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Nature volume 36, page 388 | Download Citation



THIS volume of the Annals of the Harvard College Observatory contains the description and theory of the instrument invented by Mr. S. C. Chandler, and called by him the almucantar, as well as the reduction and discussion of a series of observations made with it at the Observatory in 1884 and 1885. The instrument consists of a telescope mounted upon a base that floats in mercury, and the observation consists in noting the time of transit of a star across an almucantaral (or horizontal) circle, the particular horizontal circle which the inventor has found most convenient being that passing through the Pole, which he has called the “co-latitude” circle. If, therefore, the telescope be clamped at the given altitude, “the sight-line will mark accurately in the heavens a horizontal circle: and the transits of stars, as they rise or fall over this circle in different azimuths, will furnish the means of determining instrumental and clock corrections, the latitude, or right ascensions and declinations.” Mr. Chandler believes that an instrument on the almucantar principle is capable of giving results more free from both accidental and systematic errors than those obtained from a meridian circle, and certainly the discussion of his observations contained in the volume before us goes far to justify such a belief. The probable accidental error of a single observation in zenith distance is ± 0″.404, whilst for stars north of 60° declination it is as small as ± 0″.379; the probable accidental errors of the clock corrections from a complete transit (including the residuals for Polar stars) are ± 0.047s. and ± 0.043s. for two observers. And these results have been obtained, it must be remembered, with a telescope of only 4 inches aperture and less than 44 inches focus. The chief advantage of the system is, however, that it gives measurements of both co-ordinates of a star which are absolutely free from the effects of flexure, and also of refraction as far as it depends on zenith distance. The almucantar certainly appears to be a valuable addition to our means of attacking difficult problems of practical astronomy.

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