TRANSITS. The passage of a heavenly body between the observer and another more distant body of larger apparent surface is called a transit. The most frequent phenomena of this kind occur in the case of the satellites of Jupiter. The latter is many times larger than any of its satellites and it happens very fre quently that an observer with a good telescope can see the passage of a satellite of Jupiter passing over the disc of the planet. The times of these transits are predicted in the astronom ical ephemeris. The transits of the inner satel lites occur at nearly equal intervals of one day hours, and it takes about 2 hours and 27 minutes for the satellite to cross. The tran sit can be observed only when Jupiter is above the horizon and the sun below it, so that only about one transit in five is visible at any one place. Since the satellite is an opaque body, it casts a shadow which may be thrown upon the planet. To an observer on the latter, if the shadow passed over his position, there would be a total eclipse of the sun. The shadow appears to us as a small dark spot passing over the planet near the position of the satellite.
The satellites of all the other planets are either too small or too distant to admit of their transits being observed. For the most part they are entirely obliterated to our sight by the brilliant light of the planet itself when they ap proach the latter.
The planets Mercury and Venus, having or bits inside that of the earth, will be seen in transit across the sun whenever they pass in a direct line between the earth and sun. If the planes of their orbits coincided with the eclip tic, this would happen at every inferior con junction of the planet. But, as a matter of fact, there is a certain inclination of each of the orbits to the ecliptic. Imagining the latter plane to surround the sun, extending out to the earth, the orbits of the inferior planets each intersect this plane at a small angle at two op posite points. The line adjoining these points passes through the sun and is called the line of the nodes. If, when the planet is in inferior conjunction, the earth happens to be on or near this line, a transit of the planet will be seen across the sun's disc. In the case of Mercury the earth passes the line of nodes about 8 May and 10 November of each year. It is only within a few days of these times that transits of Mercury can be seen. When such a transit
does occur, we must generally wait several The earth passes the line of nodes of Venus on 6 June and 6 December of each year. But it very rarely happens that Venus is so near the node on these dates that a transit will be seen. Accordingly the transits of this planet occur at much longer intervals than those of Mercury. For many centuries past and to come there are four transits in every 243 years. The condition which governs their recurrence is that 13 revo lutions of Venus require almost exactly eight years. The result is, when Venus and the earth happen to pass a node at nearly the same time, they will both pass nearly the same point eight years afterward. But at the end of the second interval of eight years the conjunction will occur so far from the node that no transit will be visible. The result of this is that the transits occur in pairs, eight years apart. The interval between the last transit of one pair and the first of a pair following is either 105% years or 121% years. The dates of these transits for several centuries past and to come are as follows: 1631 December 7 1882 December 6 1639 December 4 2004 June 8 5 2012 tine 6 3 2117 mber 11 1874 mber 9 2125 December 8 It will be seen that the whole 20th century will pass away without the inhabitants of the earth having an opportunity to observe this phenomenon. But the approach of the transits of 2004 and 2012 can be watched in thought through successive cycles of eight years. An inferior conjunction of Venus occurred on 8 July 1900. At that time the earth, having passed the node on 5 June, was 34 degrees distant from it. In consequence, there was no transit, but Venus, could it be visible so near the sun, would have been seen passing below the sun. Eight years later the same thing will repeat it self, only the conjunction will take place be tween two and three days earlier, namely, on 5 July 1908, the earth being between two and three degrees nearer to the node than it was on 8 July 1900. The conjunction will go on repeating itself in 1916, 1924, etc., two or three days earlier at each repetition and a little nearer to the node, until 2004, when there will be a transit. At the conjunction of 1908, the earth will be on the opposite side of the node, but near enough to it for another transit. Then there will be no more transits for more than a century.