Sky Lights: Equinoxes

Find out how this month's 29th day halts the disorienting creep of the seasons.

By Bob Berman
Feb 1, 2000 6:00 AMApr 26, 2023 6:33 PM


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Quick quiz: how many days are there in February this year? Getting to the correct answer may seem easy, but it involves making a few detours to check out distant stars, the Earth's spin, and even a few groundhogs. Counting the days in February 2000 should mean including a leap day because any year divisible by four has an extra day.

On the other hand, century years-those ending in double zeros-ordinarily do not have a February 29; 1700, 1800, and 1900 didn't have this additional day. But once every four centuries, the double-oh year is a leap year, and 2000 is one of them. So there will be a February 29 this year.

How did this get so complicated? Basically, people like nice whole numbers, but nature does not oblige. Measured against the stars, an Earth year lasts 365.2422 days-365 days, 5 hours, 48 minutes, and 46 seconds. If the year were 365.25 days, or about 11 minutes longer, things would be simple: Add one leap day every fourth year and you eliminate the fraction. But the Earth circles the sun a tad faster than that, so we occasionally have to skip the once-every-four-year leap day.

Bypassing three of them every 400 years may be confusing, but it gets the job done. This extra tweak, introduced by Pope Gregory XIII in 1582, keeps our calendar accurate to one day every 3,300 years. Years divisible by 4,000 are not leap years, which fixes that little glitch.

If it weren't for this elaborate system of leap years, each January 1 would begin at a slightly different point in Earth's circuit around the sun. That might not seem like such a big deal, but eventually, the seasons would start at odd times. Christmas would come in the summer if this went on long enough. Our previous calendar, in which all century years were leap years, was good enough for Julius Caesar, but its annual 11-minute error did add up. By the 1500s, equinoxes and solstices took place a full 10 days too early.

In 1582, Pope Gregory boldly returned the seasons to their intended place by simply eliminating 10 days from the calendar. That year, October 4 was followed by October 15. He also decreed that all century years divisible by 400 would be leap years.

The Gregorian calendar allows equinoxes to drift behind as each century progresses, only to push them ahead a bit too much by the omitted leap year at most centuries' end. People over 30 may recall a time when the first day of spring fell on March 21. Long since then, the date slid to March 20, however. The insertion of an extra day this month keeps the start of spring on the twentieth and even allows it to slip back to March 19 later this century.

You'll have still more trouble keeping track of the days this month. February 2, Groundhog Day, is one of the ancient "cross-quarter" days marking the midpoint between the solstice and the equinox. Cross-quarter days have also shifted over the centuries. So if you want to pop leftover millennium champagne at the moment there's exactly as much winter behind you as ahead, you'll have to wait two more days, until February 4. If only Gregory were still in charge-he'd probably just make those days vanish.

Peter Meyer, a software developer and calendar expert, has written a comprehensive overview of the Julian and Gregorian calendars:

The Encyclopaedia Britannica, now available free on the Internet, has an excellent essay on the history of calendar reform:,5716,108734+2,00.html.

For a quick look at all the different calendar systems out there, and how to convert among them, go to

You can keep track of the ever-changing celestial phenomena at (it helps to have Shockwave installed on your computer):

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