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    Re: Parallax in azimuth
    From: Paul Hirose
    Date: 2020 May 29, 12:20 -0700

    On 2020-05-24 10:00, Dave Walden wrote:
    > Consider the
    > moon on 23 May 2020 at 1330 UTC from 39N 77W.  What is the parallax in
    > azimuth of the center?
    apparent azimuth & altitude:
    88°50'04.6" +32°50'16.2" geocentric
    88°49'51.6" +32°03'18.1" topocentric
    88°49'51.6" +32°03'18.2" topocentric (no diurnal aberration)
    I used the nearest midnight (2020-05-24 0 h UTC) values from IERS
    Bulletin A: -0.25412 s = UT1-UTC, (0.1115, 0.4474) = polar motion x, y.
    Calculations with DE431 ephemeris, IAU 2006 precession, 2000B nutation.
    For the geocentric angles, the coordinate origin is translated to the
    geocenter but axes remain parallel to the topocentric east / north /
    zenith axes.
    One factor to consider is the diurnal aberration. Due to Earth rotation,
    the topocenter does not have the same velocity as the geocenter, and so
    the aberration is a little different (as much as 0.3″). In the third
    line I excluded that. At the precision of my calculation, diurnal
    aberration affects only altitude (not surprising since the body is
    almost exactly east).
    There's also a difference in light time which will affect apparent
    place. I don't think it's significant at this precision. Maybe it would
    have been better to calculate geometric place at both origins to
    eliminate every effect except parallax.
    Chauvenet discusses parallax in azimuth and altitude:

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