The origins of the leap year: The celestial heavens are a complicated thing


by Jan McQuay

MANITOULIN – If the ancients could understand time, surely with our sophisticated education systems we should be able to understand it too. It’s complicated. A day is sun up to sun up, no matter how long there is sunlight or darkness. But planet Earth doesn’t circle the sun in an even number of days, it takes 365.256 days to get back to the starting point, so a year is really 365.256 days.

The moon’s orbit is even more puzzling. It takes the moon 27.3 days to orbit Earth, but the sunlight that the moon reflects varies as the Earth and moon travel around the sun so that it appears to be 29.5 days. Lunar or solar, nothing lines up evenly with the length of a day. Little wonder, then. that the ancients developed many different calendars.

The Sumerians’ writing system put them at the forefront of known history of calendars as early as the fifth century B.C. Each state had its own calendar, but it seems they were all based on the “lunisolar” system that tried to account for both the lunar and solar cycles. The beginning of the year was spring, when the sun rose exactly due east. 

Months were based on the orbit of the moon, and the beginning of each month was the new moon. “New moon” is a strange phrase now because to us it signifies no moon visible at all, more of an “empty moon,” but to the Sumerians a razor-thin crescent was the beginning of the lunar cycle, hence “new moon,” the beginning of a month. Since the lunar cycle was 29.5 days, the Sumerians alternated between months of 29 days and 30 days. But 12 lunar months only take 354.36 days. If one year starts off on a new moon perfectly, then at the end of 365 days they are already 11 days into the next lunar cycle. So, every few years, by royal decree, an extra month was added into the calendar, making 13 months in that year. You could perhaps call it a “leap month.”

The Egyptians used both sundials and stars, dividing a day up into daytime and nighttime. They divided both equally by 12, but with length of the hours changed with the seasons. In winter, a nighttime hour was longer than in the summer. Their day was like our day, except the daylight and nighttime hours grew and shrank depending on the season.

The ancient Egyptian solar calendar had 365 days, but their weeks were 10 days long, months were 30 days long, seasons were 120 days long and a year had three seasons, making a year 360 days. That wasn’t right so they added five holy days. But even at 365 days to a year, their year was far enough off the mark that over the years the weeks, months and seasons shifted. It took 1,460 years for the calendar to get back to where it started. 

The early Roman calendar had only 10 months, each month for one lunar cycle. Of course, that didn’t work with the seasons, so they had an undefined “winter period.” The calendar just started up again in spring. Along the way they fitted in two more months, January and February, but even that only made 355 days, so high-ranking priests introduced an extra month from time to time. You could perhaps also call those “leap months.” They became more or less a matter of politics to lengthen or shorten a magistrate’s term of office. 

Julius Caesar finally ended the confusion in 46 B.C. After his trip to Egypt, where he learned about the Egyptian calendar system, he added extra days to the old Roman calendar, for a total of 10 days. This made a year of 365 days, still not quite enough. So once every four years he added a day to the month of February, and every fourth year became a “leap year.”

But why call it “leap?” We’re not really leaping, we’re just adding an extra day. The name has to do with the days of the week. January 1 falls on the same day of the week every seventh year. For example, if January 2018 begins on a Monday, January 2019 begins on a Tuesday and January 2020 begins on a Wednesday. But January 2021 starts on a Friday, not a Thursday, so we “leaped” past Thursday.

Even that doesn’t quite work out right, so in 1582 Pope Gregory XIII changed the cycle of leap years according to whether the year is exactly divisible by 100 or 400. It turns out the celestial heavens were complicated for the ancients too.  (We consider a leap year to be precisely divisible by four.)