What Is a Leap Year and Why Do We Have Them?

(Last Updated On: January 27, 2020)

What Is a Leap Year and Why Do We Have Them?
What is a leap year and why do we have them? Why does February only have 28 days? And what on Earth is a leap second? We’ll answer all your “leap” questions in this post.

What Is a Leap Year?

Leap years take place every four years. During a leap year, an extra day is added to the calendar, so instead of the usual 365 days in a year, there are 366. This extra day is added to the month of February, meaning that during a leap year there are 29 days in that month.

Why Do We Have Leap Years?

The Gregorian calendar is the most widely used calendar in the world. It is named after Pope Gregory XIII, who introduced it in October 1582. This calendar is based on the Earth’s orbit around the Sun. We know, of course, that the Earth takes 365 days to make one complete circumsolar orbit. Sort of.

Actually, if we want to get precise, it takes 365.24 days. In other words, it takes the Earth 365 days and six hours to travel around the Sun.

These six hours add up, and every four years, they total an extra day. To account for this extra day, every four years, we have a leap year and add another day to the calendar. This addition is important to keep the calendar synchronized with the movement of the Sun, the solar seasons, and the patterns of the Moon and tides.

Why Does February Only Have 28 days?

In the 8th century BCE, the Romans used a 10-month, 304 day calendar that began with the spring equinox in March, and ended in December. In those days, calendars were used primarily by farmers, who regarded winter as a useless time period, so the season wasn’t even counted.

King Numa Pompilius thought this was weird. So in 713 BCE, he created a new calendar using the year’s 12 lunar cycles (spanning about 355 days). The months of January and February were introduced at the end of the calendar.

The Romans were superstitious of even numbers in those days, so King Numa Pomilius tried to make every month have an odd number of days. However, it became clear that one month would need to have an even number to stick to the lunar cycle. So February, being the last month and seen as a bit of an afterthought, got the even number of days. The months looked a little something like this:

Martius: 31 days
Aprilius: 29 days
Maius: 31 days
Iunius: 29 days
Quintilis: 31 days
Sextilis: 29 days
September: 29 days
October: 31 days
November: 29 days
December: 29 days
Ianuarius: 29 days
Februarius: 28 days

Of course, with only 355 days, seasons soon fell out of sync. So the Romans began introducing leap months every now and then, and this confused everyone.

Julius Caesar would ultimately get rid of the leap month, and change the calendar altogether. He aligned his Julian calendar with the sun, and few days were added so each year would have 365 total. Though his reason is not entirely clear, he decided to keep the 28 days in February.    

What Is a Leap Second?

Though less familiar than leap years, it is worth noting that sometimes, extra seconds are added as well. On certain days of the year, leap seconds are inserted into our clocks to account for irregularities in the Earth’s rotation.

Determining when these leap seconds occur is influenced by the moon and tides: when the moon rotates around the earth, it is slower than the earth rotating on its axis, and we have to account for this 0.002 seconds of delay each day. As such, leap seconds are added. Because it’s only a second however, most people don’t even know about them or even realize a change. Who knows, the second that just passed may have been a leap second…

Leap years, leap seconds, and lack of days in February aside, the whole synchronization of the Earth, Sun, Moon, tides, and time is pretty impressive. And having an extra day every four years seems like a small price to pay for what we get in return.