What's in the Sky: Happy Leap Year!

As most of us are aware, we have leap years because the Earth does not orbit around the sun in precisely 365 days. Instead, it orbits in 365.2426 days. That’s pretty close to 365 and a quarter days, so adding a day every four years keeps us in step with the seasons. That ensures us that the dog days of summer are in August, Thanksgiving comes in November, and Christmas is in December. Can you imagine singing, “I’m dreaming of a white Fourth of July”?

It was not always this way. Calendars were once based on the phases of the moon. The moon’s orbital period also does not fit evenly with 365 days, so the calendar was periodically adjusted by the Romans, adding a few days every couple of years. It was not only confusing, but corruptible. Pontiffs — religious figures who got to call the shots on this — sometimes added days to keep their political allies in office by lengthening their terms, or refused to add days to kick out their enemies more quickly.

That was corrected by the “Julian” calendar, a reform promoted by Julius Caesar. That calendar established 12 months, with a day thrown in every four years to keep the seasons in line. That’s where we are today, you might say.

Tossing in a leap year every four years helps a lot, but remember that number — 365.2426? Notice that it’s not quite 365 and a quarter. It’s about 11 minutes short of a quarter day. Given enough years, that begins to add up. We gained about three days every four centuries, and after a few centuries, it began to impact how the seasons align with the months. By the 1500s, the first day of spring was no longer on March 21, but rather occurred on March 11. Since the first day of spring is tied to Easter, that didn’t settle well with the Catholic Church. Pope Gregory XIII introduced a revised calendar, the Gregorian calendar, which we still use today. Because we were gaining about three days every four centuries, that calendar takes away three leap years every four centuries. See if you can follow this: Every year that is exactly divisible by 100 is not a leap year, unless it is divisible by 400. Confused? Relax. You probably won’t have to worry about it in our lifetimes. It only involves years such as 1900 (divisible by 100, but not 400 and thus not a leap year), 2000 (divisible by 100 and also 400, so it is a leap year), and 2100 (like 1900, not a leap year). It cleverly removes three leap years every four centuries.

Gregory had another problem though. On Feb. 24, 1582, he signed the decree establishing his calendar. But by then, we had already gotten out of sync by 10 days, with the first day of spring occurring in March 11 instead of 21. His solution? Simple, just drop those days! The big switch came in October 1582. We had October 1, 2, 3, 4, then 15. October 5-14 didn’t happen. If you have ever downloaded the planetarium program Stellarium (www.stellarium.com), or another planetarium program, check it out, those dates are not there.

The change was not immediately picked up in the non-catholic world, but others eventually changed for the convenience of international trade. The British Empire (including us) changed in 1752. Greece was the last to accept the change, in 1923.

So enjoy your extra day on Feb. 29, now that you have a better understanding of where it came from. Maybe you have a friend born on that day, who laments not having a birthday every year. Tell your friend that it could be worse ­— imagine the poor kids born on Oct. 10, 1581. When did they get to celebrate their first birthdays?