337 



TRANSITS OF MERCURY AND VENUS. 



TRANSLATION. 



of the observation* and computations may be judged of from the 

 printed Observations, 



If an architect should bare to construct a first-rate observatory, we 

 ahoukl advise him, after learning what initnimenU are to be accom- 

 modated, to itudy each of these observatories, and arrange the rooms 

 in the most convenient manner. The beat appearance which the case 

 admits of may be given afterwards, but he should not be very rigorous 

 a* to ouUide symmetry. It is scarcely possible to unite convenience 

 a* an observatory with a regular exterior, except at a considerable 



TRANSITS OF MERCURY AND VENUS. The inferior planets 

 aa they are called, whose orbit* are within that of the earth, may some- 

 times appear to pan over the body of the sun, eclipsing, by their 

 opacity, successive parts of the solar surface. The transits of Mercury 

 and Venus are phenomena of this kind. They do not take place very 

 often, aa they can only be when the planet is in or very near to the 

 node of it* orbit, at the time when a line drawn through the sun's 

 centre and that node passes through the earth. As the nodes of the 

 planets alter their positions on the ecliptic very slowly, it will happen 

 for many centuries together that these appearances can only take place 

 at stated periods of the year in which they happen. A transit of 

 Mercury is always either in May or November (according to the node 

 of the orbit at which it takes place), and one of Venus in June or 

 December. The first transit of Mercury which was observed took 

 place in 16S1, and of Venus in 1639, and the following are the dates 

 of those which have occurred since, or will occur for a long time to 

 come : 



TRAXUT* or MERCCBT. 



NOT. 11, 1815. 



Nor. 4, 18]]. 



May 5, 183]. 



NOT. 7, 18J5. 



Mar 8, 1845. 



NOT. 9, 1848. 



NOT. 11, 1861. 



NOT. 4, 1808. 



Mar 6, 1878. 



NOT. 7, 1881. 



May 9, 1891. 



NOT. 10, 1891. 



Of these the transits yet to come in 1861, 1868, 1878, will be visible 

 (weather permitting) in this country. 



Dc. 4, 1636. 

 June i, 1761. 

 June 3, 1769. 



TRANSITS OF VKKCS. 



Die. 8, 1874. 

 Deo. 6, 188]. 



June 7, 1004. 

 June 5, 2012. 



Of these the transits of 1882 and 2004 may be visible in this 

 country. 



The use of these transits is threefold. First, they may be employed 

 in correcting the tables of the planet in question ; secondly, in com- 

 puting the longitude of the place of observation ; thirdly, in finding 

 the actual distances of the planet and of the sun from the earth. The 

 first of the uses is shared by the transits with many other kinds of 

 observations, and as they occur so seldom, it is fortunate that in this 

 respect they are by no means indispensable ; and the same may be said 

 of the second use. As to the third, the transits of Mercury, which 

 occur with tolerable frequency, are comparatively useless, from the 

 difficulty of the observation ; but the transits of Venus are more avail- 

 able, and furnish our most precise mode of ascertaining the distance of 

 the earth from the sun. 



It cannot be shown prtcudy to any but a mathematician, how it is 

 that the observation of a transit of Venus at several different places on 

 the earth's surface is made to answer the above purpose. The phe- 

 nomenon itself obviously resembles an eclipse of the sun, as distin- 

 guished from one of the moon ; and is affected in its progress by the 

 rotation of the earth. If a spectator were placed at the earth's centre, 

 he would see Venus pass over a certain line on the sun's disc, traced 

 out by a moving line the end of which is in his own eye, and which 

 paste* through the centre of Venus. Whenever this line passes through 

 the sun's surface, Venus will appear to be projected on that surface as 

 a dark spot. At the same time a spectator on the surface of the earth 

 will refer the spot to a different point of the sun's surface, and the 

 thing to be noted is, that the difference of the lines which Venus 

 appears to pass over on the sun's surface depends jointly on the spec- 

 tator's place and the positions of Venus and the sun. The formula by 

 which the time of transit is connected with these two things the spec- 

 tator's place, and the position of Venus and the sun points out that 

 the difference of the apparent beginnings and endings of the transit at 

 different places on the earth's surface depends entirely on the difference 

 of the distances of the sun and Venus from the earth (as is sufficiently 

 evident without the formula ; for if the planet were at the same dis- 

 tance from us a* the sun, that is, if it really passed over the body of 

 the sun, the phenomena would not be sensibly different at any two 

 part* of the earth). The distances of the sun and planet enter into 

 the formula; by means of their parallaxes [PARALLAX] ; and if the 

 difference of the parallaxes is once known, the parallaxes themselves 



are known, for the proportion of the distances of the earth tad Venus 

 from the sun is sufficiently well ascertained from Kepler's laws. At a 

 place of known longitude, if only the ingress or egress of the planet be 

 observed, either of the two, and if the phenomenon can be calculated 

 as it would be seen from the centre of the earth, then the difference of 

 the parallaxes can be found ; but this supposes, first, that the longitude 

 of the place is very well known ; secondly, that the errors of the tables 

 of the sun and Venus are insensibly small. If both the beginning and 

 ending of the transit can be observed at one place, it is no longer of 

 any consequence that the longitude of the place should bn so accu- 

 rately known ; an approximate determination of it will be sufficient. 

