The elements of astronomy; a textbook . entral, it occupies about eight hours, but whenthe track lies near the edge of the disc, the duration is ofcourse correspondingly shortened. Since the sun passes thenodes of the orbit on June 5th and December 7th, all transitsmust occur near these dates, but they are very rare phenomena. Their special interest consists in their availability for thepurpose of finding the suns parallax (see Appendix, Arts. 516-519). The first observed transit (in 1639) was seen by onlytwo persons, — Horrox and Crabtree, in England, but the fourwhich have occurred since the
The elements of astronomy; a textbook . entral, it occupies about eight hours, but whenthe track lies near the edge of the disc, the duration is ofcourse correspondingly shortened. Since the sun passes thenodes of the orbit on June 5th and December 7th, all transitsmust occur near these dates, but they are very rare phenomena. Their special interest consists in their availability for thepurpose of finding the suns parallax (see Appendix, Arts. 516-519). The first observed transit (in 1639) was seen by onlytwo persons, — Horrox and Crabtree, in England, but the fourwhich have occurred since then have been extensively observedin all parts of the world where they were visible, by scientificexpeditions sent out for the purpose by the different govern-ments. The transits of 1769 and 1882 were visible in theUnited States. 327. Recurrence and Dates of Transits. — Five synodic, or thir-teen sidereal revolutions of Venus are very nearly equal to eight years,the difference being little more than one day; and still more nearly —. 328] MAES. 229 in fact, almost exactly — 243 years are equal to152 synodic, or 395 sidereal revolutions. If,then, we have a transit at any time, we mayhave another at the same node eight years earlieror later. Sixteen years before or after, it willbe impossible, and no other transit can occurat the same node until after the lapse of 235or 243 years, although a transit or pair of tran-Fig. 81. sits may occur at the other node in about half Transit of Venus Tracks, that time. Transits of Venus have occurred, orwill occur at the following dates : — j Dec. 7th, 1631. ( June 5th, 1761. ( Dec. 4th, 1639. ( June 3d, 1769. V Dec. 9th, 1874. f June 8th, 2004. 1 Dec. 6th, 1882. ( June 6th, 2012. Fig. 81 shows the tracks of Venus across the suns disc in thetransits of 1874 and 1882. MAES. 328. This planet is also prehistoric as to its discovery. Itis so conspicuous in color and brightness, and in the extentand apparent capriciousness of its movement am
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