The most favorable times for making observations are about an hour and three-quarters before sunrise in autumn, and after sunset in spring; but very clear weather and a good eye are required. At certain periods, when between the earth and the sun, on a line joining the centers of the two bodies, Mercury appears projected on the solar disk as a small, round, dark spot. This is called a _transit_, and would occur during every revolution if the plane of his orbit coincided with that of the orbit of the earth. But as one-half of his orbit is a little above that of the earth, and the other half a little below it, he passes above or below the sun to the terrestrial spectator, except at those intervals, when, being between us and the sun, he is also at one of the two opposite points where the planes of the respective orbits intersect each other. Then, stripped of all luster, the planet passes over the face of the great luminary as a black circular speck, affording evidence of his shining by reflected light, and of his spherical form. The first transit of Mercury recorded in history was predicted by Kepler, and witnessed by Gassendi, at Paris, on the morning of a cloudy day, the 7th of November, 1631. It was toward nine o’clock when he saw the planet; but owing to its extreme smallness, he was at first inclined to think it was a minute solar spot. It was then going off the sun; and made its final egress toward half-past ten. The observed time of the transit was nearly five hours in advance of the computed time. This was a very satisfactory accordance for that age between theory and observation; but it was the effect of a fortuitous combination of circumstances, rather than of computations founded upon well-established data. “The crafty god,” wrote Gassendi, in the peculiar style of his day, “had sought to deceive astronomers by passing over the sun a little earlier than was expected, and had drawn a veil of dark clouds over the earth in order to make his escape more effectual. But Apollo, knowing his knavish tricks from his infancy, would not allow him to pass altogether unnoticed. To be brief, I have been more fortunate than those hunters after Mercury who have sought the cunning god in the sun. I found him out, and saw him where no one else had hitherto seen him.” Since Gassendi’s time a number of transits have been observed. These crossings of the sun’s face, or “transits,” as they are called, have been important matters. The transit of Venus especially was once eagerly looked for by astronomers, since, by close observations of Venus’s movements and positions, the distance of the sun could at that time be better calculated than in any other way. Other methods are now coming into vogue.