May 12, 1882.] 



♦ KNOWLEDGE • 



587 



without seeming to cross the sun's face centrally. But as 

 her path is inclined to the plane in which the earth travels, 

 she is sonietiiues slightly ou one side, sometimes slightly on 

 the other side of that plane, wlien she is passing between 

 the earth and the sun, so that sometimes she passes above, 

 or north of the sun, sometimes below, or south of him, 

 and is not seen crossing liis disc. \Vhen, however, it so 

 chances that Venus comes between the earth and the sun 

 at or near the time when she is crossing from one side to 

 the other of the earth's plane, there occurs a transit. This 

 can only happen, of course, at two certnin times in the 

 year^viz., at or near the time when the earth is crossing 

 the line through the sun, along which the two orbits — the 

 earth's and Venus's — intersect One of these times in the 

 year is about June 8, the other about Dec. 7, and a transit 

 of Venus never occurs except at or near one of these dates. 

 Now, when one of these transits occurs, certain parts of 

 the earth are suitably placed for seeing, cither the whole 

 transit, or the beginning or end of it, while from certain 

 positions of the earth no parts of the transit can be seen. 

 Let us see how such places arc determined : — 

 Suppose that c (Fig. 1) represents the earth, .««' repre- 

 senting the sun. In reality, both are enormously exag- 

 gerated in dimensions compared with the distance separat- 

 ing them. Suppose a cone, sks, enclosing the earth and 

 sun, in the way shown, to travel round with the earth ; and 

 :ilso another cone, sis', touching both the earth and the sun. 



Then the transit is over : (and no more transits of Venus 

 will be seen till the year l!004.) 



Wiiile Venus is between 2 and 3, she is visible on the 

 sun's disc from every part of the earth's surface, turned sun- 

 wards, and the portion wlience bhe is thus seen at some time 

 or other is of course more than a hemisphere, seeing that 

 ^^ hen Venus is at 2, a hemisphere of the earth is turned 

 sunwards, and a dill'erent licniisphere when Venus is at 3. 



Kow, in the large chart there arc shown : — 



(1) The semicircles bounding the region (approximately 

 a hemisphere) whence the beginning of the transit is seen. 

 These are marked " transit begins at svinrise," " transit 

 begins at sunset" 



(2) The semicircles bounding the region (approximately a 

 hemisphere) whence the end of the transit is seen. These 

 are marked " transit ends at sunrise," " transit ends at 

 sunset" 



Over the region common to both (1) and (2), the whole 

 transit is seen, except in the small region (shaded in the 

 chart), where, though both the beginning and end arc seen, 

 a part of the mid-transit is not seen. Over the region 

 belonging to neither (1) nor (2) no p.art of the transit is 

 seen, except in two small regions (shaded in the chart), 

 whence, though neither the beginning of the transit nor the 

 end is seen, a part of the mid-transit is seen. 



Over region 1, a series of dotted lines are drawn, showing 

 how much the Ingress of Venus, or the beginning of the 



Fig. 1. 



but on opposite sides of the vertex L Now, imagine Venus 

 to come along, gaining on the earth, and passing (as she 

 does when there is a transit) through the double cone, at 

 the points 1, 2, 3, and 4 ; 1 being the point where she 

 first reaches the outer surface of the exterior cone; 4, 

 the point where she finally leaves that cone ; while 2 and 3 

 are the corresponding points for the inner cone. 



Now, a little consideration will show that when Venus 

 reaches the point 1, the transit begins, but only for that 

 jiarticular point where the line from s' to 1 produced 

 touches the earth. Thi'v the transit begins earliest of all. 

 As Venus passes from 1 to 2, transit begins for difl'erent 

 ]ilaces on tlie earth, until when Venus is at 2, transit begins 

 from just that point where the line from s' to 2 produced 

 touches the earth. The face of the earth turned towards 

 the sun during the short time (less than half-an-hour), 

 during which Venus passes from 1 to 2, does not change 

 much ; so that one may say that over a hemisphere of the 

 earth the beginning of the transit is seen, but for about 

 half of that hemisphere the beginning is seen earlier and 

 over the other half later than from the earth's centre (if 

 we could imagine an observer, stationed at that incon- 

 venient spot, to be able to watch the transit). 



After passing 2, some six hours elapse before Venus 

 conies to 3, when the end of the transit occurs at its very 

 earliest for the station where s3 produced touches the 

 earth ; and then, as Venus passes from 3 to 4, the end is 

 seen at different stations on the earth ; until, finally, when 

 Venus reaches 4, the end is seen at its very latest from 

 that point where the line s4 produced touches the earth. 



transit, is hastened or delayed (Ingi-ess Accelerated or 

 Retarded, marked "I A so many minutes," or " I R so 

 many minutes " ). 



Over the region 2, a series of dotted lines show how 

 much the Egress of Venus, or the end of the transit, is 

 hastened or clelayed (E A or E II). 



Then, over the region whence the whole transit can be 

 seen, a series of heavy lines show how much the duration 

 exceeds (-f ) or falls short of ( — ) the average. 



The places where the duration is greatest and least are 

 marked by heavy black dots, beside one of which is marked 

 the maximum amount of shortening of the duration. (The 

 other being on a part of the earth whence none of the 

 transit can be seen, is not so marked.) 



The map is very easily interpi-etcd : — 



Thus, in England, say London, we see that only the first 

 part of the transit can be seen, and not much of that, the 

 transit beginning shortly before sunset : ingress is retarded 

 about five minutes. At New York the whole transit can 

 be seen : ingress is retarded nearly eight minutes, and 

 egi-ess accelerated rather more than seven minutes, duration 

 falling short of the mean nearly fifteen minutes. 



And similarly for any other places where either the 

 beginning, the end, or the whole transit can be seen. 



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