Leap Years and Leap Days
Leap years help to keep our calendar aligned with the Earth’s orbit around the sun and aligned with the seasons. This article explains their purpose and why they only occur every four years.
The seasons occur as the Earth orbits the sun and the calendar tracks its progress in doing so. The calendar has to be accurate in doing this, so the seasons fall in the same months each year.
A calendar year is 365 days long, but it takes the Earth slightly longer to complete its orbit. When a new calendar year begins, the Earth is short of completing a full orbit by approximately 6 hours. This means that after four years the Earth is about 24 hours (one whole day) behind where it should be.
If this continued, after 120 years the Earth’s position around the sun would be 30 days behind the calendar. For example, the calendar would suggest it was the beginning of September when it was really the beginning of August.
The Leap Day
The solution is to add an extra day to the calendar every four years. This extra day is like a catch-up day, giving the Earth the time it needs to realign itself with the calendar once again. This is how the Julian calendar works.
The extra day, called the leap day, allows the Earth’s position to ‘leap forward’ and realign with the calendar. The leap day is 29th February, although it has fallen on different days in the past.
Over Compensating
However, adding a whole day to the calendar every four years is actually too much because the Earth is not 6 hours short of completing its orbit around the sun when a new calendar year starts. Currently, on average, the Earth is 5 hours, 48 minutes and 45 seconds short, which means that after four years it’s only 23 hours and 15 minutes behind the calendar. Adding an extra day to the calendar every four years is therefore adding 45 minutes too much allowing the position of the Earth to now get in front of the calendar, not behind it like it was before, and every four years it gets another further 45 minutes in front.
After 128 years, the calendar and the Earth’s orbit would become a whole day out. So now, the calendar suggests it’s 1st January, when it’s actually 2nd January. To prevent this from happening, every so often when a leap year is due to be added, it isn’t. This is how the Gregorian calendar works.
Calculating Leap Years
If a year is divisible by 4, it is a leap year. However, if it is also divisible by 100 then it is not a leap year unless it can also be divided by 400 in which case it is a leap year. Years that are not leap years, are common years.
Leap Years of the 20th and 21st Centuries
| 1900-1999 | 1904, 1908, 1912, 1916, 1920, 1924, 1928, 1932, 1936, 1940, 1944, 1948, 1952, 1956, 1960, 1964, 1968, 1972, 1976, 1980, 1984, 1988, 1992, 1996 |
| 2000-2099 | 2000, 2004, 2008, 2012, 2016, 2020, 2024, 2028, 2032, 2036, 2040, 2044, 2048, 2052, 2056, 2060, 2064, 2068, 2072, 2076, 2080, 2084, 2088, 2092, 2096 |
The Next Five Leap Days
- Tuesday 29th February 2028
- Sunday 29th February 2032
- Friday 29th February 2036
- Wednesday 29th February 2040
- Monday 29th February 2044
Footnote: The Solar Year
The solar year is the time it takes the Earth to orbit the sun. Currently, it is about 365.2422 days long. The Julian calendar assumes it to be 365.25 days and the Gregorian calendar, 365.2425. The differences between them is small, but they soon grow over time as can be see here:
