Why Do We Have Leap Years?

A year is 365 days… until suddenly it isn’t. 

Most of us are familiar with the concept of leap years, but unless you’re an astronomer. Mr. Scot Gill, a Dunbar science teacher helps students understand the reason why we add a day every four years.

“Day and night is caused by the fact that the Earth is spinning, and years are caused by the fact that the Earth is going around the sun, but instead of taking 365 days, it takes 365.24. We have to have a leap year every four years,” Mr. Gill said.

We add an intercalary day every four years because the Earth takes slightly longer than 365 days to orbit around the sun–365.242189 days, or one tropical year, to be exact. And because a tropical year is slightly shorter (11 minutes shorter, to be exact) than 365.25 days, there is no intercalary day in century years that aren’t divisible by 400. For example, 2100 will not be a leap year, but 2000 was.

“But if we have a leap year every four years, we’ve made up too much, so every 100 years we don’t have a leap year,” Mr. Gill said.

Without leap years, we would lose nearly six hours each year. That’s something that happened for millennia, up until the invention of the Julian calendar in 45 B.C. by Julius Caesar’s astronomer Sosigenes.

Even after the invention of the Julian calendar, there was still an 11-minute gap between the calendar and solar years. While this may sound small, by the 16th century the calendar was about 10 days off which led Pope Gregory XIII to implement the revised Gregorian calendar that we use to this day.

Because leap years are infrequent, the chances of being born on Feb. 29 are only 1 in 1,461. These birthdays have been the frequent subject of pop culture mockery as in the famous comic opera The Pirates of Penzance

At Dunbar, we have one student who has a Feb. 29 birthday. This year she’s turning four-years-old (or 16 in non-leap year terms). Happy birthday, Leslie Sanchez.