Apnlg, 1874] 



NATURE 



A^7 



THE COMING TRANSIT OF VENUS 

 I. 

 T N days of old it was supposed that the earth held 

 -*■ the central position of the solar system, and that 

 moon, sun, and planets moved round it, each in its own 

 orbit. The moon was supposed to be nearest to us, then 

 came Venus, then Mercury, after that the sun, then Mars, 

 Jupiter, and Saturn. We now know that of all these the 

 moon is the only one which revolves round the earth, and 

 that all planets go round the sun in the following order : — 

 Mercury, Venus, the earth. Mars, Jupiter, Saturn. These 

 are all the planets which were known to the ancients. 

 Since Mercury and Venus were formerly supposed to be 

 lower than the sun, and all the others higher, the name of 

 ififirior planets was given to the former, and superior 

 planets to the others. These terms are still retained by 

 astronomers, though the ideas that gave rise to these 

 terms are long since exploded. Fig. i shows the pheno- 

 mena exhibited by an inferior planet in the course of its 

 journey round the sun. V is the planet Venus in the dif- 

 ferent parts of its orbit. E is the earth, which is shown 

 in the figure always in one position, although of course it 



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also describes an orbit round the sun. We are naturally 

 led by a study of the diagram to three points of interest 

 concerning the motions of an inferior planet. 



The first is that the planet can never seem to be far 

 distant from the sun. Venus moves round the sun in the 

 direction shown by the arrow. The earth rotates in the 

 same direction. We are supposed to be looking down 

 upon the solar system from some point in the northern 

 heavens. It will be noticed that when the planet leaves 

 the point V, she will seem to recede from the sun more 

 and more, until she reaches the position V3. She can 

 never be farther from the sun than this, and is then said 

 to be at her greatest eastern elongation. She then ap- 

 proaches the direction in which the sun is seen, uritil she 

 is lost in the brightness of his rays. During all this time 

 she is seen best in the early morning before sunrise, 

 setting before the sun. When Venus has passed this posi- 

 tion her distance from the sun appears to an observer 



upon the earth to increase until she reaches Vp, her 

 greatest western elongation, when she again begins to 

 approach the sun. 



The next point to be noticed is that she is sometimes a 

 great deal closer to the earth than at other times ; and 

 when she is most near to the earth she appears to be 

 larger. At her closest approach to the earth she is only 

 about 26,000,000 of miles away ; but when farthest oft her 

 distance is 158,000,000 of miles. Her apparent size is 



therefore much greater in the first case than in the 

 second. These differences are shown at the lower part of 

 Fig. I. 



The third point to be mentioned is that she exhibits 

 phases just as the moon does. In any position that hemi- 

 sphere alone is illuminated which is directed to the sun ; 

 so that in the position V3, when we can only see one-half 

 of that hemisphere, she will have the appearance of a 

 half-moon. So in the position Vo she has a crescent 

 form, and at V, a gibbous form. The apparent size and 

 shape of the planet in its different positions are shown in 

 the lower part of Fig. i. 



The question now arises, what will happen when Venus 

 is between us and the sun ? In the first place, since her 

 illuminated hemisphere is turned away from us, she will 

 appear to be black ; so that we shall have no chance of 

 seeing her,unless shebe seen as a black spot upon the bright 

 surface of the sun. We would naturally expect that this 

 should happen every time that the planet is at its least dis- 

 tance from us. A simple consideration shows that this need 

 not be the case. The orbits of Venus and the earth do 

 not lie in the same plane. In other words, we cannot re- 

 present accurately the paths of \'enus and the earth by a 

 drawing upon a sheet of paper. The orbit of Venus 

 would have to be tilted up above the plane of the earth's 

 orbit. Both of these planes pass through the sun, but 

 they are inclined to each other at a certain small angle. 

 This will be seen by a glance at Fig. 2, where V repre- 

 sents the orbit of Venus, E that of the earth. The line 



AB, which passes through the sun S, is called the line of 

 nodes ; and it is quite clear that in order to see 

 Venus as a black spot upon the sun both the earth 

 and Venus must lie approximately on this line of 

 nodes. Now it generally happens that when V'enus is at 

 her least distance from the earth, these two planets 

 occupy some such places as V and E, so that she seems 

 to be above the sun ; and, not being illuminated, she is 

 invisible. Only twice in a century does she reach the 

 node, so nearly at the same time as the earth, as to be 

 seen as a black spot upon the sun. Such a phenomenon 



