Julian Day calendar

Julian Day calendar system of astronomical dating that allows the difference between two dates to be calculated more easily than conventional civil calendars with their uneven months. It was devised by Joseph Scaliger in 1582 and named in honor of his father, Julius Caesar Scaliger. The Julian period of 7,980 years is a product of the solar cycle, the lunar cycle, and the Roman indiction cycle and begins on Jan. 1, 4713 BC, that being the nearest past year in which the three cycles coincided. Dates are numbered consecutively from that day, regardless of the various changes made in civil calendars based on changing definitions of the year. The Julian Day number for Dec. 31, 1999, is 2,451,544; for Jan. 1, 2000, is 2,451,545; for Jan. 2, 2000, is 2,451,546; and so on. The Julian Day is from noon, universal time , on the given date to noon of the following date.

Julian calendar a calendar introduced by the authority of Julius Caesar in 46 bc, in which the Julian year consisted of 365 days, every fourth year having 366 days. It was superseded by the Gregorian calendar, though it is still used by some Orthodox Churches. Dates in the Julian calendar are sometimes designated ‘Old Style’.

Julian calendar The form of calendar first introduced in 46bc 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 The Julian calendar was superseded by the more precise Gregorian calendar in 1582.