Secular year
A secular year is the year that closes a century. Its year is divisible by 100 without a remainder. Examples are the years 1900, 2000, 2100.
All secular years except those whose year is divisible by 400 without a remainder are not leap years in the Gregorian calendar (application of the solar equation ). Therefore the years 1700, 1800 and 1900 were not leap years. But the year 2000 was a leap year (like 1992, 1996, 2004, 2008, 2012, 2016 and 2020).
Compared to the Julian calendar , the Easter calculation is corrected in the Gregorian calendar . The lunar equation required for this is only used in secular years - in a rhythm of seven times 300 and once 400 years. This process started in 1800 and will continue in 2100. It continues 2400, 2700, 3000, 3300, 3600, 3900. After that, the four-hundred-year interval follows for the first time, so that the next lunar equation does not continue until 4300.