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    Re: Confused--Can someone answer this question regarding theequation of time
    From: Stan K
    Date: 2013 Sep 17, 20:16 -0400
    Herbert,

    But is the EoT, as given in the daily pages of the Almanac, always accurate to one second?  Doesn't the EoT change throughout the course of the day?  To say that it has a single value throughout a day does not make sense.  If I am thinking about this correctly, the value (to the second) given on the daily page of the Almanac is for 12h (or 00h) UT, one particular time.  In the time it takes for the Sun to move from one longitude to another, hasn't the EoT changed?  If I am right, then the error introduced by the variation in the EoT during the day can be significantly worse (depending on the time of year, worse when the EoT is changing the fastest, often exceeding ten seconds) than the error introduced by the possible shift in the tabulated GHA of the Sun, which, by my calculation, only amounts to 0.6 seconds at the most.

    Besides all that, all the books I have seem to indicate that the GHA method is the best for determining the time of LAN with an accurate longitude.

    Where am I going wrong?

    Stan



    -----Original Message-----
    From: Herbert Prinz <herbert@heavenlysoftware.org>
    To: slk1000 <slk1000@aol.com>
    Sent: Tue, Sep 17, 2013 7:38 pm
    Subject: [NavList] Re: Confused--Can someone answer this question regarding theequation of time


    Stan,
    I would not recommend to derive Mer. Pass. from the GHA Sun if it is required to be accurate to the second. We are told on page 261 of the N.A. that the tabulated GHA Sun may be shifted by as much as 0.15' of arc. This makes your method #3 less accurate than #2 because EoT is always accurate to 1 sec of time (I would hope!).
    If anything, you may try to derive a "better" value of the GHA Sun from the given EoT. There may be some justification for this when you need the GHA Sun for a lunar distance. However, it should be pointed out that such creative uses of the N.A. are very dangerous because they require a thorough understanding of the design and the implementation of the tables. Both, you and I, have learned from our frustrated attempts to reverse-engineer the N.A. that we don't have this detailed knowledge. Best practice is to use the GHA for GHA and the EoT for EoT.
    Herbert Prinz
    Herbert
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