[LEAPSECS] Bulletin C and all that

Steve Allen sla at ucolick.org
Sun Jan 25 22:36:54 EST 2015

On Sun 2015-01-25T16:57:58 -0500, G Ashton hath writ:
> Brooks Harris mentioned, at approximately Sun 1/25/2015 21:02 UT, a
> "Gregorian timescale". I believe that since in Gregory's time there was no
> alternative to making the passage of calendar days agree with the day/night
> cycle, we must understand "Gregorian timescale" to mean either apparent or
> mean solar time (unless an ecumenical council is called and declares
> otherwise). Also, I am not at all sure that the Gregorian calendar has
> always been used together with the convention that the new day begins at
> midnight; I would like to be pointed to a reference that says that, if it
> exists.

Most references are those of the common practice within some context.
A few have the status of international recommendation.

The tabulations of the UK Nautical Almanac were done in apparent solar
time through 1832.  Then the Admiralty decided that chronometers were
ubiquitous enough that their use justified changing the tabulations to
be done in mean solar time starting with 1833.

The 1884 International Meridian Conference decided that the Universal
Day was mean solar time, and they recommended that it should start at

The UK and US almanacs started the day at noon through 1924.
Starting in 1925 the almanacs switched to the civil practice
of starting the day at midnight.

The citable references for these transitions are linked within

Steve Allen                 <sla at ucolick.org>                WGS-84 (GPS)
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