Joseph Justus Scaliger

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    David Pulley

    I am in need of some information and hoping someone on this forum can help.

    Scaliger is attributed with introducing the idea of the Julian period later adopted and modified by John Herschel for recording the date and time of astronomical events.

    1)    Scaliger was a notable historian and scholar who devised this system to reconcile, and prevent future errors, in historical dates, particularly relating back to the Greeks, Romans, Phoenicians etc.  But did historians, or any other body of people other than astronomers, use his system.

    2)    Was Scaliger the first to devise the Julian period based upon the solar cycle, Metonic cycle and cycle of Indiction  or was he influenced by others who went before him?

    Thank you



    Hi David,

    I’m not sure I can shed too much light on your questions, but I had a potter around my book collection. There’s a good discussion of “The Julian and Gregorian calendars” in section 1.10 of “Historical Eclipses and the Earth’s Rotation” by F Richard Stephenson. (Contact me off list if you’d like a photocopy of the relevant pages) I get the impression that Scaliger’s work was part of the concerted efforts around the reformation of the calendar in the late C16. But I’m not an expert (just the Historical Section director).

    Mike Frost

    Alex Pratt

    Hi David,

    There’s some discussions in past Journals of Scaliger’s work, as documented in the BAA Journal DVD Archive.

    JBAA vol 57 no 3 (1947 March) has a paper by A. K. Bennett on ‘Scaliger J. J. 1540-1609’, but it may not answer your questions.

    Best regards,


    Dominic Ford

    I also can’t provide a definite answer, but I suspect the answer to both questions is “not really”.

    The Julian Day system’s great strength is that makes it really easy to calculate the time interval between two dates/times, which is difficult in most calendars. But unless they’re assembling historical chronologies, or doing astronomy over long periods, how many people actually do much date arithmetic?

    The system’s weakness is that its epoch was nearly 7000 years ago, so Julian dates are big numbers. In the past, scribes got lots of practice at writing out big numbers that start 24…. But nowadays it leads to loss of precision in computer programs. Computers typically do calculations to 16 significant figure precision. If you waste some of those figures on 245… you have less precision on the digits that come later. A Julian Date stored in a standard-precision variable in a computer program is only stored accurate to 20 microsecond.

    Many of the applications today that involve time arithmetic involve computer operating systems where a lot can happen in a microsecond. They usually use Unix times, stored as the numbers of seconds since 1 Jan 1970. That gives you 0.1 microsecond precision. I think pretty much everyone other than astronomers use Unix times rather than Julian Dates nowadays.

    So in short, it’s not clear to me many people outside astronomy would have had much use for Scaliger’s system. And it’s not clear his decision to use an epoch in 4713 BC was ever a useful one. Is it fair to suggest today, even astronomers only use the system out of tradition, rather than because it’s good?

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