Leap day, or how to keep time on track

Monday, February 29, was a leap day, an intercalary day: a day inserted into the calendar.

Leap day represents a fudge factor to reconcile the fact that the year—or the time for the earth to revolve around the sun—and the day—the time that it takes the earth revolve around its axis—bear no connection.

As we all learned in school, the solar year is longer than 365 days, by 0.2422 days, such that almost every four years we add a day. The ‘almost’ comes from the fact that the extra time is less than a quarter of a day.

As for months, the moon orbits the earth relative to the sun in 29.5 days in what is called a synodic month.

Days just don’t fit neatly into years or months. So arbitrary conventions are required. Like months of twenty-eight, thirty, and thirty-one days.

Civilizations need calendars

For the bulk of human history, we needed calendars to know when to plant and when to harvest. We needed calendars to know when to attend fairs to trade our goods, and when to go to holy sites to partake in rituals.

Given the regularity of the moon and the sun, it is only natural that mankind looked to the heavens to define time.

Think a leap day is bad, imagine two months…

Because days don’t fit into months or years evenly, all calendars require a ‘fix’ to avoid migrating away from the sun. For some it is a day every four years, for others a month, for others five days at the end of the year.

When Julius Caesar tried to sort out the drift associated with a 365-day year, he added two months to the year. 46BCE had 445 days and was known as annus confusionis, the Year of Confusion.

The LA Times calculated that if we’d ignored leap year corrections as made since 46BCE, we would currently find ourselves in mid-July.

Defining time

The quest to improve timekeeping is one of mankind’s oldest pursuits, and has essentially been a search to find better oscillators. — NIST Primary Frequency Standards and the Realization of the SI Second.

Earth’s rotation on its axis—the succession of night and day—is an oscillator that is hard to ignore, so naturally the second was originally defined as a fraction of the day. Of course day length varies with latitude and season. It is even affected by large weather patterns.

The increased temperatures and shifts in atmospheric pressure associated with El Niño have been observed to slow the speed of rotation by 0.05 to 0.1 milliseconds.

Day length is also over time increasing due to friction from the tides. Global warming will also have a measurable impact.

The period of revolution around the sun is more stable than day length. In 1956, the standard second was defined as 1/31 556 925.9747 of the tropical year 1900. But this is impractical for real-time activities, such as synchronizing satellites.

Enter the atomic clock 

Instead of looking out to the cosmos to define time, the current standard for the second comes from the oscillation resulting from electrons shifting levels within an atom of cesium: 9,192,631,770 periods to be precise.

The year is now defined in function of the atomic clock. We have gone from peering at the heavens to looking inside an atom.

But the historical and cultural weight of astronomical timekeeping is so great that the two are kept in sync. And thus we have leap seconds, during which the universal time defined by cesium pauses for solar time to catch up.

Since 1972, 26 leap seconds have been introduced.

Why are they called leap days, or leap seconds, when nothing is being skipped?

Notwithstanding the fabulous photos of leaping animals on from National Geographic, a leap day is an extra day introduced into our calendars. There is no jumping involved.

Apparently the term has been in use since at least the 13th C. It is thought to refer to how calendar dates move from one day of the week to the next day on the following year. But on leap years, those that fall after February 29th move two days.

How did you use your extra day this year?

 There is little fuss and fanfare around the intercalary day, which I think is a mistake.

A leap day calls attention to the arbitrary nature of calendars. It is a clerical correction to a solar system where planetary rotation and revolution aren’t neatly linked.

Wouldn’t it make sense to use leap day as an actual extra day, a gift from the heavens to take the time to look above or deep inside?

leap day

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