 Still the errors of the planetary tables remain. But if both beginning 

 and ending of the transit can be observed at two different places (and 

 the greater their difference of longitude the better), then the differences) 

 of the parallaxes can be computed from the two observed durations of 

 the transit, independently both of the longitudes of the places and of 

 the planetary tables, that is, so as not to be rendered seusibly inaccu- 

 rate by any moderate inaccuracy in either. The fact is, that when the 

 transit is observed at one place only, the formula; suppose it to be 

 known at some other, either the centre of the earth, or Greenwich or 

 some other observatory. When the transit is observed at two places, 

 the second observation is inserted instead of the computed substitute 

 for observation. This explains why it was that expeditions were sent 

 by different governments to different parts of the globe to observe the 

 transits of l"t>l and 1769. If the transit should be seen both at its 

 beginning and ending at six different places, every pah- of them (and 

 there are 15 pairs) would give a determination of the difference of 

 parallaxes, and the mean of all the results would be entitled to a high 

 degree of confidence. 



The first transit of Mercury that was ever observed was seen by 

 Oassendi, November 6, 1631 ; the first transit of Venus by Horrocks, 

 December 4, 1639. Halley pointed out the use of such transits, and 

 preparation was made to observe that of 1761. Legentil was sent to 

 India, Chappe to Tobolsk, and Pingrd to the island of Rodriguez, by 

 the French government ; Maskelyne went to St. Helena, and Mason to 

 the Cape of Good Hope (he intended, but was prevented, to go on to 

 Sumatra). The weather hindered or injured most of the observations; 

 the most fortunate was that of Mason, who made the sun's parallax 

 eight seconds and a half. The transit of 1769 was still better an 

 to. The complete duration of the transit was observed at Cape \V. ,!!- 

 bus, Kola, Cajaneburg, O-taiti, Fort Prince of Wales on the north-west 

 coast of Hudson's Bay, St. Joseph, and Santa Anna in California. The 

 ingress of the planet was seen at almost all the obscrvatorii 

 the egress at Petersburg, Yakutsk, Manilla, Batavia, Pekin, Cmi-i, 

 Orska, and Orenburg. The value of the parallax was variously de- 

 duced, different astronomers preferring different values, from 8 "'5 to 

 8"-8 : Laplace used 8"-66 ; M. Encke deduced 8"'5776 from all the 

 observations. De Ferrer (' Mdm. Astron. Sue.,' vol. v., p. 253), from a 

 re-examination of the whole observations, deduces 8 "'68, and thinks 

 this cannot be wrong by so much as j&ths of a second. 



Most of the observers who saw the ingress of the planet unite in 

 stating that after the planet had entered on the sun, it continue; 

 short time to appear as if it were joined to the limb or border of the 

 sun by a dark protuberance or ligament (some call it a thread). ThU 

 phenomenon appears to be of the same sort as that noticed in SUN with 

 respect to the annular eclipse. A full account of what was seen with 

 respect to Venus will be found in Mr. Baily's paper there referred to. 

 (' Mem. Astron. Soc.,' vol. x., p. 1.) 



TRANSLATION. This word is used in mechanics, as distinguished 

 from ROTATION, in the following manner : A body has motion of 

 translation when all its points move in parallel straight lines ; when, in 

 fact, all its points have the same motion. If all have not the same 

 motion, there is either simple rotation, that is, about one permanent 

 axis ; or rotation about a varying axis ; or else a compound of trans- 

 lation and rotation. 



The point which is called the centre of gravity of a system, and 

 which is of no small importance in the theory of equilibrium, has yet 

 more in that of motion. The motion of any free system is com- 

 pounded of the translation of its centre of gravity, and the rotation 

 about an axis (whether always in one direction or not) passing through 

 its centre of gravity. Now whatever the forces may be by which such 

 a system is either set in motion, or acted on while in motion, the 

 translation of its centre of gravity may always be made a distinct 

 problem from the rotation about its centre of gravity, by the foil 

 simple rules : 



1. The centre of gravity moves just as it would do if the whole 

 system were there collected, and all the forces were there a|> 



2. The rotation of a system about its centre of gravity is no other 

 than what it would be if that centre were made a fixed point, and all 

 the forces applied in their proper places. 



Suppose, for instance, a bar A n, whose centre of gravity is nt c, is 

 sent spinning into void space by a certain blow in the direction o x, 

 communicated at D. Let there be another similar bar ab, whose 

 centre of gravity c moves on a fixed pivot without friction, and which, 

 being parallel to A B, is struck at the same instant with a similar blow 

 iu the direction de. To find the position of the bar at the end of 

 any given time, say three seconds, is a twofold problem, as follows : 

 First suppose all the mass of the bar concentrated at c, and let the 



