How Leap Years are Calculated on the Gregorian Calendar
Have you ever wondered why an extra day is added to the end of February, but only once every few years? It’s all because of the Earth’s imperfect revolution around the sun. In fact, the way time is kept also plays a role. From the time a person is a child, he or she will be taught that a year is 365 days. The fact of the matter is that this isn’t exactly true.
In astronomical terms, a solar year, also known as a tropical year, is the mean time it takes the Earth to make one complete revolution around the sun. To be specific, a year is calculated by measuring how long it takes the sun to repeat being directly above the Equator following its last appearance over the same point. This event is known as the vernal equinox, and takes place around March 21 or 22nd. In the northern hemisphere, this the first day of spring, but in the southern hemisphere, it is instead the first day of autumn. Also depending on hemisphere, the vernal equinox is also halfway between either the summer or winter solstice.
So if a year is measured from March to March, why do modern calendars begin in January? You can blame Julius Caesar for that. At the time of his reign in Rome, some 50 years before the birth of Christ, it was decided that the month of January would kick off each new year. The Julian Calendar was thus adopted. Many centuries passed, and the Julian Calendar began to be ignored because it was based primarily on when Roman politicians took office. However, in the 1570s, Pope Gregory adopted the current-dayGregorian Calendar, and January 1 once again prevailed as the beginning of each new year. Oddly enough, England and the American colonies celebrated New Year’s Day on March 25 all the way up until 1752!
Regardless of when mankind decided on an arbitrary date to begin a new year, the issue of Earth’s imperfect revolution around the sun still needed to be addressed. A problem existed, and would worsen as more time passed. That solar year discussed above is not 365 perfect days in length, but rather 365.2422 days. Thus, using 365 days every successive year would throw things off. For starters, around 6 hours would be lost after every year. If a whole century went by, calendars would then be off by some 24 days. Needless to say, this would be quite confusing. EnterLeap Year, a sensible way to offset this anomaly.
By adding a 366th day to each year every 4 years, the difference between the calendar and the occurrence of the vernal equinox is more or less kept in line. Since the vernal equinox takes place before the end of March, a 29th day was added to February during these “leap” years. It would be just too weird to have a date of January 32, and adding a 31st or 32nd day to months following March would throw the equinox off in the opposite direction.
To make matters more complicated, no calendar is perfect. Due to the gravitational pull of other planets, Earth’s orbit around the sun is not completely consistent. This isn’t noticeable from day to day, because the differences are quite tiny. Currently, the error is about 27 seconds per year. However, this will eventually add up, and the Gregorian Calendar will have to be modified in a few thousand years.
Interestingly, what many may not know is that while leap years usually occur every 4 years, this isn’t always the case. Every 200 years or so, 8 years will pass instead. The last time this occurred was between 1896 and 1904. The year 1900 was thus not a leap year as it normally would have been. The next time 8 years will pass between leap years will be between 2096 and 2104.
