Leap year

From RationalWiki

Leap years are misnamed because nothing is actually leaped over. In fact it could be argued that all the other years are leap years because they "leap over" February 29th. A leap year is a year in the Gregorian calendar with 366 days instead of 365. This is done because people with mad math skills must have everything line-up exactly. If the Earth orbits the Sun in over 365.25 days, then dagnabbit, the calendar must reflect it! So we add extra day to the month of February which is already really short, and a leap year doesn't do a whole lot to help that.

[edit] Calculating

Calculating when a leap year occurs is more Byzantine than it looks on the surface. A leap year occurs in any year evenly divisible by four, unless that year is also evenly divisible by 100. If a year is divisible by 100, then it's not a leap year unless it is evenly divisible by 400.

This results in 2000 being a leap year, but not 1900 or 2100.

[edit] Why?

The Gregorian calendar, and its predecessor the Julian calendar, are solar calendars and thus use the Earth's orbit around the Sun as its base. As it turns out, Earth takes a little less than 365.25 days to orbit the Sun. To compensate for the almost quarter day, leap year was invented by Roman Astronomer Sosigenes in the year 46 BCE for then Roman Emperor Julius Caesar. This leap year didn't have the rule regarding years divisible by 100, thus all years divisible by 4 were leap years. Julius Caesar then added extra days to 46 BCE to bring the seasons inline. This brought 46 BCE in at 445 days, the longest year on record. The resulting calendar is called the Julian Calendar (not to be confused with the Julian Date. The Julian date is the number of days into the year it is, rather than a month/day/year system. That's what some people mean when they say Julian calendar. And if you are an astronomer, then you measure the Julian date from noon-to-noon rather than midnight to midnight).

The Georgian calendar came about because the Julian calendar overcompensated and seasons started to drift (can't have the spring equinox in June, that's unnatural!). To restore Easter to its proper place, Pope Gregory XIII decreed that ten days would be dropped from October 1582 CE. The calendar went from October 4th to October 15th. This change didn't take effect immediately all over the world, making some odd discrepancies in calculating time across multiple years. By the 18th century, most places had adopted the Gregorian calendar, the most notable exception being Russia.

Historical evidence suggests that this was not an attempt to screw-over people unlucky enough to be born on February 29th.