Abstract
A SMALL expedition from the Royal Observatory, Greenwich, under Dr. R. d‘E. Atkinson, is proceeding to Mombasa to observe the eclipse of the sun on November 1 ; the purpose is to try out a method of correcting the moon‘s place from observations on the thin crescent of the sun seen from a station where the eclipse just fails to be total. Measurements of the position-angle of the line of cusps, as a function of the time, can give corrections to the differential co-ordinates of sun and moon which are independent of any assumption about their apparent radii, and independent also of irradiation if the sky is uniform. If a large number of such measures is made, the effects of irregularities of the moon‘s limb also cancel out to a considerable extent. In the neighbourhood of Mombasa, where the eclipse is nearly 98 per cent total, the position-angle swings round through about 70° in 2½ minutes ; it is hoped to obtain 3,000 timed pictures on 35-mm. film in this interval. The camera will be stationary, and the trail of successive images produced by the sun‘s diurnal motion will give the zero of position-angles. An ordinary small visual telescope-lens, stopped down to about ƒ/40, is being used, in conjunction with a green filter and exposure times of 1/1,000 sec. Special time-signals will be sent from Rugby, and these will be recorded, together with the instants of the shutter-openings, on tape-chronographs which have been made at Abinger. The accuracy of the results will be limited by uncertainties about the contour of the moon‘s limb ; it seems certain that if the limb were smooth the moon‘s place would be determined with consmb-effects miderably greater accuracy than that with which it is at present computed in the Nautical Almanac. Whatever the liay be, the accuracy should be higher than that which results from six thousand ordinary occultations, since the timing is considerably better than is possible with visual methods, while all effects due to personality, systematic time-differences between stations, and real changes of libration are eliminated. If the method works out as well as is hoped, it will probably be used for long-range geodetic purposes ; the accuracy will then be more severely limited by our ignorance of the moon‘s limb, since considerable changes in the libration are inseparable from a long shadow-path. However, the accuracy should still be considerably greater than can be obtained, for the same purpose, by merely timing the instants of contact, since in that case all the emphasis is on some one point of the limb at each contact ; a chance error at that point can cause a large error in the time.
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Solar Eclipse of November I : British Expedition to Mombasa. Nature 162, 521–522 (1948). https://doi.org/10.1038/162521d0
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DOI: https://doi.org/10.1038/162521d0