The form of calendar first introduced in 46 bc by the Roman emperor Julius Caesar, after whom it is named. It was prepared in consultation with the Greek astronomer Sosigenes (1st century bc). Each month was assigned the number of days it has today, and a normal year had 365 days. It was intended to be a solar calendar in which the date remained in step with the seasons. To preserve this link, every fourth year was a leap year with February having an additional day. The average length of a year is then 365.25 days, close to but not exactly equal to the tropical year. The Julian calendar was superseded by the more precise Gregorian calendar in 1582.