. The transit of Venus. when he should be And thus, in addition tohis celebrated prediction of a comet, he left a secondlegacy to his successors, who, as Englishmen, mightbe entitled to be proud of his foresight though hecould not live to reap the glory of it. It is a matter of some difficulty to show, in anelementary manner, the way in which the value of the .suns parallax can be found from observation of 1 Hookes Lectures and Collections, 1678. 2 Catalogus Stellaram Australium ; also Phil. Trans. 1694 and I7IS- II.] THE TRANSIT OF VENUS. 25 contact. We will try, however, to put it in
. The transit of Venus. when he should be And thus, in addition tohis celebrated prediction of a comet, he left a secondlegacy to his successors, who, as Englishmen, mightbe entitled to be proud of his foresight though hecould not live to reap the glory of it. It is a matter of some difficulty to show, in anelementary manner, the way in which the value of the .suns parallax can be found from observation of 1 Hookes Lectures and Collections, 1678. 2 Catalogus Stellaram Australium ; also Phil. Trans. 1694 and I7IS- II.] THE TRANSIT OF VENUS. 25 contact. We will try, however, to put it in a lightwhich anyone, with a little attention, will under-stand. 1. It must be thoroughly understood, from whathas already been said, that if we know the amount ofthe suns parallax ; in other words, if we know theangle subtended by any known distance on the earthssurface, at the sun, we know the suns distance. 2. We know that the relative positions of the earth,Venus, and the sun, are given by supposing the earth r/o. /o. to go round the sun in 365 days, and Venus in 224days. Or, if we please, we may take no account ofthe earths revolution, but suppose it fixed, in whichcase the revolution of Venus relatively to the earth(ie. the synodical revolution) is 584 days. 26 THE TRANSIT OF VENUS. [chap. 3. If, then, Venus moves round the sun through 3600 relatively to the earth in 584 days, she moves 1 ^° through —- of that in one day, and through s84X2~ of a degree in one hour ; which is at the rate of aboutii second of arc in a minute of time. Now we are ready to understand H alleys A (Fig. 10) be the position of an observer onthe earth at the time of first internal contact. S is thesun, and V, is now the position of Venus. Thisobserver sees the contact earlier than a hypotheticalobserver at the earths centre would see it, by thetime Venus takes to move over v3 v2. If we knew bycalculation the instant when an observer at E wouldsee it, and the observer at A saw it 8 m
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