       Re: Julian Day Calculation - Plea for Help

• To: mathgroup at smc.vnet.net
• Subject: [mg3983] Re: Julian Day Calculation - Plea for Help
• From: Ken Starks <straton at kjstarks.demon.co.uk>
• Date: Tue, 14 May 1996 02:11:35 -0400
• Sender: owner-wri-mathgroup at wolfram.com

```searchme at earthlink.net (Matthew K.) wrote:

> I need , if possible, to get the equation for calculating the Julian Day
> well into the future (we're talking 1000 years here).

There is no need to worry overmuch about being '1000 years hence'.
The Julian Calendar repeats exactly every 400 years, which
is also an exact number of weeks.
( 365 x 400 ordinary days + 97 leap days
= 146097 days
=  20871 weeks )

Once you have a conversion for any 400 year interval, it is trivial to
add or subtract a few multiples of 146097 on one side of your function
and a matching number of 400 years on the other.
The fact that it is an exact number of weeks means that you
also get the day ofthe week without even trying.
( Exercise: what proportion of '13th of the month'
happen on a Friday ? )

Beware of using Julian dates as a measure of scientific/absolute
time ( as measured on an atomic clock ) because civil time
sticks in extra 'leap seconds' whenever the powers
that be happen to find it convenient. They are neccessary because
of the gradual slowing in the rotation of the earth on its axis,
and that is a bit hard to predict accurately  - in the long
term - because it depends on how much icecap melts Etc.

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