Time Libraries Rough Draft

Gregory Wright gwright at comcast.net
Wed Feb 9 22:16:28 EST 2005

On Feb 9, 2005, at 9:44 PM, Ashley Yakeley wrote:

> In article <5d8a02884c87a98fb6e4f4ae8c98d5a9 at comcast.net>,
>  Gregory Wright <gwright at comcast.net> wrote:
>> Are the Julian Dates to be true Julian dates (date changes at noon) or
>> Modified Julian Dates (changes at midnight)?
> I'd originally thought JD, but MJD might be better if zero starts at 
> midnight. In either case JulianDay and JulianDate should match. 
> Probably
> we should call them ModJulianDay and ModJulianDate if so.
> Which is more commonly used?

MJD is most commonly used, especially among people with a professional
interest in timekeeping.  The definition is

	MJD = JD - 2400000.5

Where the JD is the day count from the Julian Epoch, 4712 BCE January 1 
Greenwich Noon. Julian Dates are represented as decimal fractions.

The _Explanatory Supplement_ states: "The midnight that begins the 
civil day
is specified by subtracting 0.5 from the Julian Date at noon." So MJD 
lags the
JD by half a day.


> -- 
> Ashley Yakeley, Seattle WA
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