Happy Leap Day (opens in new tab), world! Today (Feb. 29) marks a rare event in the calendar — an extra day that occurs each "Leap Year" to keep our calendar in check. Google is celebrating with a cute Google doodle.
Leap days happen every four years (opens in new tab) (with a few exceptions) and adds an extra day to the short month of February.
"Today’s Doodle is jumping for joy on Leap Day, the 29th day of February that only occurs about every four years, to keep our calendars in alignment with the Earth and sun," Google representatives said in a statement (opens in new tab). "We HOP you have a good one—Happy Leap Day!"
Leap days are an essential part of keeping the Gregorian calendar (opens in new tab) — the calendar that most of the world uses to keep time — in alignment with the Earth's motion around the sun. Our calendar is normally 365 days long under the Gregorian scheme, but the actual year of the Earth is 365.25 days.
Therefore, adding an extra day every four years to create a 366-day year keeps our calendar in sync with the seasons.
"Every culture has to make choices on how they organize its calendar," Kenneth Coles, a geoscientist at Indiana University of Pennsylvania, said in a statement.
"We ignore the cycles of the moon, understanding that they can come anytime in the calendar – but we add the 'leap day' every four years to keep us in sync with the Earth’s revolution around the sun, which we call a solar year. There's no perfect solution, but it's the one we’ve developed."
If we didn't put it in a leap year, what's the worst that could happen? To answer that question, we need to journey back 2,000 years to the end of the Roman Republic. The Romans had a few calendar versions over the years; at the turn of the millennium (opens in new tab), however, they had been using a 355-day year for a few centuries.
All seemed to go well for 700 years, because high-ranking state priests (Pontifices) would add extra days in some years (a process called intercalation) to keep the calendar in sync with the seasons. But there were flaws in the system, as LiveScience explains (opens in new tab):
"Because a Roman magistrate's term of office corresponded with a calendar year, the power of intercalation was prone to abuse: the priests could lengthen a year in order to keep an ally in office, or shorten it when an opponent was in power. Also, since intercalations were often determined so close to their announcement, the average Roman citizen often did not know the date, particularly if they were some distance from the capital."
Julius Caesar was appointed dictator in 49 B.C. By 46 B.C., the calendar was three months out of alignment because there had only been five intercalary years preceding him, instead of the needed eight. Caesar (on the advice of mathematicians and scientists) added the needed extra months back in temporarily to put the calendar into alignment again. He also made permanent changes to the calendar fixing it at 365 days, with a "leap day" every four years.
The system worked well for 1,600 years, but it didn't perfectly match the seasons. By the late 16th century, the calendar was about 10 days out of alignment and threw the celebration of the Christian holiday Easter off from the actual spring equinox, when Easter is supposed to be celebrated. Pope Gregory XIII commissioned scholar Aloysius Liliusa to make changes – specifically, making century years that were divisble by 400 exempt (such as the year 2000) and cutting 10 days from the calendar in 1582.
The Gregorian calendar is still used today, with some modifications. In the mid-20th century, scientists changed the second to be based on the natural vibrations of the cesium atom, which were a feature of newly invented atomic clocks. This pulls the clock a little out of alignment with the Earth's naturally slowing rotation, so "leap seconds" are periodically added. The last leap second took place in December 2016 (opens in new tab).
- Alien planets would likely have leap years, too
- Why do we need Leap Days? (opens in new tab)
- The Gregorian calendar: Why we have Leap years and April Fools' Day (opens in new tab)
OFFER: Save at least 56% with our latest magazine deal! (opens in new tab)
All About Space magazine (opens in new tab) takes you on an awe-inspiring journey through our solar system and beyond, from the amazing technology and spacecraft that enables humanity to venture into orbit, to the complexities of space science.