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Leap day: Why do we do this every four years?

As we await the arrival of the total solar eclipse on Case Western Reserve’s campus this spring, our calendars have another astronomical event to hurdle first: leap day.

Observed Feb. 29, the day occurs every four years and extends the length of our shortest month. But why do we observe leap day at all?

To uncover the complex historical and astronomical significance of Feb. 29, The Daily sat down with faculty members at the College of Arts and Sciences. Read on to learn more about leap day from Paul Iversen, associate professor and chair of the Department of Classics and director of undergraduate studies, Glenn Starkman, Distinguished University Professor and vice-chair of the Department of Physics, and Stacy McGaugh, professor and director of the Department of Astronomy.

1. Leap day is a matter of timekeeping.

Earth orbits the sun every 365 days, six hours and nine minutes—slightly more than our typical 365-day calendar.

“One tropical year is not evenly divisible by an integer number of synodic days, so the remainder accumulates,” explained Stacy McGaugh. “If we want spring to arrive at the same seasonal time every year, we have to occasionally tweak the length of the year to cope with the remainder.”

2. The leap day doesn’t always occur every four years.

“We don’t quite need a leap year every fourth year—a little less will do, which is why the Gregorian calendar omits Feb. 29 on years divisible by 100 (like 1900), except (as in 2000) in years divisible by 400,” said Glenn Starkman. “That is good enough that the Gregorian calendar will take over 3,000 years to be off by a day.”

The next time we will skip adding leap day to our calendars will be in the year 2100.

3. The Gregorian calendar addressed Roman priests’ miscalculations.

Though the concept of leap day was formally introduced in Rome by Julius Caesar in 46 BCE, the pontifices (the priests at Rome also in charge of the calendar) misunderstood the rule and counted the number four inclusively rather than exclusively; as a result they inserted, or intercalated, an extra day every third year. 

“Our sources state the pontifical error persisted for 36 years, in which 12 days were intercalated rather than the proper nine,” said Paul Iversen. “The easiest way to reconcile all the available evidence is to argue that the last of these 12 intercalary years happened in 9 BCE, the intercalation was omitted for 12 years, and then a proper intercalary day was inserted in 8 CE, and from then on it has been inserted every fourth year, as it was meant to be.”

In 1582, Pope Gregory XIII reformed the Julian calendar to correct this error and named it after himself, thereby creating the Gregorian calendar that most people around the world use today.

4. The Easter holiday largely drove the decision for change.

Before the introduction of the Gregorian calendar, the spring equinox would change—and as a result, so would Easter celebrations.

“The astronomical spring equinox was an important marker in determining the date of Easter, which is a moveable feast,” said Iversen. “But it’s supposed to be in the spring—and it was getting later and later in the spring, and if nothing was done, eventually it would fall in the summer.”

5. Not all institutions follow the Gregorian calendar.

Most Orthodox Churches still use the Julian calendar for dates such as Christmas and Easter, Iversen explained, and Greek and Russian Orthodox dates for these Christian holidays vary from those of the Roman Catholic Church and Protestant Churches. 

“Recently, this became an issue in Ukraine, where they had been following the Julian Calendar as the Russian Orthodox Church still does, but they switched to following the Gregorian calendar or another calendar known as the Revised Julian Calendar to tie themselves more closely to the West,” said Iversen.

6. Leap days had medieval significance.

“Since the concept of leap days and intercalary months were somewhat late additions to calendars, there tends to be nothing special about them in antiquity—apart from the fact that both intercalary days and years were often manipulated for political purposes by some politicians,” said Iversen. “But by the Medieval period, leap days were associated with things such as marriage proposals or the reversing of gender roles in some cultures.”

7. There are better calendars for keeping track of time.

While calculations are good enough that the Gregorian calendar will take over 3000 years to be off by a day, there are more precise calendars that exist.

“The Maya calendar was off by one day in about 6,000 years, and the Persian calendar is off by only one day in 110,000 years,” said Starkman.