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Tropical, Anomalistic, and Sidereal years
A '''leap year,''' in the [[Gregorian Calendar|Gregorian]] and [[Julian Calendar|Julian]] calendars, is a year containing 366 days instead of the ordinary 365. Normally, this occurs every four years, where [[February]] has an extra day, bringing the number of [[day]]s in that [[month]] to 29. Leap years occur when a year's number is a multiple of 4 (1992, 2144, 1852, etc.). However, when a year's number is a multiple of 100, it is only a leap year if the number is also a multiple of 400. Thus 1900 was not a leap year while 2000 was and 2100 will not be.
A '''leap year,''' in the [[Gregorian Calendar|Gregorian]] and [[Julian Calendar|Julian]] calendars, is a year containing 366 days instead of the ordinary 365. Normally, this occurs every four years, where [[February]] has an extra day, bringing the number of [[day]]s in that [[month]] to 29. Leap years occur when a year's number is a multiple of 4 (1992, 2144, 1852, etc.). However, when a year's number is a multiple of 100, it is only a leap year if the number is also a multiple of 400. Thus 1900 was not a leap year while 2000 was and 2100 will not be.<br />
Leap year takes place because the earth takes about 365.25 days to complete one rotation around the sun. Therefore, every four years the earth ends up a full day's travel short of one rotation. Making things slightly more complicated, however, is the variety of definitions for a year. In general, those using the Gregorian Calendar go by the Tropical year, which measures from equinox to equinox, resulting in 365.24219 days per year. However, a Sidereal year (based on relative star location) measures at 365.25636 days, and an Anomalistic year (based on least distance from the sun) measures a year as 365.25964 days. Although the variations are minor, they can make a significant difference over time.<ref>https://maas.museum/observations/2010/09/15/how-long-does-the-earth-take-to-go-around-the-sun-and-what-causes-the-seasons</ref>
[[Category:Dating Systems]]
[[Category:Dating Systems]]