Clocks and calendars are easy to use, even if they don’t keep up with the astronomical world.
Earth’s actual orbit around the sun takes six hours and nine minutes longer than the strict 365 days preferred by our regular scheduling mechanisms. To synchronize the natural world with our calendars, we add a leap day every four years on February 29 — today.
This all seems like simple chronological housekeeping, but there are other concerns at play, according to Judah Levine. He 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 to coordinate the world’s clocks so they are in sync not only with each other but also with the physical world. He sat down with The New York Times to discuss what’s most at stake on Leap Day.
This conversation has been edited and condensed for clarity.
So does this all start with Julius Caesar?
He was the guy who started the original jump business, around 46 BC
Did he just declare a leap year?
I think he just said, “Every four years.” He was Caesar — he didn’t have to get a vote. Although he proclaimed it, it didn’t happen until 30 or 40 years later.
The goal was to make the vernal equinox happen in spring, and the problem was that the equinox fell in winter. that was not nice.
The vernal equinox in many societies was associated with a harvest festival. to have a harvest festival, you must have a harvest. Passover, around the time of Jesus, was a harvest festival, so Passover had to be in the spring. it had to be loosely hooked at the equinox. The same applies to the Christian Easter.
But Caesar came before that.
Julius Caesar must have used a similar argument – that when we don’t do the leap year, these harvest holidays are pushed closer to winter. It may have been responding to a Roman demand.
Then, in the Christian environment, the leap year created a problem in relation to Christmas, because Easter moved back towards Christmas.
By definition, Easter falls on the Sunday after the first full moon after the vernal equinox.
Correctly. The biggest problem is that Christmas is a fixed date, but Easter is a movable holiday. And Easter, loosely speaking, is similar — it must be spring.
The Jewish calendar does not have a leap month, but it does have a leap month. Happens seven times in 19 years. It affects the Jewish calendar — it affects all holidays. There is a long debate in the Talmud about how you decide when to have a leap month, and the root of the debate is that Passover should be a harvest festival.
Then came the Gregorian fix. What was that?
It was made by Pope Gregory to correct Julius Caesar’s rule, which was okay but not quite right. From the time of Julius Caesar to the time of Pope Gregory was, approximately, 15 centuries. At that point, the equinox was at least 10 days off target—just under one day per century. Easter was now in the summer. Pope Gregory removed these 10 days from the calendar and removed three leap days every 400 years from the system. This made a small adjustment to prevent the problem from recurring.
The idea was to keep the equinox on March 21, plus or minus a day.
Was it enough to keep Passover and Passover and harvest festivals at the desired point?
At least for several thousand years.
Because small time differences still add up?
I’m sure there is a rounding now, and there will be a problem in 10 centuries, but loosely speaking, the break is at the right time and will be for the foreseeable future.
What is important to understand nowadays about Leap Day?
That the fundamental reason for this is to keep the seasons and the calendar linked together. That’s why there are bisects.
Why does this happen with the vernal equinox but not with the winter or summer solstice?
Could. But once you determine the length of the year, it doesn’t matter how you fix it. Once you sync the astronomical year with the calendar, you could do it either 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 held a special place in people’s hearts. It’s a special moment.
