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    Re: Welcome to Spring (or Fall)
    From: Herbert Prinz
    Date: 2006 Mar 20, 19:03 -0500

    Dan Allen wrote:
    > The vernal equinox occurred a few minutes ago according to my
    > calculations (Mon 20 Mar 2006, 18:20:51 GMT), so it is now officially
    > Spring in the Northern hemisphere and Fall in the Southern hemisphere!
    > Dan
    Oops! You are some 5 minutes early. The correct value is 18:25:33. If
    you compute this from the N.A.you should be able to get the time within
    an accuracy of better than 15 seconds. I don't have a N.A. handy to
    check it out.
    In sync with the equinox related traffic that occurs with a period of
    0.5 tropical years on this list, I posted a diagram showing the apparent
    place of the sun around today's equinox with respect to the two
    fundamental reference frames:
    In the diagram, the ecliptic is blue. The true equator and equinoctial
    colure of date are green. The Sun (painted red) moves currently on a
    parallel of ecliptical latitude of -0.7".
    At 18:25:26 UT the Sun hits the equinoctial colure (i.e. R.A. = 0) at a
    declination of -0.73".
    At 18:25:33 UT it reaches ecliptical longitude 0. This is equinox.. At
    this point the decl. is 0.62"
    At 18:26:10 UT the sun reaches declination 0, at a longitude of 1.5" and
    a R.A. of 0.1s.
    All data computed with MICA Vers. 2, U.S.N.O.
    Conceptually, equinox implies that the declination of the sun equals
    zero. Hipparchus supposed this to happen in the equinoctial point, i.e.
    at zero longitude. When it was realized after Newton that the apparent
    Sun does not necessarily have to move through the equinoctial point each
    year, one had to make a decision whether to choose zero longitude or
    zero declination for the definition of equinox. Longitude was more
    How come the Sun has latitude? Hint: The latitude of the Sun is always
    very roughly (give or take 0.2") that of the Moon, divided by 32000.
    Have fun.
    Herbert Prinz

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