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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.The Julian day number is based on the Julian Period proposed by Joseph Scaliger, a classical scholar, in 1583, at the time of the Gregorian calendar reform, as it is the least common multiple of three calendar cycles used with the Julian calendar: Its epoch falls at the last time when all three cycles (if they are continued backward far enough) were in their first year together — Scaliger chose this because it preceded all historical dates. In point of fact, finding the year is a very straightforward arithmetical procedure. Although many references say that the Julian in "Julian Period" refers to Scaliger's father, Julius Scaliger, in the introduction to Book V of his ", which translates more or less as "We have called it Julian merely because it is accommodated to the Julian year." Thus Julian refers to Julius Caesar, who introduced the Julian calendar in 46 BC.Years of the Julian Period are counted from this year, 4713 BC, which was chosen to be before any historical record. Originally the Julian Period was used only to count years, and the Julian calendar was used to express historical dates within years.He chose noon because the transit of the Sun across the observer's meridian occurs at the same apparent time every day of the year, unlike sunrise or sunset, which vary by several hours.Midnight was not even considered because it could not be accurately determined using water clocks.Without an astronomical or historical context, a "Julian date" given as "40" most likely means the fortieth day of a given Gregorian year, namely February 9.

It has been used by historians since its introduction in 1583 to convert between different calendars.

Julian day is the continuous count of days since the beginning of the Julian Period and is used primarily by astronomers.

The Julian Day Number (JDN) is the integer assigned to a whole solar day in the Julian day count starting from noon Universal time, with Julian day number 0 assigned to the day starting at noon on Monday, January 1, 4713 BC, proleptic Julian calendar (November 24, 4714 BC, in the proleptic Gregorian calendar), The Julian date (JD) of any instant is the Julian day number plus the fraction of a day since the preceding noon in Universal Time.

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).

Astronomers adopted Herschel's "days of the Julian period" in the late nineteenth century, but used the meridian of Greenwich instead of Alexandria, after the former was adopted as the Prime Meridian after the International Meridian Conference in Washington in 1884.

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