The History and Science of Leap Days


A Leap Year is a calendar year that contains an additional day – a Leap Day – compared to common years. In other words, each leap year has 366 days instead of 365. The 366th day is added to keep the calendar year synchronized with the solar year, the actual time it takes the Earth to circle once around the Sun.

Because astronomical events and seasons do not repeat in a whole number of days, calendars that have a constant number of days in each year will unavoidably drift over time with respect to the event that the year is supposed to track, such as seasons.

By inserting an additional day into some years, the drift between the calendar and the physical properties of the solar system can be corrected. A leap day is added to the shortest month of the year – February.

2024 is a Leap Year! Upcoming Leap Years will be: 2028, 2032, 2036, 2040, 2044, and 2048.

Too Many Leap Years

Throughout history, the Julian and Gregorian calendars have been two of the most commonly used calendars in the world. One of the biggest differences between the Julian and Gregorian calendars is the way they calculate leap years.

The Julian calendar was developed by Julius Caesar in 45 BC, and it was based on a solar year of 365.25 days. The Julian calendar was in use for centuries, but it wasn’t perfect. Over time, the calendar gained a few days due to small inaccuracies in the calculation of the solar year. In the Julian calendar, a leap day was added every four years, which is too frequent.

The Gregorian calendar, also known as the Western or Christian calendar, is the most widely used calendar in the world today. The Gregorian calendar was developed by Pope Gregory XIII in 1582 to adjust for the small error in the Julian algorithm. It is based on a solar year of 365.2425 days. Although it is not perfect either, today’s Gregorian calendar uses a much more accurate rule for calculating leap years.

The Gregorian calendar only adds an extra day to years that are divisible by four. However, there are some exceptions. If the year is divisible by 100 and not divisible by 400, leap year is skipped. The year 2000 was a leap year, but the years 1700, 1800, and 1900 were not.

Lost Days

Over the centuries since its introduction in 45 BCE, the Julian calendar had gradually drifted away from astronomical events like the vernal equinox and the winter solstice. To make up for this error and get the calendar back in sync with the astronomical seasons, a number of days had to be dropped when the Gregorian calendar was adopted. In North America, the month of September 1752 had only 19 days, as the day count went straight from September 2 to September 14!

Leap Month

The Chinese calendar, which is one of the oldest calendars still in use, operates on a lunisolar system. The traditional Chinese lunisolar calendar is a hybrid timekeeping method that takes into account both the moon’s orbit around the earth AND the earth’s 365-day orbit around the sun.

The regular Chinese calendar has 12 months. However, a whole extra month is inserted into the calendar when a leap year occurs. Therefore, leap years in the Chinese calendar have 13 months, unlike leap years in the Gregorian calendar in which an extra day is included. A leap year in the Chinese calendar does not necessarily fall at the same time a leap year occurs in the Gregorian calendar.

Moreover, the leap month’s place in the Chinese calendar varies. To determine when a leap month is added, consider the number of new moons between the 11th month in one year and the 11th month in the following year. If there are 13 New Moons from the start of the 11th month in the first year to the start of the 11th month in the next year, a leap month is inserted. This happens approximately once every two or three years.

So every second or third Chinese calendar year has 13 months and 383-385 days. According  to, the last Chinese calendar leap month was the second lunar month in 2023. The next leap month will be in 2025, when the sixth lunar month will be observed as leap month. The name of the leap month is the same as the previous lunar month.

Fascinating stuff! Were you born on a Leap Day? I bet you’re excited to have another Leap Year come around. When do you celebrate your birthday in the other years?

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