisible by 400. So 2000 was a leap year, while 1700, 1800 and 1900 were not, and 2100 will not be. This makes the average calendar year 365.2425 days, against a true year of about 365.2422. The Bureau of Meteorology estimates the remaining drift at one day in around 3,300 years. [4] [5]

The Gregorian system remains an approximation. Timeanddate.com says it is still not perfect, though the resulting deviation is very small. Physicist Pauline Barmby of Western University adds that Earth's rotation is gradually slowing, so days have grown slightly longer over the past few millennia, which also affects calendars over long periods. The sources give slightly different values for the year's exact length, such as 365.2422, 365.24219 and 365.242189 days, reflecting rounding and the way the year is defined. [3] [6] [5]