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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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Ahead of America250, Villanova Historian Reveals How Independence Hall Almost Didn't Survive featured image

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100 Years After the "Launch" of Aerospace Industry, Villanova Faculty Continue to Innovate the Sector featured image

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In 1926, Robert H. Goddard launched the world’s first liquid-fueled rocket in Auburn, Mass. Goddard’s 10-foot-tall rocket was airborne for just 2.5 seconds, reaching speeds of 60 miles per hour before landing 184 feet away from the launch site. A century later, the aerospace industry is booming, with new technology and missions making headlines every day—some with incredible success, and others encountering challenges that send scientists back to the drawing board. In February 2026, NASA delayed the launch of Artemis II—its next mission to the moon—citing issues with helium flow in the rocket’s systems. By April, the mission was on track again, and Artemis II completed the first crewed flight to the moon in more than half a century. Crew members for the upcoming Artemis III mission were also recently announced, as well as a timeline and overview for Artemis IV, the first planned crewed mission to the lunar South Pole in 2028. 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Villanova Researcher Authors Comprehensive Look at Large Outdoor Gatherings and Severe Weather Risk featured image

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If Benjamin Franklin had modern climatological data to consult, he may have scheduled his famous kite and key experiment for July of 1753 in Philadelphia, instead of a month sooner. July, after all, is Philadelphia’s most active month for lightning strikes, noted Stephen Strader, PhD, associate professor of Geography and the Environment and severe weather geographer at Villanova University. Lightning has been on Dr. Strader’s mind recently. In Spring 2025, he and a colleague published a study in Weather, Climate, and Society where they developed a metric to comprehensively rank more than 475 “large outdoor gatherings” in the U.S. in terms of greatest lightning risk, weak (EF0+) tornadoes and strong or significant (EF2+) tornadoes. The rankings factored in time of year, frequency of events, attendance and capacity, which was then compared to tornado/lightning climatology data for the venue location. 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In 2011, a spectator at Pocono Raceway was struck and killed by lightning in the parking lot after the race was halted due to weather. One year later, a sudden strong wind from an approaching storm collapsed a grandstand at the Indiana State Fair, causing seven fatalities and dozens of injuries. The objective of Dr. Strader’s research was to find what venues are most likely, comparatively, to experience these extremely rare worst-case scenario exposures. By doing so, he hopes that it will help venues and organizers to understand how they can plan, execute or update their preparedness and protocol, so that their impacts are less deadly when they do occur. “The idea isn’t to scare people,” he said. “Most of these venues and entities know that they're at risk for inclement weather. My hope is that those who do have a plan in place re-evaluate them, and think, based on the climatology, how they can adapt. For those who don’t, the goal is to make them aware of their risk so they can begin to plan. “There's a mental side to this too. Every year that goes by where there's not an issue or incident, people get increasingly relaxed. It’s a reminder to stay vigilant.” Many of the large outdoor gatherings that appear high in the research rankings do have robust protocols and infrastructure. For example, amusement parks score very high for lightning risk because they are open almost every day of the year to huge crowds, and many are located in states that experience frequent severe weather, like Florida and Texas. “But places like Disney World have comprehensive preparedness plans—they have people hired to understand and be aware of these threats,” Dr. Strader said. “Attendees have ample places to seek shelter.” The same is true, to varying extents with sporting venues, especially major professional sports stadiums. Even though gatherings held there may rank higher for these weather events, the chances of a catastrophe are lessened because of preparedness, protocol and infrastructure. Where Dr. Strader is more concerned is with gatherings like parades and fairs, especially when they last for multiple days and/or draw large crowds. “A state fair tends to be more ‘mom and pop’ at times, and they lack permanent infrastructure,” he said. “Vendors set up in temporary structures and a lot of times there are no places for attendees to seek shelter when thunderstorms threaten. You're also dealing with a transient population—people who don't know the area, and don’t know where to go when severe weather hits. “Parades are kind of a free for all in that you can come and go as you please. This is one of the reasons venues take tickets—so they understand their capacity to deal with an emergency. But if you have a place that doesn't do that, it can be tricky.” How Philadelphia’s 2026 Events Rank So, what exactly did Dr. Strader find when factoring in this summer’s events in and near the City of Brotherly Love, and what does it mean? In terms of lightning risk, several events and venues rank in the 80th percentile and greater for potential exposure compared to all other documented U.S. large outdoor gatherings. Citizens Bank Park, the host of the MLB All-Star Game in July, ranks in the 95th percentile. America 250 celebrations on July 4 rank in the 94th percentile, while FIFA World Cup events at Lincoln Financial Field rank 87th in June and 94th in July. For tornado threats to all nationwide events, Citizens Bank Park tops all local functions in July at the 96th percentile. The America 250 celebration ranks in the 95th, FIFA World Cup the 94th and MLB All-Star Game the 78th percentile for tornado exposure in that same month. According to Dr. Strader, lightning is often the bigger concern because it is far less predictable than tornadoes. While events are frequently canceled ahead of a credible tornado threat, they are less likely to be canceled due to the possibility of lightning. “Although lightning [strikes] and tornadoes are not very common in southeastern Pennsylvania, the large number of people gathered for these events sets up a ‘what if’ situation where tens of thousands of individuals could be threatened by severe weather,” he said. “Based on these findings, venue operators and governing bodies should continue to develop risk reduction strategies aimed at improving lightning and tornado safety, especially as lightning and tornado hazard threats evolve over time and across geographic space.” It is not only operators and organizers who should be prepared. Patrons, he says, can also devise and enact their own personal preparedness plan in the event of sudden severe weather. Teaching children simple sayings such as “when thunder roars go indoors” and scoping out potential places for shelter can go a long way in the event of severe weather—especially at come-and-go, transient events like America 250 festivities. “And if you see approaching clouds on July 4, maybe find another way to pay homage to Ben Franklin, and pull down your kite.”

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