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Leap Years and the Connection Between Astronomy and Our Lives

Feb 23, 2024

5 min

Most of us know February 29 as a whimsical anomaly—nothing more than a chance to tease our friends or colleagues born on this day as technically being a quarter of their purported age.


But how often do we think about the origins of the day as we now know it? Or about the near-universal implementation of this specific way to keep track of time? Do we ever consider the impact a leap year could have on everyday life?


Frank Maloney, PhD, an associate professor of astronomy and astrophysics at Villanova University, has been teaching for nearly 12 leap years (47 years). He is an expert in timekeeping and calendaring, calling them a “fundamental connection to our lives, ruled by the motion of objects in the sky,” because “everyone has to agree what day and time it is.”


Dr. Maloney currently teaches a course called “Earth: Our Habitable World,” where he discusses this and other connections between astronomy and people’s lives.


In the case of leap years, the astronomical phenomenon from which they originate—the Earth’s time to orbit the sun—is a very important one to accurately track.


“You want your calendar to keep pace with the seasons,” Dr. Maloney explained. “There are all sorts of ways of measuring the pace of the Earth around the sun, but the way that [also] keeps pace with seasons is called the tropical year, and unfortunately, there’s not an integer number of days in that year. We can’t ignore it, because after the first year you’re off by a quarter of a day and after four years off by a full day, and so on.”


Ancient civilizations were aware there were slightly more than 365 days in a solar year but didn’t know exactly how much more. Gradually, the seasons would become unsynchronized with the calendar, and those various civilizations added days back in at random times to realign.


“In those days, it might be possible to leave one area in April, and arrive [somewhere else] the previous December,” Dr. Maloney joked.


The concept of a leap year began with the Roman Empire’s implementation of Julius Caesar’s namesake calendar on January 1, 45 B.C.E., at his behest. The Julian calendar was a solar calendar, which consisted of a 365-day year, and a 366-day leap year every four years, without exception. It was often added as a duplicate day in the middle of February.


“But a year is not exactly 365 and a quarter days. It’s a little bit less,” Dr. Maloney explained. “By the Middle Ages, it was 10 or so days out of whack with the tropical year. Astronomers would have seen that very easily... but the reason to change it was not there.”


Not until the late 16th century, that is. And the reason it did change was because Easter had moved out of line with the vernal equinox.


“Nearly all calendars have a mystical, religious or theological component,” Dr. Maloney said. “In the Roman Church, Easter is reckoned as the first Sunday after the first full moon on or after the vernal equinox, or first day of spring.”


In order to have Easter fall back in line with the equinox, Pope Gregory XIII issued a papal bull in 1582, which declared a year to be a more accurate 365 days, 5 hours and 49 minutes in length (roughly).


What that meant for leap years was that, instead of every four years without exception, they would now occur every four years except on century marks, unless that century mark was divisible by 400. For example, 1900 was not a leap year, but 2000 was. The years 2100, 2200 and 2300 will not be leap years.


The global switch to a new calendar was not easy or done in haste. To enact the initial calendar change, 1582 went from October 4 to October 15 to eliminate extra accumulated days. Catholic countries mostly followed suit soon after, but many others resisted, as citizens feared it was a political trick. It took centuries to get to the near-universal use of the Gregorian calendar we have today.


Great Britain and other Commonwealth nations did not adopt the Gregorian calendar until 1752. An individual such as George Washington could have been considered to be born on one day in the Julian calendar and have a different birthday in the Gregorian calendar. In the American colonies, September 1752 skipped to the 14th day of the month from the second.


The most recent country to switch from the Julian to Gregorian calendar was Greece in 1923. By then, the calendar was roughly two weeks off from the tropical year. In the early 1900s, when globalization was commencing, this was a big deal.


“You could get in an airplane and fly someplace, and not even know what day you’d be landing. According to the calendar, it’d be time travel,” Dr. Maloney said.


Saudi Arabia still used a few elements of the Islamic calendar for fiscal purposes until 2016, and Afghanistan, Iran, Nepal and Ethiopia are the only countries in the world that do not officially use the Gregorian calendar currently.


So, what does all this mean for people today? For starters, historians and genealogists must be careful when studying historical dates and events. For example, a country may have still been using the Julian calendar during a particular time period, or perhaps an event might have occurred during the time days were skipped to make the switch from the Julian to the Gregorian calendars.


“If an infant were born [in the American colonies] on the second of September 1752, for example, and died on the 14th, they were not really 12 days old,” Dr. Maloney said. “Or if a war began in a country one day but started on a different day in a different country, it causes confusion.”


Leap years and other adjustments to timekeeping can also cause a plethora of computing and software issues, impacting multiple industries. This is especially true in the digital age where time-stamping is so ubiquitous. Case in point, on occasion, we actually have to add a leap second to time to account for the slowing of Earth’s rotation. These leap seconds are added after 11:59:59 on either December 31 or June 30, when needed.


“There’s a great deal of controversy about this particular practice,” Dr. Maloney said. “It really confounds software. A jet airplane, for example, can travel a fairly long distance in one second. Time has to be kept now to fractions of seconds, [even for things like] lawsuits and insurance policies. Timekeeping is a very important task for astronomers.”


It seems those astronomers have it figured out... for now. Even the Gregorian calendar will eventually need an adjustment, as its margin of error is about 27 seconds per year. That means every 3,236 years—so sometime in the early 4800s—an additional extra day will need to be added somewhere to correct it.


Luckily, we have some time to plan ahead.


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