About every four years our calendars add an extra day to February. It's not random, it's astronomy.
Leap days are caused by a mismatch between the number of calendar days and the actual time it takes the Earth to orbit the Sun. One calendar year is 365 days, but technically one solar year is 365 days plus 5 hours, 48 minutes and ten seconds.
“There's a quarter of a day there,” said Sky and Telescope senior editor Kelly Betty Ali explained Boston Public Radio.
Humans have known about this mismatch since the days of Julius Caesar in 46 BC, who decreed that the Romans should celebrate a leap day every four years.
“This idea of leap days has become ingrained in our calendar system. Every four years we catch an extra day, and then there’s still a little bit left over,” Beatty said.
Then, 1,600 years after Julius Caesar, Pope Gregory XIII reformed the calendar and the leap day system again, creating Gregorian calendar Which most of the world uses today.
In the Gregorian calendar, years that mark the beginning of a new century are not leap years, although they are divisible by four. But Beatty said: “Every century that is divisible by 400, like the year 2000, has a leap year.”
“It's confusing, but in the end it keeps our calendars… in sync with where the sun is in the sky,” Beatty said.
