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    The Moon and Parallax
    From: David Pike
    Date: 2019 Oct 14, 08:07 -0700

    I’ve called this question The Moon and Parallax rather than Lunar Parallax to avoid anyone confusing it with lunars, which are still a bit outside my pay-grade.  I’ve decided to break the habit of a lifetime and start observing the Moon.  Why not when I can kneel against a radiator and observe it most nights through a south facing open bedroom window?  However, there are one or two things which puzzle me.

    One.  How is it that air navigators are allowed to get away with a relatively simple P in A correction in the Air Almanac, while marine navigators are forced to chase parallax though HP on the daily page to two corrections from the Altitude Correction Tables?  I presume this is because a. with all the other problems we have to deal with deflecting our bubble, we’re not interested in working to the nearest 0.1 of a minute of arc; we’re happy to accept corrections to the nearest minute, and b. being simple souls, we prefer to aim our bubble at the middle of any large object in the sky.

    Two.  The top Altitude Correction Table ought to reduce from a maximum at zero altitude like P in A, but it doesn’t.  It reduces either side of about 20 degrees altitude.  I presume this must be, because designed for operating at sea level, the Altitude Correction Table can also correct for atmospheric refraction, whereas the P in A tables don’t.  The air navigator must correct for atmospheric refraction separately, because it changes with aircraft height as well as star altitude. 

    Three.  This is the bit which always baffles me.  Why do they use the name Horizontal Parallax when it leads to a vertical correction?  DaveP

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