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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
tree.

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
instrument.

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.

George.


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or from within UK 01865 820222.
Or by post- George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13
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