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Well, you just never know what you are going to find here. I really found your posting on Julian calendar most interesting and enjoyable. Thanks much -- and Merry Christmas!
Karl
:
:The so-called Julian Calendar was developed by Julius Caesar with the advice
:of the astronomer Sosigenes. The Julian Calendar discards the lunar month
:used in more ancient Arabic and Jewish calendars and adopts 365.25 days as
:the length of a year. This year is divided into twelve periods (months) of
:30 or 31 days. (February doesn't have 28 or 29 days in the Julian Calendar.
:) The normal year was 365 days. To make up the extra 1/4 day, an extra day
:was intercalated (put into the normal calendar) every four years.
:
:365.25 was amazingly close to the exact actual period of a year. The actual
:duration of a tropical year is only 0.0078 day less than Sosigenes'
:calculations. Even so, with a 0.0078 day per year error rate, after 1,000
:years the Julian Calendar was in error 7.8 days. By 1582 the date of the
:vernal equinox was March 11, instead of March 21, so Pope Gregory decided to
:return the sun to its proper position by slipping the calendar forward ten
:days and by modifying the Julian Calendar calculations in such a way that
:the error would not reappear.
:
:We still use Pope Gregory's modified Julian Calendar today. It is identical
:to the Julian Calendar, except only such century years are leap years as are
:divisible by 400. This is roughly equivalent to dropping 3 days every 400
:years, resulting in an average year length of 365.2425 days. That value
:differs from the exact tropical year length by only 0.0003 days, so that by
:the year 2582 (1,000 years after its creation) Pope Gregory's calendar will
:be in error only slightly less than 1/3 of a day. Our decedents can have an
:extra leap day 2,000 years after that, in the year 4582, to correct 3,000
:years of accumulated error and return the sun back to exactly where it
:belongs.
:
:The Gregorian Calendar was immediately adopted in 1582 by all Roman Catholic
:countries, but the Greek Church and most predominately Protestant countries
:refused to recognize it until long after that time. The confusion following
:the change persisted down to the present century. Both calendars were in
:common use in different parts of the United States early in its history,
:causing confusion for modern historians. Romania did not change from Julian
:to Gregorian until 1919. Some parts of the world, such as predominately
:Moslem countries still use various forms of ancient lunar calendars.
:
:Astronomers number dates with "Julian Day Numbers" to avoid the need to
:correct for the various changes that have been made to other calendars over
:the years and because they often are concerned with dates prior to the
:beginning of the Gregorian Calendar. Julian day zero was a very long time
:ago, so the count is now very high. The Gregorian Calendar date January 1,
:2000, will be Astronomical Julian Day Number 2,451,545. 2,451,545 / 365.
:2422 = 6,712.1077, so that date will be 6,712 years and 39 days from day
:zero of the Astronomical Julian Calendar.
:
:Astronomical Julian Day Numbers are widely used internally by computer
:programs to record dates and to perform date arithmetic. Date arithmetic is
:greatly simplified with Julian Day Numbers, because the number of days
:between any two dates can be determined by simply subtracting one from the
:other. Compare that to the problem of determining the number of days
:between two Gregorian dates, like April 17, 1998, and November 3, 1983,
:where calculation would have to take into account the varying number of days
:in each month and the extra February day in intervening leap years.
:
:Julian Day Numbers also make it easy to calculate the day of the week.
:Julian Day Numbers change at noon, rather than at midnight, as in the
:Gregorian Calendar. If you know the Julian Day Number at noon, the day of
:the week can be obtained by adding one and then taking the result modulo
:seven. A zero result corresponds to Sunday, one to Monday, two to Tuesday,
:etc. Compare that to the difficulty of calculating the day of the week of a
:Gregorian Calendar date like May 4, 1999.
:
:For more information about Astronomical Julian Day Numbers, see:
:
:Meeus, J. 1982, Astronomical Formulae for Calculators, 2nd ed., revised and
:enlarged (Richmond, VA: Willmann-Bell).
:
:Also, Hatcher, D.A. 1984, Quarterly Journal of the Royal Astronomical
:Society, vol. 25, pp. 53-55; and op. cit. 1985, vol. 25, pp. 151-155, and
:1986, vol. 27, pp 506-507.
:
: -Bob Brickey
: Scientific Approaches
: sci@xxxxxxxxxx
:
:
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