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. In Moslem countries the new day still conventionally begins at sundown, and this convention is pervasive. As my wife and I learned during our years in Iran, a dinner invitation for Tuesday night is an invitation for what we should call Monday night. John Gilmore, Ashland, MA 01721 - USA ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
