Guy's Elements of astronomy : and an abridgment of Keith's New treastise on the use of the globes . suppose, v/hen speaking of thel^lane of the ecliptic, or plane of the earths orbit, thatit is a real and visible flat surface; nor in speaking ofthe orbits of the planets, that we mean solid rings; forthe planets perform their revolutions with the utmostregularity in unbounded space. The Transits of Mercury and Venus. If Venus were in her ascending node at F, (plateVIIL fig. 1,) when the earth is at a, or in her descend-ing node at H, when the earth is at b, she would be ina line with the sun, a
Guy's Elements of astronomy : and an abridgment of Keith's New treastise on the use of the globes . suppose, v/hen speaking of thel^lane of the ecliptic, or plane of the earths orbit, thatit is a real and visible flat surface; nor in speaking ofthe orbits of the planets, that we mean solid rings; forthe planets perform their revolutions with the utmostregularity in unbounded space. The Transits of Mercury and Venus. If Venus were in her ascending node at F, (plateVIIL fig. 1,) when the earth is at a, or in her descend-ing node at H, when the earth is at b, she would be ina line with the sun, and on the suns disc she wouldappear a dark round spot passing over it. These appearances are called transits; they happen very sel-dom, because Venus is very seldom in or near her nodesdt her inferior conjunctions. That there are great variations in the apparent dlameter of Venus may be demonstrated thus: suppose S(plate Vn. fig. 1.) to be the sun, E tiie earth in its or-bit, and abed, ^^ i/ Plains. THE ECLIPTIC, ZODIAC, AND EQUATOR, &C. 41 She Wt/uld, if visible, appear much larger than whenshe is at 4 in superior conjunction, because so muchnearer in the former case than in the latter ; being inthe situation a, but 27,000,000 miles from the earthE ; but at e, 163,000,000. As Venus passes from a i^lirough h c d \o e^ she may be observedby a good telescope to have all the same phrases as the moon h«?\ji [)assmg from new to full; therefore when she is at e she is , during her journey from c througrh/to ^,she will appear to havea direct motion in her orbit; from g to I, and from b to : to be nearlyst/2tio?iary, but from h to h, her motion, thtugh sL^ll really direct, ;ear to a spectator at E, to be goijig back again, (j7re^rograt^e,»« w^^•shown before. Mercury is seen in the same manner, which i» a piixvthat their orbits must be within that of the eaith- CHAPTER XV. THE ECLIPTIC, ZODIAC, AND EQUATOR, ETC. The Ecliptic is an imaginary great circle in rh
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Keywords: ., bookcentury1800, bookdecade1860, booksubjectastronomy, bookyear18
