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

Earth’s actual orbit around the sun takes six hours and nine minutes more than the strict 365 days that our regular scheduling mechanisms prefer. To sync the natural world to our calendars, we add a leap day every four years, on Feb. 29 — today.

This all seems like mere chronological housekeeping, but there are other concerns at play, according to Judah Levine. He’s is the head of the Network Synchronization Project in the Time and Frequency Division at the National Institute of Standards and Technology, or NIST, in Boulder, Colo. He is one of dozens of time experts around the world who work on coordinating the world’s clocks so they are in sync not only with one another but with the natural world. He sat down with The New York Times to discuss what more 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 guy who started the initial leap day business, in something like 46 B.C.

Did he just declare a leap day?

I think he just said, “Every four years.” He was Caesar — he didn’t have to take a vote. Although he proclaimed it, it didn’t happen until about 30 or 40 years later.

The goal was to make the spring equinox happen in the spring, and the problem was that the equinox was bumping into winter; that was not cool.

The spring equinox in many societies was associated with a harvest festival; in order to have a harvest festival, you have to have a harvest. Passover, roughly in the time of Jesus, was a harvest festival, so Passover had to occur in the spring; it had to be loosely hooked into the equinox. The same thing is true of the of the Christian Easter.

But Caesar came before that.

Julius Caesar must have used a similar argument — that when we don’t do the leap day, those harvest holidays get pushed closer to winter. He may have been responding to a Roman requirement.

Then, in the Christian environment, the leap day produced a problem relative to Christmas, because Easter was moving back toward Christmas.

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

Right. The larger problem is that Christmas is a defined date, but Easter is a movable feast. And Passover, loosely speaking, is similar — it’s got to be a springtime thing.

The Jewish calendar doesn’t 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 the holidays. There’s a great discussion in the Talmud about how you decide when to do the leap month, and the root of the discussion is that Passover has got to be a harvest festival.

Then came the Gregorian fix. What was that?

It was made by Pope Gregory to correct the Julius Caesar rule, which was OK but not exactly right. From the time of Julius Caesar to the time of Pope Gregory was, like, 15 centuries. At that point, the equinox was at least 10 days off target — a little less than a day per century. Easter was now moving into the summer. Pope Gregory dropped those 10 days from the calendar, and he removed three leap days every 400 years from the system. That made a small adjustment so that the problem wouldn’t recur.

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

Was it enough to keep Passover and Easter and harvest celebrations in the desired spot?

At least for several thousand years.

Because small time differences still accumulate?

I’m sure there’s a round-off now, and there will be a problem in 10 centuries, but loosely speaking, the holidays occur at the right time and will for the foreseeable future.

What’s important to understand nowadays about Leap Day?

That the fundamental reason for it is to keep the seasons and the calendar linked together. That’s why leap days are there.

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

It could. But once you fix the length of the year, it doesn’t matter how you fix it. Once you’ve synchronized the astronomical year with the calendar, you could do it any way and it would be equally good.

We could have leaped in winter, summer or autumn?

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



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