Editing Julian calendar (section)

===Year length; leap years===
The Julian calendar has two types of year: "normal" years of 365 days and "leap" years of 366 days. There is a simple cycle of three "normal" years followed by a leap year and this pattern repeats forever without exception. The Julian year is, therefore, on average 365.25 days long. Consequently, the Julian year drifts over time with respect to the [[tropical year|tropical (solar) year]] (365.24217 days).<ref name="Richards">Using value from Richards (2013, p. 587) for tropical year in mean solar days, the calculation is {{nowrap|1/(365.2425-365.24217)}}</ref>

Although Greek astronomers had known, at least since [[Hipparchus]],<ref name=":0">[[Ptolemy|Claudius Ptolemy]], tr. [[G. J. Toomer]], ''[[Almagest|Ptolemy's Almagest]]'', 1998, Princeton University Press, p. 139. Hipparchus stated that the "solar year ... contains 365 days, plus a fraction which is less than {{sfrac|1|4}} by about {{sfrac|1|300}}th of the sum of one day and night".</ref> a century before the Julian reform, that the tropical year was slightly shorter than 365.25 days, the calendar did not compensate for this difference. As a result, the calendar year gains about three days every four centuries compared to observed [[equinox]] times and the seasons. This discrepancy was largely corrected by the [[Gregorian calendar|Gregorian reform]] of 1582. The Gregorian calendar has the same months and month lengths as the Julian calendar, but, in the Gregorian calendar, year numbers evenly divisible by 100 are not leap years, except that those evenly divisible by 400 remain leap years<ref name=":1">[http://aa.usno.navy.mil/faq/docs/calendars.php Introduction to Calendars] {{Webarchive|url=https://web.archive.org/web/20190613115330/http://aa.usno.navy.mil/faq/docs/calendars.php |date=2019-06-13 }}. (15 May 2013). [[United States Naval Observatory]].</ref> (even then, the Gregorian calendar diverges from astronomical observations by one day in 3,030 years).<ref name=Richards />