The Making of a Leap Year
February 18, 2016
Sarah Blount

Keep your eyes peeled this week for a rare sighting, one that comes only once every four years (or so): the 29th of February. Why does the calendar include this infrequent addition to the month? Because for Earth, one lap around the sun doesn't take exactly 365 days, and this inconsistency has thrown a wrench in different societies' attempts to create a calendar for thousands of years.

What is a Year?

For the calendar used in the United States today, one year is defined by the length of time it takes Earth to orbit the sun, measured from one vernal equinox to the next: about 365.242 days. These extra hours make setting a calendar of 24-hour units tricky—if this fragment of a day is not included, eventually the equinox, which should remain in essentially the same spot on an accurate calendar, begins to shift.

Much like noon is easy to pinpoint because the sun is at its zenith in the sky, the equinoxes are recognizable because they are the two times of year when the sun is directly over Earth's equator, giving the entire planet equal amounts of daylight and darkness. This alignment occurs during the transition between seasons—in the Northern Hemisphere, summer months are marked by the sun's concentrated light on the northern half of the planet, and the winter months' chill comes from the sun being more directly focused on the Southern Hemisphere. As Earth rotates around the sun on its tilted axis, the sun's direct focus shifts between these two areas of the planet, passing over the equator on its route.

Hail Caesar! Well, almost...

In the modern calendar, the autumnal equinox falls around the 22nd of September, marking the beginning of autumn, and the vernal equinox occurs around the 21st of March, signifying the start of spring. However, in early calendars, the seasons and their accompanying equinoxes slowly migrated through the civil schedule due to inconsistencies in the established length of the civil calendar.  In an attempt to correct this discrepancy, Julius Caesar moved the Roman people from a lunar calendar to a solar one in 46 BCE, establishing a calendar system that set the number of days in a year at 365, with every fourth year having 366 days. This creation of a bissextile, or a leap year, set the median length of the year at 365.25 days, and reordered the calendar such that the vernal equinox appeared at the 25th of March, closer to where it had been historically. While Caesar's calendar was very close to the actual length of Earth's orbit, it wasn't quite right—so the equinox once again crept backwards on the civil calendar.

Religious Events and the Lunar Calendar

Several major religious events, such as Passover and Easter, are marked by the lunar calendar in relation to the vernal equinox, so having this astronomical event correctly identified on the calendar was a priority for Pope Gregory XIII. To attempt to keep the civil calendar accurate, the pope mandated that years that were divisible by 100 would not be bissextile, and to bring the equinox closer to its historical position, 10 days would be skipped that year from the month of October, meaning Catholic countries went from October 4, 1582 to October 15, 1582. The new calendar, referred to as the Gregorian calendar, slightly reduced the mean length of a year, bringing it closer to its true value. This change was adopted slowly across Europe, and England and the American colonies did not adopt this new calendar until the middle of the 18th century.

Today, the system of leap years used to determine the calendar in the United States is:

  • Years that are divisible by four have an extra day (bissextile, such as 2016)
  • Years that are divisible by 100 do not have an extra day (exceptions to the leap years, such as 1900)
  • Years that are divisible by 400 have an extra day (an exception to the above rule, making them bissextile, such as 2000)

To learn more about different calendar systems throughout history, check out this article from Rice University: The Gregorian Calendar.