In sci.astro message <4uadnbgKKvbyaITVnZ2dnUVZ8q-rnZ2d@[EMAIL PROTECTED]
>, Thu, 1
May 2008 17:53:03, Mike Dworetsky <platinum198@[EMAIL PROTECTED]
>
posted:
>
>Astronomers use something called the Julian Day number which is a
>running count. The Julian period starts on 1 January 4713 BC (Julian
>calendar) and lasts for 7980 years. It starts on the day when the
>Roman Indiction, Golden Number, and Solar number all had a value of 1.
>This allows them to avoid having to use dates in one calendar or
>another for calculated events in the distant past (like eclipses). For
>accurate timings they of course have to use a dynamical time scale
>independent of variations of Earth rotation. When they define
>somenthing to be measured in years they usually use a unit of Julian
>years of 365.25 days, e.g., if discussing the period of a visual binary
>star, or the orbit of the Sun around the galaxy.
Julian Day Number starts at noon GMT = 0.0 on that day, which was Monday
BC 4714 Nov 24 proleptic Gregorian, or -4713-11-24.
Chronological Julian Day Number can be used to count local days from the
local midnight which started that date.
--
(c) John Stockton, nr London, UK. ?@[EMAIL PROTECTED]
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