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    Re: Lunar mechanics and Double Alts.
    From: George Huxtable
    Date: 2003 Apr 30, 17:43 +0100

    Fred Hebard's letter was fine, until he got to-
    >The moon's declination changes radically from day to day, so the
    >ecliptic is only a first approximation as to the best location for a
    >star.  You could pick Regulus, which is smack dab on the ecliptic, and
    >still have it be 35 degrees of declination out of the moon's orbit.
    Not so, Fred. The Moon stays within about 5 degrees of the Ecliptic, even
    though the Moon declination can change greatly over a month. Some years, it
    can swing nearly 29 deg North and South of the Equator, in the course of a
    month. It's not the declination that matters, though. Regulus, being so
    close to the ecliptic, can never be more than about 7 deg North or South of
    the Moon's path, as it passes Regulus. As long as Moon-Regulus observations
    are made with a distance between them exceeding 20 deg or so, the speed of
    the Moon approaching Regulus or leaving it will be quite adequate.
    The Sun, by definition, is always on the ecliptic, so the Moon will always
    be within 5 deg or so of the Sun, as it passes.
    Other navigational planets will also be near the plane of the ecliptic:
    Venus can't get more than 8 deg away, Mars, Jupiter, and Saturn closer
    still. So it can be assumed that these bodies, like the Sun and Regulus,
    are always suitable candidates for a lunar distance, as long as the lunar
    distance exceeds 20 deg or so.
    The "standard" list , from which would be selected appropriate stars to
    provide lunar distance in the almanac on a particular day, was-
    Altair, Fomalhaut, Hamal, Aldebaran,Pollux, Regulus, Antares, Spica, and Markab.
    contact George Huxtable by email at george@huxtable.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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