On Thursday 14 February 2013 10:11:39 John Gilmore wrote:
> Joel C. Ewing also wrote:
>
> <begin extract>
> Astronomers are a perverse lot for choosing an epoch origin at noon
> rather than midnight, which makes their concept of "same day"
> different from normal discourse.
> <end extract>
>
> This is interesting because it reifies what the international lawyers
> call a merely municipal convention into a universal one.
>
> Calendars differ widely in how they define ther durations and
> endpoints of days. The classic statement of these differences is in
> Copernicus's De revolutionibus orbium coelestium, englished for the
> benefit of latin dropouts:
>
> Some, like the Chaldeans and the ancient Hebrews, defined the day as
> the time between two successive sunrises; others, like the Athenians,
> as that between two successive sunsets; others, like the Romans, as
> that between successive midnights; and still others, like the
> Egyptians, as that between successive noons.
>
> Copernicus adopted the 'Egyptian' convention, which the Egyptians had
> in fact borrowed from the Babylonians along with sexagesimal time
> units and this convention was the standard astronomical one until UTC
> was adopted. This noon-to-noon converntion had and has one sovereign
> virtue. It is free of the sharp seasonality that sunrises and sunsets
> exhibit. It yields a "mean and equal" day.
>
And I might point out that the methods used in marine navigation (which
is
closely associated with astronomy), requiring noon measurements for
determination of latitude, perpetuated the use of the noon-to-noon paradigm.
Leslie
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