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Astronomers often need to know the difference between two dates, or to be able to calculate the next date of a periodic event. For events that are quite far from one another, such as comet appearances, the regular calendar is not well suited, due to the different number of days in months and leap years, as well as calendar reforms (Julian/Gregorian) and so on.
Thus, Joseph Justus Scaliger, a French astronomer (1540 - 1609) invented Julian dates or Julian days, named after his father, Julius Scaliger. And, just in case the thought occurred to you, it is not about the Julian calendar at all.
Julian days are the counter, each day incremented by one. So, if you know the value of the Julian day for one date and the value of the Julian day for another, you can simply subtract one from another and find the difference.
The start of Julian days, called the start of the Julian era, is defined as noon of January, 1st, 4713 B.C. in the Julian calendar. With this date, all known historical astronomical observations have positive Julian day numbers, so all calculations are simple additions and subtractions.
A Julian day is a fractional number, where the whole part corresponds to midday, 0.25 is 6:00pm, 0.5 is midnight, 0.75 is 6:00am, etc.
Because the first two digits of a Julian day remain constant for about three centuries, sometimes a shorter version of a Julian day, the Modified Julian Date is used. The start of Modified Julian days (MJD) is defined as midnight of November, 17th, 1858, and