Transits of Venus

This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1875 edition. Excerpt: ... CHAPTER III. THE TRANSIT OF 1769. The general impression among astronomers, after the observations of 1761 had been discussed, was that too much reliance had been placed on Delisle's method. ' Experience, ' wrote J. D. Cassini, later, in his ' Histoire du Passage de 1769, 'is our chief instructor; the fruit of its lessons indemnifies us for the value of the years they cost us. The principal end had been missed, in 1761, for want of observations in places where the durations differed sufficiently. It was essential not to experience a second time the same disadvantage.' Among the first statements published respecting the transit of 1769 was that by the ingenious Ferguson, who wrote as follows in 1762: ' On the 3rd of June, in the year 1769, Venus will again pass over the sun's disc, in such a manner as to afford a much easier and better method of investigating the sun's parallax than her transit in the year 1761 has done. But no part of Britain will be proper for observing that transit,1 so as to deduce anything with respect to the sun's parallax from it, because it will begin but a little before sunset, and will be quite over before two o'clock next morning. The apparent time of conjunction of the sun and Venus, according to Dr. Halley's tables, will be at thirteen minutes past ten o'clock at London, at which time the geocentric latitude of Venus will be full ten minutes of a degree north from the sun's centre; and therefore, as seen from the northern parts of the earth, Venus will be considerably depressed by a parallax of latitude on the sun's disc; on which account the visible duration of the transit will be .lengthened; and in the southern parts of the earth she will be elevated by a parallax of latitude on the sun, which will shorten...