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    Re: NavPac
    From: George Huxtable
    Date: 2005 Jan 8, 01:15 +0000

    Jim Thompson wrote-
    >I guessed wrong as I was reading along, because I thought the pamphlet would
    >have told George that it was Apparent time.
    I will explain below what has caused Jim's quite-understandable confusion
    about this matter.
    >Does not GHA Sun in the printed Nautical Alamanac that we use in North
    >America relate directly to position of the apparent sun, rather than  the
    >mean sun,
    I agree. The almanac's time scale, as with all such almanacs since the
    1830's, is in Greenwich Mean Time, and for the Sun it's predicting the
    position of the apparent Sun (that is, the real Sun as you actually see it
    in the sky) which isn't exactly on the Greenwich meridian at Greenwich
    >hence we need to use the EqT to convert from GHA sun to get the
    >position of the mean sun, for example?
    Well, yes, the Equation of time gives you the time difference between the
    position of the apparent Sun and the Mean Sun. But can't you get the GHA of
    the Mean Sun more easily,  just from a clock set to GMT, without looking
    anything up?  At Greenwich Noon, the Mean Sun IS exactly on the Greenwich
    meridian, so its GHA is 0 degrees at Greenwich noon, and moves Westward at
    15 degrees per hour.
    >I thought that all the GHA values
    >for the various bodies in the Almanac related to apparent time, not mean
    I'm not sure what Jim is saying here. Not since the 1830s has the Almanac
    used apparent time for its "argument"; everything is related to Greenwich
    Mean Time. The tables for GHA and Dec refer to the positions of the real
    bodies, as they would be seen from the centre of the Earth, in terms of
    Greenwich Mean Time (I avoid small effects such as aberration). A table of
    the GHA of the Mean Sun in terms of Greenwich Mean Time would be very
    boring, simply increasing at 15 degrees per hour (which may be why there
    isn't such a table).
    The discussion above was in response to matters that Jim raised in his posting
    Now to get back to the problem that Jim saw, which prompted that posting,
    and we will see that his concerns were irrelevant, though understandable.
    Astronomers tend to bandy words such as Mean and Apparent in different
    contexts, in which they mean quite different things. This adds to the
    mystery, which keeps the astronomers' trade in business.
    And where they are referring to Mean Sidereal Time and Apparent Sidereal
    Time, the difference between these times is NOTHING to do with the Equation
    of Time, which corrects the irregular motion of the Sun. In the case of
    Sidereal Time, Mean Sidereal Time (or Uniform Sidereal Time, as Smart calls
    it in his "Textbook on Spherical Astronomy") allows for a steady
    precession, but assumes that there is no Nutation (the tiny wobble of the
    Earth's axis and equator with (mainly) a 26-year period, caused by the
    Moon). But that wobble causes sidereal time to be slightly irregular, and
    that irregularity is taken into account in Apparent (or True, as Smart
    calls it, just to complete the confusion) Sidereal Time. The difference
    between these two is never much more than a second of time, or thereabouts,
    in contrast to the Equation of Time that Jim was thinking about, which can
    reach 16 minutes of time.
    Have I understood and answered Jim's question, I wonder?
    I hope I've got the astronomy right. As I've said before, any astronomical
    knowledge is what I happen to have picked up along the way.
    >> -----Original Message-----
    >> From: George Huxtable
    >> I wanted to calculate GHA Aries rather
    >> precisely. I knew that it corresponded to Greenwich Sidereal Time, but was
    >> that Greenwich Mean Sidereal Time, or Greenwich Apparent Sidereal Time
    >> (there's a small difference)? None of my texts on spherical astronomy
    >> helped, and (though I thought I knew which) I didn't want to guess. But on
    >> page 59 of the booklet I found - "Table 5 may be used to calculate GHA
    >> Aries which is equivalent to Greenwich mean sidereal time (GMST)." Just
    >> what I needed to know.
    contact George Huxtable by email at george@huxtable.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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