TRANSITS OF MERCURY AND VENUS. The inferior planets as they are called, whose orbits are within that of the earth, may some times appear to pass over the body of the sun, eclipsing, by their opacity, successive ports of the solar surface. The transits of Mercury and Venus are phenomena of this kind. They do not take place very often, as they can only be when the planet is in or very near to the node of its orbit, ?t 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 Juno or December. The first transit of Mercury which was observed took place in 1631, and of Vent l in 1639, and the following are the dates of those which have occurred since, or will occur for a long time to Como :— Of these the transits of 1882 and 2004 may be visible in this country.
Tho 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 iu 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 precisely 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 passes through the centre of Venus. Whenever this lino 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 pas over on the sun's surface depends jointly on the spec tater'. 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 as the sun, that la, if it really passed over the body of the sun, the phenomena would not be sensibly different at any two parts of the earth). The distances of the sun and planet enter into the formulae by means of their it:wall:ma [Pattatsstx]; and if the difference of the parallaxes is once known, the parallaxes themselves are known, for the proportion of the distances of the earth and Venus from the sun is sufficiently well ascertained from Kepler'a 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 end Venue 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 be 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 sensibly 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 twn places, the second observation is inserted instead of the computed substitute for observation. This explains wby it was that expeditions were sent by different governments to different parts of the globe to observe the transits of 1761 and 1769. If the transit should be seen both at its beginning and ending at six different places, every pair 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 over observed was seen by Gassendi, 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 Iudia, Chappe to Tobolsk, and Pingre to the island of Rodriguez, by the French government ; 31askelyno 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 attended to. Tho complete duration of the transit was observed at Cape Ward hus, Kola, Cajaneburg, 0-taiti, Fort Prince of Wales on the north-west coast of Hudson'e Bay, St. Joseph, and Santa Anna in California. The ingress of the planet was seen at almost all the observatories of Europe; the egress at Petersburg, Yakutsk, Manilla, Batavia, Pekin, Gurief, 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. Eneko deduced S"•5776 from all the observations. De Ferrer (‘ 316m. Astron. Soc.,' voL v., p. 253), from a re-examination of the whole observations, deduces 8"-58, and thinks this cannot be wrong by ao much as Aths 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 continued for a 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). This 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. Rally's paper there referred to. (` M6m. Astron. Soc.,' vol. x., p. 1.)