Transit, in astronomy, the passage of a planet across the disk of .the sun, or of a satellite across the disk of its primary; also, the passage of a heavenly body across the meridian of the place of observation, sometimes called its culmination. Of the planets, only Mercury and Venus, having orbits within the orbit of the earth, can present this phenomenon. The transits of Venus are employed for the determination of the sun's distance; they recur at alternate intervals of 8 and 105½, and 8 and 121½ years. The earliest transit of the sun's disk of which we have an account is that of Venus in 1639, predicted and observed by Jeremiah Horrox, an amateur astronomer of Lancashire, England. The transits of the last century, in the years 1761 and 1769, were observed with great care, expeditions having been equipped for the purpose by the chief European states. But the results then obtained were not so trustworthy as had been anticipated. Two methods of observation were relied on, both depending on time, though not in the same way.
It had been suggested by Halley, early in the century, that instead of observing the position of Venus on the sun's face at any assigned instant (for the purpose of thence determining her relative parallactic displacement and so her distance), the observers should note the interval of time occupied by the planet in completing her transit. As the effect of parallax would be to cause her to traverse different chords, as seen by observers at northern and at southern stations, there would result a difference in the duration of transit, the amount of which would enable astronomers to deduce the sun's distance. De-lisle, when the transit of 1761 was approaching, discovered that there would be on that occasion disadvantages in applying Halley's proposed method, which requires that both the bednninc: and end of the transit should be seen; and he proposed another method, requiring only that one or other of these phases should be noted. According to this plan, two observers were both to note the beginning (or else both to note the end), one observing the phase where it occurred as early as possible, and the other observing it- where it occurred as late as possible; then, by noting the difference of time between their two observations, they would be able to estimate the sun's distance.
Halley's method was manifestly the easier, since each observer had to note the duration between two phenomena both of which were observed by him, and the difference between the two durations thus noted could be determined at once; whereas in Delisle's method each observer had to determine the absolute time of a single phenomenon, and a comparison between their results could only be effected satisfactorily if these results could be referred to some common standard time of reference, as Greenwich or Paris time. But in the actual application of both methods another difficulty obtruded itself into notice. It was found that the moment when Venus was in internal contact, either at ingress or egress, could not be determined, as Halley had hoped, within a single second, or indeed within several seconds. Accordingly doubt had long rested on the determination of the sun's distance obtained from the observations made in 1761 and 1769. In fact, from the first, the results were found to be widely discordant according to the manner in which the observations were interpreted.
The values of the sun's distance deduced from the transit of 1761 ranged from 77,846,000 m. to 96,163,000 m.; those deduced from the transit of 1769, though not ranging quite so widely, yet differed by more than 4,000,000 m., the greatest being as before 96,163,000 m., the least 92,049,650 m. Strangely enough, all this was forgotten when (after Encke had published his result from the combination of both series of observations, viz., 95,265,000 m.) a long period had elapsed during which the text books and ephemerides had published the same value for this important element. Accordingly, much surprise was expressed when other methods of observation showed that this value so long received was too great by three or four million miles, the true value appearing to be nearer 92,000,000 m. Although this surprise was by no means justified by the facts of the case, yet it was natural that much attention should be attracted to the transits of 1874 (Dec. 8) and 1882. Accordingly great preparations were made for the observation of the earlier transit, the United States in particular taking a distinguished part in the work. It has been estimated that nearly $1,000,000 must have been expended on the various expeditions.
Stations were occupied in Siberia, China, Japan, the Hawaiian islands, northern India, Persia, Turkis-tan, and Egypt in the northern hemisphere, and at a number of islands in the Indian and Southern oceans, from Kerguelen on the east to Chatham island and New Caledonia on the west, Australia, Tasmania, and New Zealand being also occupied in force. Various success attended the observers, but on the whole the results obtained were excellent. Delisle's method and Halley's, the heliometric method, and photography were applied at many of the most important stations; and though bad weather prevailed at other stations, the object of the expeditions was achieved. So far as can be judged at present, the sun's distance indicated by these observations is about 92,000,000 m. The next transit of Venus will occur Dec. 6, 1882, and is looked forward to with great interest for verifying these determinations. - The transits of Mercury are much more frequent than those of Venus, in consequence of the former planet being nearer the sun, and having thus a narrower orbit and a shorter year; but they are not available for the determination of the solar parallax.
The transit of stars is employed in the determination of longitude. (See Longitude.) The precise relative situation of the heavenly bodies in respect to their right ascension is determined by comparing their exact times of transit. For the means by which these times are ascertained see Transit Circle.