Why Leap Day is actually about party planning

Clocks and calendars are useful even if they are out of step with the astronomical world.

Earth’s actual orbit around the sun takes six hours and nine minutes longer than the strict 365 days favored by our regular scheduling mechanisms. To sync the natural world with our calendars, we add a leap day every four years, on February 29 – today.

This all seems like just chronological housekeeping, but according to Judah Levine, there are other concerns at play. He is the head of the Network Synchronization Project in the Time and Frequency Division of the National Institute of Standards and Technology, or NIST, in Boulder, Colorado. He is one of dozens of time experts around the world who work to coordinate the world’s clocks. so they are not only synchronized with each other, but also with the natural world. He spoke with The New York Times to discuss what else is at stake on Leap Day.

This conversation has been edited and condensed for clarity.

So this all starts with Julius Caesar?

He was the man who started the first Leap Day activities in about 46 BC

Did he just declare a leap day?

I think he just said, “Every four years.” He was Caesar; he didn’t have to vote. Although he proclaimed it, it didn’t happen until about thirty to forty years later.

The goal was to have the vernal equinox occur in the spring, and the problem was that the equinox bumped into winter; that wasn’t cool.

The vernal equinox was associated with a harvest festival in many societies; To have a harvest festival, you must have a harvest. Passover, roughly in Jesus’ time, was a harvest festival, so Passover had to occur in the spring; it was to be hooked loosely into the equinox. The same applies to Christian Easter.

But Caesar came for that.

Julius Caesar must have made a similar argument: if we don’t do leap day, the harvest holidays will move closer to winter. He may have been responding to a Roman demand.

Then, in the Christian environment, leap day posed a problem with regard to Christmas, as Easter moved back toward Christmas.

By definition, Easter falls on the Sunday after the first full moon after the spring equinox.

Right. The bigger problem is that Christmas is a fixed date, but Easter is a movable holiday. And Passover is, loosely speaking, similar; it must be something spring-like.

The Jewish calendar does not have a leap day, but it does have a leap month. It happens seven times in 19 years. It affects the Jewish calendar – it affects all holidays. There is a big discussion in the Talmud about how to decide when to do the intercalary month, and the core of the discussion is that Passover should be a harvest festival.

Then came the Gregorian solution. What was that?

It was created by Pope Gregory to correct Julius Caesar’s rule, which was okay, but not entirely correct. From the time of Julius Caesar to the time of Pope Gregory there were about 15 centuries. At that time, the equinox was at least ten days off target – just under one day per century. Easter now entered summer. Pope Gregory removed those ten days from the calendar, and he removed three intercalary days from the system every four hundred years. A small adjustment was made so that the problem would not repeat itself.

The idea was to keep the equinox on March 21, plus or minus a day.

Was it enough to celebrate Passover and Easter and hold the harvest festivals in the desired location?

At least for several thousand years.

Because small time differences keep piling up?

I’m sure there is a resolution now, and there will be an issue ten centuries from now, but roughly speaking, the holidays are timely and will continue to be so for the foreseeable future.

What is important to understand about Leap Day today?

That the fundamental reason for this is to keep the seasons and the calendar connected. That’s why there are leap days.

Why does this happen during the spring equinox, but not the winter or summer solstice?

It could be. But once you’ve determined the length of the year, it doesn’t matter how you do it. Once you have synced the astronomical year with the calendar, you can do it any way and it would be just as good.

Could we have jumped in winter, summer or fall?

Yes. But spring is always exciting because it is a time of harvest and rebirth. It has always had a special place in people’s hearts. It is a special time.