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    Re: additional planet corrections...
    From: Chuck Taylor
    Date: 1999 Nov 23, 4:50 PM

    On Tue, 23 Nov 1999, Russell Sher wrote:
    > Does anyone know why the additional altitude corrections in the nautical
    > almanac are needed for Venus and Mars at certain times of the year? Is this
    > for refraction?
    Mostly for parallax.  The Sun, Moon, Venus, and Mars are the only bodies
    close enough to earth for parallax to be a factor.  Parallax for the Moon
    is treated separately.  For the Sun, there is about 0.1' built into the
    main correction for altitudes up to 60 degrees.  In the cases of Venus and
    Mars, whether or not we are close enough depends on whether we are on the
    same side of the Sun or opposite sides of the Sun (which depends on time
    of year).  If on opposite sides, the distance is too far for parallax to
    matter. Jupiter, Saturn, and the stars are always too far away for
    parallax to matter.
    Chuck Taylor
    SEASCAPE GB 32-2
    Everett, WA, USA

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