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- Path: sparky!uunet!cis.ohio-state.edu!ucbvax!CMR.NCSL.NIST.GOV!roberts
- From: roberts@CMR.NCSL.NIST.GOV (John Roberts)
- Newsgroups: sci.space
- Subject: Re: Calendar and Zodiac
- Message-ID: <9207281206.AA27240@cmr.ncsl.nist.gov>
- Date: 28 Jul 92 12:06:03 GMT
- Sender: daemon@ucbvax.BERKELEY.EDU
- Organization: National Institute of Standards and Technology
- formerly National Bureau of Standards
- Lines: 32
-
-
- -From: stgprao@xing.unocal.com (Richard Ottolini)
- -Subject: Re: Calendar and Zodiak
- -Date: 27 Jul 92 22:07:16 GMT
- -Organization: Unocal Corporation
-
- -No, the calendar stays the same, but the sky changes.
- -The yearly calendar until @1950 was defined as the time between extremal
- -positions of the sun: furthest north or south of the year etc.
- -However, the position of sun with respect to the stars at the extremal point
- -moves slightly each year- about the diameter of the moon per 36 years.
- -The vernal equinox (spring) now occurs when the Sun is in the constellation
- -Aquarius. During the Roman Empire the sun was one constallation over in Pisces
- -during the equinox.
- -After 1950 the length of the year is defined in terms of vibrations of cesium
- -atoms which are ten million times more stable than the length of a year.
-
- Thanks for the explanation. I would like to add a refinement: It's the
- *second* which is defined by atomic vibrations - larger units of time are
- roughly defined by number of seconds, but the passage of the larger units of
- time is still subject to celestial motions. You can't say "there are x seconds
- in a year, so the instant corresponding to this one 100 years ago was
- exactly 100x seconds ago". The Naval Observatory keeps track of the passage
- of the sun (and possibly the stars - I'm not sure), so there's no ongoing shift
- between local solar noon and noon by the clock. Since the Earth's rate of
- rotation is not constant, the clock time has to be adjusted every now and then.
- The year is *probably* defined by the background of distant stars, at least
- in the long run.
-
- John Roberts
- roberts@cmr.ncsl.nist.gov
-
-