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    Re: Unexpected USNO height correction precepts
    From: Paul Hirose
    Date: 2018 Dec 24, 02:02 -0800

    On 2018-12-22 7:01, Mehmet Guzey wrote:
    > 1- phase angle = 139.313052 degrees
    I get 139.30. No great accuracy is necessary since the center of light
    correction is small. The exact phase angle depends on geometric place
    vs. astrometric place vs. apparent place. The JPL Horizons online
    calculator uses the apparent place of the Sun as seen from the body,
    i.e., it applies the aberration for an observer at the center of Venus.
    I think that's correct but I also think it's not necessary to be so
    precise. I use the heliocentric geometric place of Venus.
    For the other vector in the phase angle computation — the vector from
    Venus to the observer or its inverse — I don't know what's strictly
    correct so I use the geometric place.
    Anyway, your phase angle is accurate enough.
    > 2- position angle = 50.820597 degrees
    Position angle is zero in the direction of the "north pole" and
    increases east. So it depends on the coordinate system: ICRS, true
    equator and equinox, or horizontal. (In the horizontal system, "north"
    is the zenith.) I computed the position angle (Venus to Sun) all three ways:
    233.9 ICRS
    233.8 true equator
    000.9 horizontal
    These are very different from your value.
    > 3- sd = 0.42 minutes
    > 4- offset = 0.312019 minutes
    Correct. Everything is correct except position angle.

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