A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Position-Finding
From: Paul Hirose
Date: 2018 Nov 4, 22:18 -0800
"In the zenith telescope, or Horrebow-Talcott, method of determining the latitude, there is substituted for the measurement of the absolute zenith distance of a star the measurement of the small difference of meridional zenith distances of two stars culminating at about the same time, and on opposite sides of the zenith. The effect of this substitution is the attainment of a much higher degree of precision, arising from the increased accuracy of a differential measurement, in general, over the corresponding absolute measurement; from the elimination of the use of a graduated circle for the essential part of the measurement; and from the fact that the computed result is affected, not by the error in estimating the absolute value of the astronomic refraction, but simply by the error in estimating the very small difference of refraction of two stars at nearly the same altitude." "A sufficient number of pairs should be observed at a station to make it reasonably certain that the probable error of the mean result is not greater than ±0″.10." (Typically, about 15 pairs were enough.) "The stars observed upon should be taken from "The Preliminary General Catalogue of 6188 Stars for the Epoch 1900" by Lewis Boss, which was published by the Carnegie Institution at Washington in 1910." [https://archive.org/details/preliminarygener00carnrich/page/n15] "Among the requisites for a pair of stars for an observing list, are, that their right ascensions shall not differ by more than 20m to avoid too great errors arising from the instability in the relative positions of different parts of the instrument; nor by less than about 1m, that interval being required to take the readings upon the first star and prepare for the second star of the pair; that their difference of zenith distances shall not exceed the half length of the micrometer comb, 20′ for most instruments; that each star shall be bright enough to be seen distinctly, not fainter than the seventh magnitude for the larger instruments; and that no zenith distance shall exceed 45°, to guard against too great an uncertainty in the refraction." William Bowie, "Determination of Time, Longitude, Latitude, and Azimuth," U.S. Coast and Geodetic Survey Special Publication No. 14, 1917. (In later years that was replaced by Special Publication No. 237, "Manual of Geodetic Astronomy.") Since a transit instrument is free to move only in altitude in the plane of the meridian, the reversal to observe the second star of each pair was accomplished with a mechanism which lifted the telescope trunnions from their V-blocks, rotated 180° about a vertical axis, and lowered them in the opposite blocks. In the bottom photo on this page the telescope assembly has been lifted and partly rotated. http://amhistory.si.edu/surveying/object.cfm?recordnumber=758936 The only graduated circles on that instrument are the Talcott-Horrebow levels attached to the sides of the telescope tube. Those are set to the expected altitude of the first star. No great accuracy is necessary, it being sufficient that the star is well positioned in the field of view. What is necessary is that the telescope is accurately returned to the same inclination for the second star. Exact centering of the bubble is not assumed. Its position is recorded immediately after each observation, and the corresponding dislevelment factored into the computation. Both stars are measured with a movable horizontal wire of a micrometer as they pass through the meridian. From the difference in micrometer readings, the difference in zenith distance is computed, from which latitude is computed. For example, if the readings are identical, latitude equals the mean declination of the pair. Note that zenith distance itself is not measured in this technique. The illustrated instrument is quite old, but I chose it because the reversing mechanism is shown so well. By the time of the 1917 publication, one of the favored instruments of the Survey had a 3-inch f/15 telescope and was normally operated at 100x. Somebody mentioned the polar motion. Corrections for that were applied: "About the middle of each year the Latitude Service of the International Geodetic Association publishes in the Astronomische Nachrichten provisional values of the coordinates of the instantaneous pole for the preceding calendar year, together with tables to reduce observed latitudes, longitudes and azimuths to the mean position of the pole."