| Re: Julian Day Question [message #48872 is a reply to message #48871] |
Fri, 26 May 2006 09:20   |
James Kuyper
Messages: 425
Registered: March 2000
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Senior Member |
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Paul Van Delst wrote:
...
> The example on that page is:
>
> IDL> print, julday(1,1,1,0,0,0), julday(1,1,1)
> 1721423.5 1721424
>
> The *input* date, 0001-01-01, /should/ be based on how we define dates /now/, starting at
> midnight. But the reference point for the input date seems to change (to 12noon) when the
> hours/minutes/seconds are not supplied.
It is based upon how dates are defined now - by astronomers. The Julian
day starts at 12:00 noon, because that means an entire night's data
get's tagged with the same Julian date. The Julian date system was
originally invented to help astronomers match up ancient records of
astronomical events with modern observations, to get more accurate
figures for things like the orbital period of a comet. The starting
point was chosen because calender cycles associated with several
different popular historical calendar systems all come together on that
date. This simplifies the process of converting between the Julian date
and any one of those calendar systems.
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