Leap years happen because our paper calendars say that a year lasts 365 days, but that’s not actually true—the Earth takes something like 365.25 days to orbit the sun, and those extra quarter-days conveniently add up to a leap day every four years.
But even those adjustments aren’t as precise as we’d like. During the Roman Empire, Julius Caesar instituted a calendar year of 365.25 days, but in the 16th century, Pope Gregory XIII acknowledged the truth: Each year actually contains about 365.2422 days. Caesar, with his Julian calendar, had rounded up to that extra quarter of a day.
“Even though we add a Leap Day in to balance the calendar, it’s still a bit off,” explains Phil Plait (B.S. ’87), an LSA astronomy alumnus and the author of Bad Astronomy, a blog that runs on Slate. “It’s a lot better, for sure, but it’s still just a hair out of whack.” Adding a full leap day to the calendar amounts to just a little too much extra time—every four years, our calendar gains about 45 extra minutes. To measure time correctly by the Julian calendar, we technically need to subtract one day every 128 years. But the Pope’s Gregorian calendar, which we use today, corrected Caesar’s rounding errors. We’ll spare you the gory mathematical details and note only that century years are never leap years, unless they’re divisible by 400. This means that 2000 was a leap year. 1900 wasn’t.
Why? “Because if there is a stupid way to do something, that’s how it will be done,” says Plait. “So every four years, February 29th magically appears on the calendar, and once again the calendar is marginally closer to being accurate.”
Accuracy aside, what does any of this mean to us? We reached out to LSA researchers, artists, and faculty across campus to ask: What’s the deal with leap years and some of those strange leap-year customs?
A leap year is divisible by 4 but not by 100—except, of course, if it’s divisible by 400. Why did we adopt such a complicated, atrocious solution?
Professor Rudi Lindner of the departments of astronomy and history says, “It could be much worse. Consider the lunar calendar used in Islamic history. A lunar year, based on lunar phase cycles, is 354.37 days. The solar calendar, based on the Earth’s trip around the sun, is 365 days. This means that for every lunar century, there are 97 solar years. If lunar January begins in solar January, over time, lunar January will begin in solar December, then November, and so on.
“In Islamic society, there is a month of fasting,” he continues. “If that month occurs in solar January, when the days are short, then fasting during the day is not very difficult. But if that month occurs in solar June, then the fast occurs during a very long day.
“The leap year is designed to prevent such wanderings. So every now and then, we add a day just to keep our seasons and our months together.
“Otherwise, even though we don’t have a lunar calendar, we might find ourselves celebrating the Fourth of July during a blizzard.”
“All these calendar systems are attempts to fit cycles of agriculture into a reasonably simple model of astronomical events. The problem has always been complex. In the Middle Ages, you proved that you were a genius not only by writing a book that explained all the competing calendar systems, but also by showing how to convert dates from one system into another.”
Surprisingly, earthquakes can influence the Earth’s rotation—large tremors redistribute mass in the Earth’s core, which makes the Earth spin more quickly around its axis. Enormous undersea earthquakes in 2004 in Indonesia and 2011 in Japan shortened days on Earth by slightly less than three microseconds. Do we need to account for earthquakes in our calendar?
“The shift in mass from earthquake movement affects Earth mechanics, but only very very slightly,” says Ben van der Pluijm, a professor in LSA’s Department of Earth and Environmental Sciences (EES) and Program in the Environment.
“Large earthquakes with a large amount of movement along faults can change the rate of Earth’s spin,” agrees Jeroen Ritsema, another professor in EES. “Think of an ice dancer moving her arms closer: She will spin around faster. If the mass movement is asymmetric, Earth’s rotation axis can change.” However, says van der Pluijm, “A single earthquake probably has no more effect on mass distribution change, and thus rotation, than the ice dancer slightly bending her pinky.”
Shifts in mass don’t stay asymmetric. Because earthquakes happen all around the world, any mass shifts tend to balance out across the Earth over very long periods of time. Gravity is more important. “Significant wobbles of Earth result from its interaction with other large masses, like Jupiter, which causes natural climatic variations,” says van der Pluijm.
As does the moon. “While small, the effect of Earth-moon interaction on Earth's spin rate is significantly larger than the effect of a few magnitude-9 earthquakes that occur in a century,” says Ritsema. “The drift of the moon from Earth—about 1.5 inches per year—makes the Earth spin slower by about 0.005 seconds per century. When dinosaurs walked on Earth, a day was a few hours shorter than it is now.”
Leap day babies—such as (left to right, above) motivational speaker Tony Robbins, 1940s celebrity Dinah Shore, rap artist Ja Rule, and actor Dennis Farina—technically celebrate their birthday just once every four years. Some are members of the Honor Society of Leap Year Day Babies, which boasts nearly 10,000 members.
Irish custom (with variants elsewhere) says that leap years provide the opportunity for women to propose to their beaus. Why?
“It’s the one day out of every four years when women can defy traditional gender roles,” says Rita C. Seabrook, a Ph.D. candidate in psychology and women’s studies. “In heterosexual relationships, men have been expected to take the initiative—to ask out a woman in the first place, to initiate sex, and to propose marriage. So this is a woman’s one opportunity to take matters into her own hands.
“The Sadie Hawkins dance is somewhat similar, but I don’t necessarily see a lot of value in either,” Seabrook continues. “It’s great that there is one day out of every four years when women can propose … but if we’re striving for gender equality, shouldn’t it be like this every day?”
Finally, a couple of professors shared their favorite work that deals with time—its absurdity, manipulation, and confusion.
Naomi André’s thoughts go straight to opera—she studies it as an associate professor in LSA’s Residential College, Department of Afroamerican and African Studies, and Women’s Studies Department. “In the Gilbert and Sullivan operetta Pirates of Penzance,” André says, “the lead tenor character was born on February 29, and his age becomes an odd plot device.”
For Susan Crowell, a ceramic artist who teaches in LSA’s Residential College and the Stamps School of Art & Design, “One artist who comes immediately to mind is my colleague Isaac Wingfield, who uses a pinhole camera and long exposure for landscape photography. A relatively ordinary site, such as an exit ramp, can become profoundly beautiful by virtue of the long exposure, much as the lingering gaze finds beauty in ordinary things. Slowing down is one of the first lessons we teach our students in art classes—getting them to see and to notice.”
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