Have you ever wondered why the month of February has only 28 days most years, but occasionally has 29 days, as it does this year? 2024 is a leap year, and it’s time to take up the slack in our calendar.

Our use of the leap year system started back in the days of the Roman Empire, under the reign of Julius Caesar. Even that long ago, sky watchers realized that there was not a whole number of days in a year.

The Earth spins on its axis 365 times in one year, plus another one-quarter spin — an excess of six hours. It wouldn’t make any sense to have the last day of the year be only six hours long, so Julius Caesar decreed in 44 BC that we would let the one-quarter days accumulate and then add in one whole day, a leap day, every fourth year. This would take up the slack between our calendar and Earth’s actual orbit around the Sun. Or so Caesar thought.

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This calendar reform by Julius Caesar assumed that the year was exactly 365.25 days long, but it isn’t. It’s actually 365.2422 days long — about 11 minutes and 14 seconds shorter than Julius Caesar assumed. So, using Julius Caesar’s method, we were adding in too many leap days over the centuries.

By the 1500s, all those 11 minutes and 14 seconds had added up to a full 10 days on the calendar, causing the vernal equinox, or first day of spring, to shift from March 21 to March 11. If that error continued, we’d eventually be celebrating the first day of spring in December. Not good.

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In 1582, Pope Gregory XIII sought to fix this problem by reforming the Julian leap year system. First, he cut 10 days out of the calendar and declared that Oct. 5 was actually Oct. 15. This brought the date of the vernal equinox back to March 21, where it should be. He then declared that any year evenly divisible by 4 would remain a leap year, unless it was a century year, like 1900 or 2000.

In order to be a leap year, a century year must be evenly divisible by 400, not four. So, the century years 1600 and 2000 were leap years, but the century years 1700, 1800 and 1900 were not, according to the new rule. Using Pope Gregory’s method, we leave out three leap days every four centuries, just enough to keep the calendar in synch with the seasons. The Gregorian reform assumes that a year is 365.2425 days long, very close to the actual length of 365.2422 days. It will take 3,300 years for this small discrepancy to add up to a full day, so we needn’t worry about it for a long, long time.

Why did the extra leap day get attached to February instead of some other month? The original Roman calendar contained 10 months, not 12: Martius, Aprilus, Maius, Iunius, Quintilus (the “fifth month”), Sextilus (the “sixth month”), Septembris (the “seventh month”), Octobris (the “eighth month”), Novembris (the “ninth month”), and Decembris (the “tenth month”).

Januarius and Februarius were added to the end of the calendar year in 713 BC and Februarius, the last month, was given only 28 days. It seemed logical to tack the extra leap day onto the end of the calendar year, giving us an occasional Feb. 29. In the year 154 BC, the first month of the year was changed from Martius backwards to Januarius. Quintilus was renamed July to honor Julius Caesar, and Sextilus was renamed August to honor Augustus Caesar, and thus we are left with our modern calendar, where Decembris, literally the “10th month,” is actually our 12th month. Strange, but true.

Did you know that in leap years, Halloween always occurs on the same day of the week as Leap Year Day? That’s also true. Both fall on Thursday this year.

Children born on Leap Year Day are sometimes called “leaplings.” Their real birthday only rolls around once every four years, so leaplings usually celebrate their birthdays on March 1 in non-leap years.

Put your extra day this year — Feb. 29 — to good use.

Jimmy Westlake is adjunct Professor of Physical Sciences at Colorado Mountain College and former Director of the Rollins Planetarium at Young Harris College in Georgia and the St. Charles Parish Library Planetarium, in Luling, Louisiana. His Celestial News column appears monthly in the Steamboat Pilot & Today. Check out Jimmy’s astrophotography website at JWestlake.com.