Year is the time taken by the Earth to revolve around the sun, and hence contains a complete cycle of the seasons. In early times it was reckoned as 12 lunar months, but this gave 354 days as the year, and the annual error of the 11 days soon became so apparent that they were added on to the lunar months at various times in order to keep the year and the seasons in agreement. The Greeks added on a whole month every few years, and the Romans constantly changed their method of supplying the necessary days. Julius Caesar, however, had the wisdom to see that the year should be reckoned by reference to the sun and not to the moon, and the Julian year of 365 days six hours was henceforth adopted by the Romans and all people under their rule. In the 16th century the year was found to be 365 days five hours 49 minutes, and this, known as the Gregorian year, now replaced that of Caesar. For ordinary purposes, however, the year is considered as possessing 365 days, and extra hours, etc., are allowed for by means of an extra day introduced every four years.