'modern' calendar is based upon the amount of time it takes the
Earth to go through one seasonal cycle (called a 'Tropical Year'
- about 365.25 days). More than 2,000 years ago, the Romans were
aware of this and made every fourth year have 366 days which was
accurate enough for a timespan of a few centuries. However, the
Tropical Year is not exactly 365.25 days - it is slightly
less. So, by the 1500's, the calendar was 10 days out of step
with the seasons. Most of Europe adjusted their calendars by eliminating
10 days of the calendar in 1582.
countries eventually compensated for this and a new calendar rule
was adopted. Every 4th year would still be a Leap Year with speacial
exceptions for certain "Century Years' (1700 1800, etc). If such
a year is divided by 400 and has no remainder, then it will be
a leap year as is the case with 2000 and 2400. If there is a remainder,
then it will only be a 365 day year as is the case with 1700,
1800, 1900, 2100, etc. You may test this by inputting February
for these years.
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JD, MJD and TJD
JD stands for Julian Day Number which had its starting
point at NOON January 1, 4713 BC, making NOON January 2, 4713
BC Julian Day Number 1. Millions of days have passed since then
and as of NOON January 1, 2000 the Julian Day Number was 2,451,545.
Astronomers use Julian Day Numbers because of the immense time
span covered and because each 'day' begins at noon. Since the
vast majority of astronomical events and activities occur at night,
it is very convenient to have one night's observations not split
between 2 days. Also, the Julian Day Number can conveniently incorporate
the time as well. For example, January 1, 2000 at 3:00 a.m. is
Julian Day Number 2,451,544.625. Well, that's all well and good
for astronomy, but rather awkward for everything else. After all,
having a day expressed as a 7 digit integer that changes at noontime
does not fit in well with most human activities. So, a slightly
different JD Numbering system was conceived called:
Modified Julian Days
Here the 'zero' point is November 17, 1858 at MIDNIGHT. (or expressing
it mathematically - Modified Julian Day Number = JD# -2,400,000.5).
Now there were fewer digits to deal with and the day changed at
midnight just as it does in the civil calendar in common use today.
Still another sytem was introduced called:
Here the zero point is May 24, 1968 at MIDNIGHT. (Expressed mathematically,
TJD = MJD# -40,000 and TJD = JD# -2,440,000.5).
© 2000 1728
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