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Re: Local solar time [message #39973 is a reply to message #39904]

Tue, 29 June 2004 12:13

Craig Markwardt
Messages: 1869
Registered: November 1996

Senior Member

James Kuyper <kuyper@saicmodis.com> writes:
>
> I know of at least two different things that are referred to as Julian
> days. The first of these is indeed a time system; it is a single number
> for every single day, starting at Jan. 1, 4713 B.C.E. at 12:00:00, a
> time when three different cycles associated with three ancient calendar
> systems were all synchronized. Nobody was using any of those three

James, as a mini-tutorial on julian day numbers, your post was great.

However, I will take issue on your claim that Julian days is a "time
system." In fact, Julian day numbers are simply a means of counting
whole and fractional days, referred to some offset, but it is the
definition of those days that are important!

For example, we could define a day as
number of solar meridian crossings + (local solar time)/(24 hr) [LAST]
or
number of solar meridian crossings + (Greenwich solar time)/(24 hr) [GAST]

Both have meaning, and both are Julian day numbers, but one is a local
solar time, one is a the Greenwich solar time. [and both will have
irregular lengths]

But why stop there? "Day" could be mean solar days. UT1 is close to
this, with the exception of leap seconds, and the fact that the length
of day varies.

"Day" could also be 86400 seconds, as defined by atomic clock
standards [TAI], in which case every day is a fixed length. In fact
there are systems where the clock is placed at different locations
(barycenter time, TDB), or even ticks at slightly different rates
(coordinate times TCG, TCB).

So, in addition to saying "Julian days," which provides the unit of
time we also need to specify the time system in use, such as UTC, TAI,
solar time [local vs. Greenwich], etc.

Why is this important? The original poster did not specify which
system he was converting from, so there was some ambiguity at the time
of what conversions would need to be done.

Craig

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Craig B. Markwardt, Ph.D. EMAIL: craigmnet@REMOVEcow.physics.wisc.edu
Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response
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[Message index]

		Local solar time By: David Oesch on Mon, 28 June 2004 02:01
		Re: Local solar time By: David Fanning on Wed, 30 June 2004 06:29
		Re: Local solar time By: kuyper on Wed, 30 June 2004 05:59
		Re: Local solar time By: Craig Markwardt on Tue, 29 June 2004 16:36
		Re: Local solar time By: James Kuyper on Tue, 29 June 2004 13:19
		Re: Local solar time By: Craig Markwardt on Tue, 29 June 2004 12:13
		Re: Local solar time By: Craig Markwardt on Tue, 29 June 2004 11:58
		Re: Local solar time By: Craig Markwardt on Tue, 29 June 2004 11:54
		Re: Local solar time By: Michael Wallace on Tue, 29 June 2004 08:28
		Re: Local solar time By: Haje Korth on Tue, 29 June 2004 08:00
		Re: Local solar time By: Haje Korth on Tue, 29 June 2004 05:12
		Re: Local solar time By: David Oesch on Tue, 29 June 2004 05:12
		Re: Local solar time By: Haje Korth on Tue, 29 June 2004 05:09
		Re: Local solar time By: James Kuyper on Tue, 29 June 2004 04:51
		Re: Local solar time By: Craig Markwardt on Tue, 29 June 2004 02:48
		Re: Local solar time By: David Oesch on Mon, 28 June 2004 23:31
		Re: Local solar time By: Craig Markwardt on Mon, 28 June 2004 20:46
		Re: Local solar time By: Haje Korth on Mon, 28 June 2004 08:07

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