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    Re: Timing error impact on Hc
    From: Gary LaPook
    Date: 2015 Jun 6, 22:22 -0700

    If we ignore "v" for the sun then the sun moves to the west exactly 1' every four seconds increasing it's GHA by exactly 1'. This will cause a change in Hc (not necessarily Hs due to possible change in refraction) of exactly 1' only IF the observer is exactly on the equator and the azimuth of the sun is exactly 90° or 270°. For any other location or azimuth the change in Hc will be less than 1'.

    Aries, and all the stars in lockstep with Aries, move to the west 15° 02.5' (902.5') every hour so the GHA of all stars increases 1.028' in four seconds creating a 1.028' change in Hc for the same conditions as for the sun. So if you want to be absolutely accurate you must use this slightly different rate for shooting stars. If you are not a purist then the rate of 1' change in GHA works well for all celestial bodies, at least for short periods of time. (For planets we have to apply the "v" correction.) And the moon marches to the beat of a different drummer. 

    The MOB table that I posted shows the variation of changes in Hc with time for different latitudes and azimuths. The tabulations are for even minute periods so just divide by 15 to find the change for a four second period. See:


    See my post:




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