The last leap year was 2016, and the next one is 2020! Leap days are extra days added to the calendar to help synchronize it with Earth’s orbit around the sun and the actual passing of the seasons. Why do we need them? Blame Earth’s orbit around the sun, which takes approximately 365.25 days. It’s that .25 that creates the need for a leap year every four years.
During non-leap years, aka common years – like 2019 – the calendar doesn’t take into account the extra quarter of a day actually required by Earth to complete a single orbit around the sun. In essence, the calendar year, which is a human artifact, is faster than the actual solar year, or year as defined by our planet’s motion through space.
Over time and without correction, the calendar year would drift away from the solar year and the drift would add up quickly. For example, without correction the calendar year would be off by about one day after four years. It’d be off by about 25 days after 100 years. You can see that, if even more time were to pass without the leap year as a calendar correction, eventually February would be a summer month in the Northern Hemisphere.
During leap years, a leap day is added to the calendar to slow down and synchronize the calendar year with the seasons. Leap days were first added to the Julian Calendar in 46 B.C. by Julius Caesar at the advice of Sosigenes, an Alexandrian astronomer.
Celebrating the leap year? Take a moment to thank Christopher Clavius (1538-1612). This German mathematician and astronomer figured out how and where to place them in the Gregorian calendar. Image via Wikimedia Commons.
In 1582, Pope Gregory XIII revised the Julian calendar by creating the Gregorian calendar with the assistance of Christopher Clavius, a German mathematician and astronomer. The Gregorian calendar further stated that leap days should not be added in years ending in “00” unless that year is also divisible by 400. This additional correction was added to stabilize the calendar over a period of thousands of years and was necessary because solar years are actually slightly less than 365.25 days. In fact, a solar year occurs over a period of 365.2422 days.
Hence, according to the rules set forth in the Gregorian calendar leap years have occurred or will occur during the following years: