Solar cycle (calendar)

The solar cycle is a 28-year cycle of the Julian calendar, and 400-year cycle of the Gregorian calendar with respect to the week. It occurs because leap years occur every 4 years, typically observed by adding a day to the month of February, making it February 29th. There are 7 possible days to start a leap year, making a 28-year sequence.

This cycle also occurs in the Gregorian calendar, but it is interrupted by years such as 1700, 1800, 1900, 2100, 2200, 2300, 2500, 2600, 2700, 2900, and any year that is divisible by 100, but not by 400. These years are common years and are not leap years. This interruption has the effect of skipping 16 years of the solar cycle between February 28 and March 1. Because the Gregorian cycle of 400 years has exactly 146,097 days, i.e. exactly 20,871 weeks, one can say that the Gregorian so-called solar cycle lasts 400 years.

Calendar years are usually marked by Dominical letters indicating the first Sunday in a new year, thus the term solar cycle can also refer to a repeating sequence of Dominical letters. Unless a year is not a leap year due to Gregorian exceptions, a sequence of calendars is reused every 28 years.

Sun-based calendars are first thought to be used by the Egyptians, who based it around the annual sunrise of the Dog Star and flooding of the Nile River.