The form of calendar that is now in almost world-wide civil use. It was devised with the help of the German Jesuit mathematics teacher Christopher Clavius (1537–1612), and was introduced by Pope Gregory XIII in 1582 to replace the Julian calendar, which had got out of step with the seasons. In that year 10 days were omitted, Thursday October 4 being immediately followed by Friday October 15. This change was made in order to re-establish March 21 as the date of the vernal equinox. Britain and America did not adopt the Gregorian calendar until 1752, by which time 11 days had to be omitted. Subsequently the calendar has been kept in step with the date of the vernal equinox by introducing a leap year every four years, as the Julian calendar had done, but suppressing the leap year in century years, unless the year is divisible by 400. Thus 1700, 1800, and 1900 were not leap years, but 1600 and 2000 are. In the 400-year cycle there are therefore 97 leap years, giving an average length of the year of 365.2425 days, very close to the length of the tropical year (365.2422 days).