February 29th. You may notice that this date didn’t appear on last year’s calendar, or the year before that, or the year before that. That is because leap day happens only once approximately every four years—and 2020 is a leap year!
Dr. Kevin Krisciunas, associate professor in the Department of Physics and Astronomy in the Texas A&M University College of Science, explained that the necessity for leap years comes from the “messiness” of our solar system.
Leap days are extra days added to the calendar year to synchronize it with Earth’s solar orbit. Due to this extra day, leap years have 366 days. Without leap years, the calendar year would continue to drift away from the solar year over time—meaning that eventually the winter months would be in the summer and vice versa.
“Leap years are used as a way to reconcile the calendar,” Krisciunas said. “Adding an extra day every four years keeps our calendar aligned correctly with the astronomical seasons.”
Why Leap Years?
Most years, the calendar is made up of 365 days. But Earth’s yearly trip around the sun takes a little longer than that (365.2422 days, to be exact). This small difference is enough to cause the calendar to gradually get out of sync with the seasons.
Julius Caesar, leader of the Roman Empire, was the first person who confronted this problem and sought to create a standardized calendar that reflected the seasons. “After working with astronomers, Julius Caesar discovered a solution he believed would fix the inconsistency in the calendar system,” Krisciunas said.
In 46 B.C., Caesar introduced the Julian Calendar. This new calendar was constructed around months of the year to create the 365-day calendar year. On the advice of an astronomer named Sosigenes, Caesar also added that every four years, an extra day would be added to Februarius (February) to account for the slightly longer length of the solar year.
Although Julius Caesar was the one who introduced the idea, it was later discovered that his math wasn’t exactly right. “By the late 1500s, the Julian Calendar had contributed to a 10-day difference in the calendar,” Krisciunas said. “Caesar had the right idea, but his calendar continued to move noticeably out of sync, prompting a second calendar reform in 1582 from Pope Gregory XIII.”
Taking the new calculations into account, Pope Gregory established a new calendar that included the “century rule.” The century rule stated that instead of consistently happening every four years, leap days would not be added in years ending in “00” unless the year was also divisible by four. (So, while the year 2000 was a leap year, the years 1700, 1800 and 1900 were not.) This rule and other additional corrections created the calendar system we currently use today: the Gregorian Calendar.
“The Gregorian Calendar’s original goal was to restore many days of religious significance back to their original date, including returning the vernal equinox to March 21,” Krisciunas concluded. “Because of this, some countries rejected the new system due to its ties to the pope. It wasn’t until the mid-1700s when almost all countries officially adopted the Gregorian Calendar.”
Dr. Krisciunas joined the Texas A&M Department of Physics and Astronomy faculty in 2006 as a member of the George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy. The physics department has greatly benefited from philanthropic contributions made by the late Cynthia and George P. Mitchell ’40 and their associated family foundation. The Mitchells’ total commitments to Texas A&M exceed $95 million, including more than $88 million that helped build two new physics buildings and establish a world-class astronomy program.