What's With the Extra Day? A CofC Professor Explains Leap Year

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Physics instructor Ana Uribe explains the science behind leap year and why we need an extra day every four years – including this year.

person jumping from Earth to the Moon

We all probably first heard about the leap year from the nursery rhyme “Thirty Days Has September,” but few of us likely ever stopped to wonder just why the month of February has 29 days every four years – well, most of the time, at least.

The College Today sat down with physics and astronomy instructor Ana Uribe – a former research fellow at the Max Planck Institute for Astronomy in Heidelberg, Germany, and at the University of Chicago’s Department of Astronomy and Astrophysics – to find out more about the science behind leap year. And, with research interests in the formation and evolution of planetary systems and extrasolar planets, Uribe also weighs in on whether we’ll need a leap year on Mars when we colonize it, too!

What exactly is a leap year, and why do we do it?

Our calendar year is based on the time it takes the Earth to complete one full orbit around the Sun. It turns out that it takes a little longer than 365 days for the Earth to complete a full turn around the Sun. The Earth actually orbits the Sun every 365 days plus about six hours. So in a non-leap year, after 365 days, the Earth would be slightly behind where it started relative to the Sun. After four years, the Earth is behind by one day of travel from the starting point. After about 100 years, if we don’t correct for this effect, the Earth would be one month behind and, for example, the summer solstice would happen in July instead of June. Our seasonal changes would be out of sync with the calendar. So, every four years, we add a day to the calendar to allow the Earth to “catch up.”

What’s the history behind it – who started it and when?

The idea of a leap day or a leap month appears in the calendars of even ancient civilizations as a way to deal with the fact that our solar year is not made up of a whole number of days. Our modern calendar is based on the Julian calendar, first proposed by Julius Caesar as a modification of the previous Roman calendar that at the time had run out of sync with the seasons partly due to mismanagement for political purposes. The Julian calendar added an extra day at the end of the year every four years. Over 1,000 years later, Pope Gregory XIII further updated the Julian calendar to account for the fact that a solar year is not exactly 365.25 days long, but actually 365.2425 days long.

Is it always every four years?

Not necessarily. To account for the exact fraction of time that has to be added to the calendar, there are additional conditions on the way the leap year is determined. A leap year must be also divisible by 4. In addition, if the year is divisible by 100, it is not a leap year unless it is also divisible by 400. Sounds complicated, but we do it all to keep the calendar in sync with the seasons over long periods of time. 

Is it always in February?

It has been in February since the implementation of the Gregorian calendar, which was ordered by the pope in the 1500s. However, historically there have been different ways of adding the leap day. At some point for example, it was added by doubling up a date. So, instead of adding February 29, the calendar contained two consecutive days dated February 24.   

Speaking of which, why does February only have 28 days typically?

This goes back to Roman times as well. The early Roman calendars had only 10 months in a year. Later on, two extra months were added to the calendar with February being made the last month of the year. Before the addition of February, each month had an odd number of days – either 29 or 31 – because even numbers were considered unlucky. But an odd number wouldn’t work for February if the year was to have the right number of days, so February got stuck with the “unlucky” number of days, to keep the year the correct length.

What are leap seconds, and when and why do we do those?

Every other year over the last 50 years, without us noticing, one extra second has been added to our clocks. In total, 27 leap seconds have been added since 1972. What are we synchronizing in this case? We are making sure that the length of the day measured by ultra-precise atomic clocks agrees with the length of the day measured by the rotation of the Earth. Since the speed of the rotation of the Earth fluctuates very slightly over time, those two measurements of time don’t always agree. But the leap second might be more trouble than it’s worth; the agencies in charge finally decided to scrap the leap second completely by 2035.

Who keeps track of all of this, officially?

Keeping track of the flow of time is a constant international effort that involves different bodies. The International Bureau of Weights and Measures, an intergovernmental organization based in France, is the main agency responsible for timekeeping.

Will we have to add a leap year on Mars when we colonize it?

Since one Martian year is also not made up of a whole number of Martian days, we would have to add leap years to our Martian calendar to keep it in sync with the seasons. We would also have to pick how many months to have in a Martian year. On Earth, our monthly periods are based on the orbit of the Moon. The two moons of Mars have such short orbits (both take under two days to go around the red planet) that we can’t use them to set the monthly cycle. Still, we might choose to have 12 Martian months in a year, as a nod to our Earthly origins and to remember where we came from.

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