case is lost in the surrounding brightness. Kepler left the suggestion, therefore, of the use of the latter's transit, for the benefit of the future generation in which it should occur. The parallax even of Mars turned out to be, with the means of that day, immeasurably small; but he reached from this the conclusion that the sun's still unknown distance was, at any rate, not less than 13,000,000 miles.
To see how it is that transits are so rare, we may consider the annexed diagram (Fig. 2), where the outer circle shows the orbit of the earth, and her positions in March, June, September, and December. The orbit of Venus, lying within this, would need to be represented by a ring, inclined to the plane in which the earth moves; that part of Venus's path nearest to the earth in March being above the surface of the paper, that nearest to our place in September being below it. If the planet passed in line with us and the sun between December and June, then it would appear to go above it; if between June and the following December, below it. There are two days in each year when we are crossing the line in which the planes of the two orbits cut each other. At these times the path of Venus, if it were a visible ring, would be seen like a slanting line on the sun; but, as the planet may be anywhere else on her path (as, for instance, at V2), it is evidently only under a rare conjunction of favoring circumstances that we see her passage across the sun's face (at V1), as a black circle on a brilliant background. This phenomenon, which can only, as appears from what has been said, occur in June and December, is known as the Transit of Venus.
Fig. 2.—Orbits of Venus and the Earth.
Owing to the fact that Venus makes about thirteen revolutions in eight years, her transits frequently come in pairs eight years apart, though with an interval of over a century from one pair to the next; and thus transits have occurred in December, 1631 and 1639, in June, 1761 and 1769, and will occur in December, 1874 and 1882. That of 1631, though predicted by Kepler, passed unobserved; that of 1639 was the first known to have been seen by any one, and the circumstances of this epoch in the history of our subject deserve mention.
Jeremiah Horrocks, a young man, devoted to astronomical studies, though without counsel or support, had found from his own computations that a transit was likely to occur, though none had been looked
