Q&A: Astronomy Professor Explains ‘Leap Year’ and Jumps Into ‘Leap Second’ Debate

Ken Seidelmann doesn’t know anybody who was born on leap day, but one of his biggest career accomplishments occurred on one.

On Feb. 29, 1976, Seidelmann was promoted to department head at the United States Naval Observatory.

“My boss had great fun joking about the fact there wouldn’t be a 29^th the next year – so I wouldn’t be getting a raise,” Seidelmann recalled with a laugh.

As fate would have it, Seidelmann went on to an illustrious career that involved many of the things that contribute to the existence of leap year.

On the eve of another leap day, UVA Today caught up with the longtime researcher in the University of Virginia’s astronomy department for the low-down on the phenomenon.

Q. Cutting right to the chase, why do we have leap years?

A. In general, for various technical reasons, you want the calendar to fit the actual motions of the Earth and the moon. It’s unfortunate that both the motions of the Earth and the moon aren’t convenient. They don’t calculate to an even number of days.

At one point, I said to someone, “I could have designed a better solar system that wouldn’t have all these complications,” and the person’s response was, “If you did, you wouldn’t have a job.” They had a good point.

Basically, the motions of the moon, the sun and the Earth are the most well-observed things. A long time ago, people started determining time periods based on those motions. They fairly early on figured out that the orbits of the period of the moon aren’t an even multiple that comes out to be equal to the motion of the Earth – so they developed a lunar calendar. They had to put in lunar months periodically to make the year come out equal to the solar motions.

Generally, things were sort of erratic until Julius Caesar came up with this system for a leap day or leap year, to make every fourth year have an extra day. That was used from 45 B.C. until the 1500s. At that point, things were off by about 10 days. That’s when Pope Gregory XIII set up a committee to do various studies, and they realized that they should eliminate leap year from three out of four century years.

And then that’s when they made the centurial years have to be divided equally by 400. So 1600 and 2000 are leap years, while 1700, 1800, 1900 and 2100 are not. The system works. Many countries have retained their country calendars for religion or other reasons, but basically everyone for business, civil and international reasons now use the Gregorian calendar.

Q. Can you discuss a little more how people kept track of time before the Julian calendar?

A. To a large extent, the calendars in those days were driven by farming requirements. You had an annual cycle to deal with as far as planting, harvesting and flooding of the Nile, these sort of things.

Generally, they observed the rising of stars and the patterns that took place. They knew that when certain constellations reappeared, it was the time of year to do planting and so forth. They basically had a farming-driven calendar. This was for the Greeks, the Babylonians, the different countries.

Q. Is there any research that suggests the motions of the Earth and moon will ever change – maybe not in our lifetimes – and eliminate the need for a leap year? And what kind of factors could cause that? Is that something you look at in your work?

A. Basically, you’re talking about determining the ephemeris [the trajectory of naturally occurring astronomical objects and artificial satellites in the sky] for the sun, moon and planets. The motions are all affected by the other bodies in the solar system. The ephemeris of Mars, for instance, is affected by minor planets coming closer to it that affect its orbit. So the orbit of Mars is the one that needs refinement most frequently – in part because of the space missions going to Mars.

But basically the variations are pretty well known for long periods of time. We’re talking millions of years. So we know how much they’re going to change with time. At this point, there’s no need to change the current leap year designation and practice.

Q. If you could get rid out of leap year, how would you do it?

A. I don’t see any way we can get rid of it. We can’t change what the Earth’s orbit is. We’re not going to change the length of a day significantly. So we basically have to live with what we have and deal with it the best way you can. As far as I can tell, what we’re doing now is the best we can do.

The issue these days is the “leap second” issue and what we do to correct the time scale to keep it in step with the rotation of the Earth, but that’s a thorny subject.

Q. What is the leap second issue?

A. Generally, the establishment of Coordinated Universal Time, or UTC, is that it should stay within 9/10 of a second of the time based on the rotation of the Earth. The Earth’s rotation varies unpredictably as we observe, so that, at times, you have to introduce a leap second to stay within that 9/10 limit.

This can’t be predicted long in advance. At this point, maybe we can do it a little over a year in advance. But it’s not like you know it’s going to happen at a set time. It generally happens at the end of the year. Some people object to basically having this unpredictable change having to be introduced. So people have proposed eliminating leap seconds and just let the time scale differ from the actual rotation of the Earth so that, with time, the sun being directly overhead at noon, is off, to a certain extent.

Getting rid of leap seconds was proposed back in 2007 or 2008 and came up for a vote in the International Telecommunications Union in 2012 and then in 2015 and was turned down. It’s not to be considered again until 2023. It’s a question of 194 countries voting whether to do this or not do it.

There are issues that concern the effects of this on people’s everyday lives, astronomical observations, pointing at satellites, and the Islamic and Jewish religions. It is an ongoing situation. In general, if the leap second were eliminated, the difference between UTC and Earth rotation time would continue to increase, but nothing would need to be done in the lifetime of people who are alive today – so they say let future generations figure it out.

Google has a system where they make the one-second change not at one time, but they change the length of a second for a period ahead of the leap second, so that by the time the leap second has taken place, they’ve already taken care of it. That’s fine for an internal organization, but it doesn’t work with international comparisons of time.

Q. Where do you stand on this?

A. My position is to leave the system as it is. People who need a time scale without a leap second have one available because the Global Positioning System doesn’t introduce a leap second in their time scale.

The objection to this, in part, is that the GPS time scale is the U.S. military time scale and some countries don’t want to use that as a basis. But that argument is somewhat going away because the Galileo satellite system created by the European Union is going to use the same time scale. That will be more and more widely available. But there are people who don’t like having another time scale available.