448 



NA TURE 



{April (), 1874 



is called a transit of Venus. If it happen that Venus 

 seems to pass across the centre of the sun she takes 

 about eight hours to complete the passage. The earth 

 occupies the position A always in June, and the position 

 B in December. If there be a transit of Venus when the 

 earth is at B, Venus is said to be at the descending node, 

 because then she is descending from the northern portion 

 of her orbit to the southern. When Venus is at C she is 

 at her ascenduiir node. 



It has been said that there are, roughly speaking, two 

 transits of Venus in a century. The following table shows 

 all the transits of which we know anything : — 



1631. Predicted by Kepler, but not observed. 



1639. Predicted and observed by Horrox. 



1761. Predicted by Halley ; observed by many. 



1769. Observed generally. 



1874. 



1882. 



It will be noticed that the transits occur in pairs, eight 

 years apart ; the reason of this can easily be rendered 

 clear. The earth takes 365 '2 56 days to go round 

 the sun ; Venus takes only 2247 days. Suppose then 

 that at any particular date Venus and the earth are 

 at the node simultaneously, viz. at V and E, Fig. 3, a 

 transit of Venus over the sun's diameter will be seen. 

 When Venus has completed a revolution the earth will 

 have moved away to E,, and Venus will not overtake the 

 earth until they reach the positions V, and Eo. This is 

 583'920 days from the time when they were at V and E ; 

 but v., and Ej do not lie upon the line of nodes ; hence 

 there can be no transit. After another 5 84 days Venus 

 will again be in conjunction with the sun, but still not on 

 the line of nodes. But the fifth conjunction occurs after 

 2919'6 days (5X583-920) ; and the earth completes eight 

 revolutions in 2922'o5 days. Thus it appears that, at 

 this conjunction of Venus with the sun, the earth and 

 Venus are very near to their old positions V and E. 

 Hence they are almost on the line of nodes. In this case 

 we can without difficulty examine the possibility of a transit. 

 If we suppose the motion of the earth to be stopped, the 

 apparent motions of the sun and Venus may be repre- 

 sented as in Fig. 4, whei'e a portion of the orbit of Venus 

 and of the ecliptic are shown near the nodes. When the 

 sun and Venus are on the line of nodes simultaneously 

 S represents the sun and V Venus. At the filth conjunc- 

 tion the sun will not quite have reached S, but will be 2I 

 days behind at S' ; Venus will then be at V. Now irl 

 this case theie can be no transit visible, for here Venus is 

 quite out of range of the sun. But if in the original 

 transit the sun was a little past the node as at S (Fig. 5), 

 then eight years after he will be at S', and there will be 



another transit. It follows from this that there will be a 

 pair of transits eight years apart, only when in the first 

 one Venus does not pass close to the sun's centre. This 

 equality of eight revolutions of" the earth, with thirteen of 

 Venus, is very interesting, because this fact was shown by 

 the present Astronomer Royal to account for an inequality 

 in the earth's motion due to the attracting influence of 

 Venus. The result of a short calculation informs us that 

 for positions of Venus and the earth near the line of 



nodes, Venus is at one conjunction 22' 16" distant from 

 her position at the conjunction which occurred eight 

 years previously,* this distance being measured at right 

 angles to the ecliptic. Now the sun's diameter is 32'. 

 This shows why, generally, there are two transits eight 

 years apart. 



The first prediction of a transit of Venus was made by 

 Kepler,f and was calculated from his Rudolphine 

 tables. In 1631, the year predicted, astronomers of 

 Europe were eagerly on the watch for so rare a spectacle. 

 But the calculation was in error, so that it took place 

 when the sun was below the horizon in Europe, and was 

 consequently invisible. 



After this no astronomers seem to have interested them- 

 selves about the possibility of such an occurrence, with one 

 exception. Jeremiah Horrox, a curate of the village of 

 Hoole, near Liverpool, was much devoted to astronomical 

 pursuits. + He possessed some tables for calculating the 

 places of the planets ; but his observations did not agree 

 at all with them. He had, however, before discovering 

 the faults of Lansberg's tables, calculated from them the 

 future positions of the planets. This work, with correc- 

 tions deduced from his own observations, led him to pre- 

 dict a transit of Venus, visible in England, for the year 

 1639. He acquainted his friend Crabtree, of Manchester, 

 with the results of his calculation, and then prepared 

 himself for the observation. He considered the best 

 method to be the employment of a telescope to throw an 

 image of the sun on a white sheet of paper in a darkened 

 room. A circle was drawn, of about 6 inches diameter, 

 upon the paper, to make the sun's image exactly fill the 

 circle. A plumb-line would give him the direction of the 

 vertical, and by marking successive positions of the planet 

 on the sun's disc, he would be able to calculate many of 

 the elements of Venus. Such an observation is of course 

 peculiarly well suited for determining the diameter of the 

 planet, the inclination of its orbit, the position of the node, 

 and the true time of passing this node. His calculations 

 showed that the transit ought to commence on the after- 

 noon of November 24 (old style) ; but to guard against 

 disappointment, and because of discrepancies in various 

 tables, he kept a watch from the 23rd. On returning 

 from some clerical duties on the 24th (Sunday) he was 

 gratified by beholding a black spot on the sheet of paper, 

 which indicated the presence of Venus on the sun's disc. 

 He made three observations before sunset and has left us 

 a drawing to illustrate the observations. § 



It is curious to find an astronomer supporting the 

 opinions of the astrologers ; but in his treatise we find 

 that the chance of a clouded atmosphere caused him 

 much anxiety, for Jupiter and Mercury were in conjunc- 

 tion with the sun almost at the same time as Venus. 

 This seemed to him to forbode great severity of weather. 

 He adds, " Mercury, whose conjunction with the sun is 

 invariably attended with storm and tempest, was espe- 

 cially to be feared. In this apprehension I coincide 

 with the opinion of the astrologers, because it is confirmed 

 by experience ; but in other respects I cannot help de- 

 spising their more than puerile vanities." But we must 

 not laugh at Horrox for his opinion. In our own day 

 there is a considerable number of diligent astronomers 

 who believe that the cyclones in the Indian Ocean, cer- 

 tain other winds, the growth of vines, and various other 



* For at the fifth conjunction the earth is a "45 days distant from her place 

 at the original conjunction. This is equivalent to 2* 24' 59", when viewed 

 from the sun, from which subtract 2' 44" {= the retrogression of the node ol 

 Venus in eight ye.irs), and we have 2° 22' 15" =the angul.ir distance of the 

 earth from its corrected original position, as seen from the sun. The ratio 

 of this to the angular distance of Venus from her original position as seen 

 dist. of Venus from earth _ 277 



from the earth = d.st. of earth from sun 7^3' M"l'>P'y'n8 = " '5 



by -ji-^, and dividing by 277, we have 6° w' 17". Multiplying thii by '06 = 

 tan 3' 23' 31", which is the inclination of the orbit of Venus, we have 

 22' 16" = the latitude of Venus at the fifth conjunction. 



t ' AdmonitiuDcula ad Ciu-iosos renim Ccelestium/' Lcipsic> 1626. 



t See Natuke, vol. viii. p. 113, 



§ Venus in Sole Visa. 



