a continuous count of days beginning with January 1, 4713 BC (-4712 CE), which is start of what is called the Julian period. The French scholar Joseph Justus Scaliger (1540-1609) introduced the Julian period in 1582 (the same year the Gregorian calendar was proclaimed), defining it to be 7980 years, the product of the 28-year cycle of the Julian calendar (after which the days of the week recur on the same dates), the 19-year Metonic cycle (after which the phases of the Moon recur on the same dates), and the 15-year indiction cycle (a unit of civil time in ancient Rome). It happens that 4713 BC is the last year in which all three cycles started simultaneously. In 1849 the British astronomer John Herschel introduced the Julian day as a means of providing an exact date for astronomical events independent of all calendars. The Julian day begins at noon Universal Time, and exact times of observations are expressed using decimal fractions of the Julian day. The first moment of the year 2004 CE, Universal Time, was JD 2 453 005.5. See also modified Julian day.
The day beginning at noon, Greenwich time, and ending at the next Greenwich noon. Set up in that way so that in Europe, where the idea originated, the Julian Day Number does not change during the night.
a time period used in astronomical circles, defined as the number of days since 1 January, 4713 BCE ( Before Common Era), with the first day defined as Julian day zero. The Julian day begins at noon UTC. Some scientists use the term julian day to mean the numerical day of the current year, where January 1 is defined as day 001.