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    Re: JPL ephemeresis and Nautical Almanac - speed of light question
    From: John D. Howard
    Date: 2018 Apr 3, 12:10 -0700


    I have read your questions several times and am somewhat confused.  Are you trying to generate an almanac for navagators?  If so, then rember that what is important is where we see something,  not where the thing is.  The main argument going into the almanac is TIME and the time navagators use is UT1 ( GMT or Zulu ).  When you SEE the sun cross over the prime meridian it is Noon Greenwitch.  ( Yes, I know that now it is determined fron UT and TT and leap seconds and .... )  It does not mater where the sun is, just when you saw it.  No need to compisate for the speed of light because it is built into our common use of time.

    GHA of the sun is where you see the sun.  A sundial is a direct measure of GHA sun.  Our watch goes by mean time and almanac data does also so we convert what we see to what our watch reads with the Equation of Time.  Also think about the sun's declination - we see it change because the Earth changed in it's orbit not because the sun's position changed.  No need for speed of light because it is built into time.

    If the speed of light was important for navigation almanacs how would you figure the position of a star that is 2000 light-years away?

    John H.

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