Transit, in astronomy → , specifically describes a planet passing over the sun.

The transit of the moon in front of a star is called more specifically the occultation of this star by the moon. See Occultation.

In their transits across the sun, Mercury and Venus appear as black or dark spots.

The transits of Mercury across the sun are quite frequent; since the invention of large telescopes, which is to say since 1610, [1] eleven have already been observed. See Mercury. Gassendi was the first to observe one in Paris on November 7, 1631. As this philosopher said, this observation was according to Kepler’s wish and warning, for Kepler had predicted this transit and published or written about it in 1630, the year of his death. [2] It is true that Kepler had reported in his Optics, [3] referencing an old account of Charlemagne’s life, that in 807 or 808 the planet Mercury appeared in the sun as a black spot for eight days. This, however, is evidently false or questionable since Mercury cannot remain at the most five or six hours over the solar disc. Furthermore, according to Kepler, even though octoties, eight times must be read instead of octo dies, eight days, it is known today that it is not possible that in such a short time Mercury passed eight or even two times over the sun. Thus, it appears that a large sunspot had appeared and been mistaken for Mercury. Three other transits of Mercury across the sun allegedly occurred in 1615, 1618, and 1628, all visible from some place on earth, and that of 1618 could even be seen from various places in Europe. But either astronomers were not at this point in time fully informed about the theory of Mercury, or they were not certain enough of these phenomena to prepare to observe them, and even less to search for them in distant countries. The English astronomer Shakerley was the first to go to Surat in 1651expressly to observe a transit of Mercury across the sun, which would only happen at night in Europe. [4] This was the second of nine observations; it was followed by six others in 1661, 1677, 1690, 1697, 1723, 1736, 1743, 1753, and finally the last one was in 1756. We would have had five or six more had others been as zealous as Shakerley.

All of Mercury’s transits of the solar disk fall at the start of May or November, and their recurrences have thus far have fallen within these limits. [5] These recurrences have periods of different lengths, of six to seven years, of ten years, of thirteen years, etc., [6] but all the same they recur after a specific number of years in conformity with the theory of Halley, the first person to have studied this subject deeply. [7]

The transits of Venus are not nearly as frequent. It appears that the first to be observed was on December 3, 1639, reported in Bouillaud’s philolaic astronomy → . [8] Another one occurred in 1761, and M. Halley alerted astronomers to prepare for it because of its utility in determining the parallax of the sun. [9]

The observations of the transits of Mercury and Venus are very useful for determining different points in the theory of these planets. One finds in Monnier’s Institutions Astronomiques a paper by M. Picard on this subject. Histoire de l’académie des Sciences (1743), and the Institutions of M. le Monnier. [10] See Mercury and Venus.

Transit also describes a planet or a star passing by the meridian, or by some other circle. See Culmination and Meridian.

1. Galileo Galilei was the first to use a telescope in 1609 to view celestial objects, though his first telescope had only a maximum magnification of 8x. By January of 1610, he had developed a more powerful 20x telescope with a large enough field of vision to view many celestial objects at once.

2. Johannes Kepler (1571-1630) published the Rudolphine Tables  in 1627 which detailed logarithmic calculations and the perpetual tables used to calculate planetary positions for any past or future dates. Using these tables, he predicted the upcoming transits of Mercury and Venus.

3. Johannes Kepler, Astronomiae Pars Optica [ Optical Part of ← Astronomy → ] (1604). In reference to the astronomical events of 807, Kepler calculated that the next transit of Mercury since 807 would occur on May 28, 1607.

4. Jeremy Shakerley was the second person after Gassendi to observe a transit of Mercury in Surat, India.

5. Mercury’s transits occur within an interval of a few days around either May 8 ^th or November 10 ^th .

6. Mercury’s May transits recur in different intervals than those that fall in November. November transits recur in 7, 13, or 33-year intervals, and May transits recur in 13 and 33-year intervals.

7. Edmond Halley (1656-1742), English astronomer after whom Halley’s Comet is named.

8. Ismail Bouillaud (1605-1694), Astronomia Philolaica (1645).

9. Halley was the third person to observe a transit of Mercury in 1677, and he was able to view one in all of its phases. It was after this observation that he devised a method of determining the parallax of the sun utilizing Mercury’s and Venus’ transits.

10. Pierre-Charles Le Monnier (1715-1799), Institutions astronomiques (Paris, 1746). According to a handwritten note inside this copy, this is in part a translation of John Keill (1671-1721), Introductiones ad veram Physicam et veram astronomian (London, 1725). Wikipedia calls it an “improved translation.”