Re: Time Libraries Rough Draft

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

In article <5d8a02884c87a98fb6e4f4ae8c98d5a9@comcast.net>, Gregory Wright 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 UTC 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 at 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. Greg

-- Ashley Yakeley, Seattle WA

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