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    Re: Planet positions and light travel time
    From: Paul Hirose
    Date: 2017 Dec 18, 13:25 -0800

    On 2017-12-17 13:33, David Pike wrote:
    > how do the tables allow for the fact that when you observe a planet, the 
    light you see comes from where the planet was several minutes before, not 
    where it was at the time of observation?
    The question has to do with the astronomical concept of "place."
    Geometric place is where a body actually is. It's what you get from the
    JPL planetary ephemerides.
    Astrometric place is where a body was, when it emitted the light that
    you see now. It's the geometric place adjusted for light time to the
    Apparent place is where a body appears to be. It's astrometric place
    adjusted for aberration (the apparent displacement of a body due to the
    observer's velocity) and relativistic light deflection due to
    gravitation. The coordinates in the Nautical Almanac are apparent places.
    Here are geocentric coordinates of Jupiter on 2017 Dec 18 1400 UTC. For
    easy comparison, all coordinates are in the same system (the ICRS).
    Distance is about 920 million km, and light time 51 minutes.
    14h48m36.76s -15°08'28.7 geometric
    14h48m36.20s -15°08'26.1 astrometric
    14h48m35.18s -15°08'21.8 apparent
    Computation of light time is iterative. Begin with the geometric place
    of the body at time t. Divide distance to the body by the speed of light
    to obtain a first estimate of tau (Greek letter), the light time. Move
    the target body to where it was at t-tau. Compute the new distance and
    divide by speed of light to get an improved value of tau. Repeat until
    the movement of the body between iterations is insignificant for your
    purposes. The result is the astrometric place of the body at time t.
    The procedure is the Astronomical Almanac includes a tiny correction for
    the retardation of light due to the Sun's gravity.

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