Century leap year
In the Gregorian calendar every year that is divisible by 400 is due to be an end-of-century leap year (often called a century leap year), with the intercalation of February 29 yielding 366 days instead of 365. End-of-century years that are not divisible by 400 are common years (with 365 days) and not leap years. For example, the years 1600, 2000, and 2400 are end-of-century leap years, while 1700, 1800, 1900, 2100, 2200, and 2300 are common years despite being divisible by 4. Leap years divisible by 400 always start on a Saturday; thus the leap day February 29 in those years always falls on a Tuesday (dominical letter BA).
The Gregorian calendar yields an average year that currently tracks the annual revolution period of the Earth more closely than the older Julian calendar, in which every fourth year (including end-of-century years) is a leap year. The Julian formula adds too many leap days (3 every 400 years), causing the Julian calendar to drift gradually with respect to the astronomical seasons. Over time, natural events such as the spring equinox began to occur earlier and earlier in the Julian calendar.
The Gregorian calendar was introduced in 1582, but was adopted by various countries over several centuries, with the result that some countries still used the older Julian calendar while others used the Gregorian calendar. Dates prior to 1582 are generally calculated using the Julian calendar, and different countries have different conventions about dates between 1582 and their adoption of the Gregorian calendar. (See, for example, Old Style and New Style dates.)
- Spofford, Thomas (1835). A new system of practical astronomy: made plain and easy to those who have not studied mathematics : containing the elementary principles of the science, all the rules and tables necessary for making all the calculations for an almanac …. Boston: Lemuel Gulliver. p. 28.