Since it is now certain that every possible combination of the three cyclic numbers finds its place in the Julian Period, it is evident that the first year of the Christian era, which was the 10th year of a Solar Cycle, the 2nd of a Lunar Cycle, and the 4th of a Cycle of Indiction, finds its place within this artificial era, and must answer to that particular year of the period which is characterized by the same cyclic numbers. In his book Outlines of Astronomy, first published in 1849, the astronomer John Herschel added the counting of days elapsed from the beginning of the Julian Period: The period thus arising of 7980 Julian years, is called the Julian period, and it has been found so useful, that the most competent authorities have not hesitated to declare that, through its employment, light and order were first introduced into chronology.Hence, to refer the Christian era to the Julian Period is the same thing as to find out what year of that period it is which, when divided by 28 will leave a remainder 10, divided by 19 will leave a remainder 2, and divided by 15 will leave a remainder 4. We owe its invention or revival to Joseph Scaliger, who is said to have received it from the Greeks of Constantinople.The Julian calendar year 2018 is year 6731 of the current Julian Period. The term Julian date may also refer, outside of astronomy, to the day-of-year number (more properly, the ordinal date) in the Gregorian calendar, especially in computer programming, the military and the food industry, or it may refer to dates in the Julian calendar.For example, if a given "Julian date" is "October 5, 1582", this means that date in the Julian calendar (which was October 15, 1582, in the Gregorian calendar—the date it was first established).Historical Julian dates were recorded relative to GMT or Ephemeris Time, but the International Astronomical Union now recommends that Julian dates be specified in Terrestrial Time, and that when necessary to specify Julian dates using a different time scale, that the time scale used be indicated when required, such as JD(UT1).The fraction of the day is found by converting the number of hours, minutes, and seconds after noon into the equivalent decimal fraction.(update) The Heliocentric Julian Day (HJD) is the same as the Julian day, but adjusted to the frame of reference of the Sun, and thus can differ from the Julian day by as much as 8.3 minutes (498 seconds), that being the time it takes the Sun's light to reach Earth.To illustrate the ambiguity that could arise, consider the two separate astronomical measurements of an astronomical object from the earth: Assume that three objects—the Earth, the Sun, and the astronomical object targeted, that is whose distance is to be measured—happen to be in a straight line for both measures.
The astronomical day had begun at noon ever since Ptolemy chose to begin the days in his astronomical periods at noon.
An error of about 1000 light-seconds is over 1% of a light-day, which can be a significant error when measuring temporal phenomena for short period astronomical objects over long time intervals.
To clarify this issue, the ordinary Julian day is sometimes referred to as the Geocentric Julian Day (GJD) in order to distinguish it from HJD.
Julian dates are expressed as a Julian day number with a decimal fraction added.
It has been used by historians since its introduction in 1583 to convert between different calendars.
Time intervals calculated from differences of Julian Dates specified in non-uniform time scales, such as Coordinated Universal Time (UTC), may need to be corrected for changes in time scales (e.g. Because the starting point or reference epoch is so long ago, numbers in the Julian day can be quite large and cumbersome.