# [coords] Question - Time domain coordinates and frames

Paul Harrison paul.harrison at manchester.ac.uk
Fri Apr 13 14:42:39 CEST 2018

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> On 2018-04 -13, at 01:10, Arnold Rots <arots at cfa.harvard.edu> wrote:
>
> Since MJD is absolute, the client should ignore the timeOrigin. Actually, it should not even look to see whether there is one.
>
> In terms of implementation:
> The common implementation of time is to keep it as either JD or MJD (my code uses MJD, the JPL ephemeris uses JD), as either two doubles or an integer and a double.
> JD and MJD times would then need either nothing or the JD-MJD fixed offset in order to be stored; ISOTime requires a little more; but only when a TimeOffset if provided would the code actually look for a timeOrigin value.
> Or, if that feels more comfortable, you can (arbitrarily) assign a default value for the timeOrigin; can be zero or 10^10 or your favorite prime number.

I don’t think that in fact things are as simple as this summary presents, both in theory and certainly in practice. I think that it is incorrect to say that MJD is absolute -  it is tied to the UTC timescale for working out the whole day, but then there is the problem of leap seconds - is the fractional part of the MJD for 12 noon on a day with a leap second 1/2 or 43200/86401? The Astronomical Almanac in fact discusses MJDs in different timescales, where there fractional part of the day is calculated in the timescale indicated - so I get the feeling that in general there is not even a consensus of definition of MJD that means that a given MJD double value is completely unambiguous on days with leap seconds - The real problem is the measuring in units of day - only seconds have unambiguous definition, and then you do want to specify an origin to be sure what is meant.

ISOTime is tied to UTC and does have unambiguous representation of time on days with leap seconds, and the textual representation can include any timezone offsets in the representation, so need no origin - and for this reason is probably superior to MJD for representing instants of time in UTC.

Paul.

Dr. Paul Harrison
JBO, Manchester University
http://www.manchester.ac.uk/jodrellbank

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