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    Re: longitude and time: was [NAV-L] Star-sight discrepancy
    From: George Huxtable
    Date: 2005 Aug 27, 23:14 +0100

    Fred Hebard asked-
    >  In this
    >post, I'm returning to the land-based observations, repeating the
    >following question:
    >How did astronomers arrange their telescopes to swing precisely in a
    >North-South plane?  What technique did they use to achieve the
    >alignment?  I know it's off topic, although similar technique also was
    >used by surveyors, who might be regarded as navigators on land, to
    >bring it slightly back on topic.
    In W.Chauvenet, "Spherical and Practical Astronomy", vol. II, chapter V is
    all about Transit Instruments. It occupies pages  131 to 282 in my 5th
    edition (University Edition). Plate IV shows an observatory instrument and
    plate V a portable one (which is probably most relevant to Fred's
    question). That portable transit has its two bearings, along an azimuth
    V,carried on an iron frame which sits on three pointed adjusing-screw feet,
    and Chauvenet suggests that it should be placed on the stump of a cut-down
    In para 125, which starts on page 141, Chauvenet having previously defined
    the quantity a as the error of the axis of rotation from true East-West
    (which is what Fred is after, I think) states-
    "To reduce a to a small quantity, or to place the instrument very near to
    the meridian, we must have recourse to the observation of stars. The
    following process will be found as simple as any other with a portable
    Compute the mean time of transit of a slow moving star (one near the pole),
    and bring the telescope upon it at that time. For the first approximation,
    the time may be given by a common watch, and the telescope may be brought
    upon the star by moving the frame of the instrument horizontally. Then
    level the axis [this is done by adjusting the screw feet according to a
    sensitive striding spirit-level placed across the two bearings - George],
    and note the time by the clock of the transit of a star near the zenith
    over the middle thread. It is evident that the time of transit of a star
    near the zenith will not be much affected by a deviation of the instrument
    in azimuth, and therefore the difference between the star's right ascension
    and the clock time will be the approximate error of the clock on sidereal
    time. With this error, we are prepared to repeat the process with another
    slow-moving star, this time employing the clock and causing the middle
    thread to follow the star by moving only the azimuth N. When the clock
    correction has been previously found by other means (as with the sextant),
    the first approximation will usually be found sufficient. The instrument is
    now sufficiently near the meridian, and the outstanding small deviations
    can be found and allowed for as explained below."
    I'm doubt if I understand all of that text, and hope that it makes more
    sense to Fred. I can't decide whether that procedure requires a precise
    knownedge of lat and long, and GMT, or not. If anyone can enlighten me,
    please do.
    Chauvenet goes on to point out that it's not necessary to remove all error
    a from the azimuth of the East-West bearings, but shows how such error can
    be measured and used to correct observations.
    Contact George at george@huxtable.u-net.com ,or by phone +44 1865 820222,
    or from within UK 01865 820222.
    Or by post- George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13
    5HX, UK.

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