Written on Mercury's transit of 1631

Kepler 1629, Admonitio [[ Kepler-Opera-7 p.589-597 ]]

Using his Tables Rodolphines (1627), Johannes Kepler (1571-1630) published a "Warning dedicated to Astronomers" (Admonitio ad Astronomos) in which he announced two transits of planets for the year 1631 : Mercury (November 7), and Venus (December 6). The results of his calculations are presented page 591 of the present edition by C. Frisch (Kepler Opera Omnia, vol 7, 1876).

The transit of Mercury was observed in Paris by Gassendi. Kepler predicted a conjunction of Venus and the Sun at 9 :41pm on December 6, 1631 in the Gregorian calendar (which corresponds to November 26 in the Julian calendar, ancient style). The time is in Ura­niborg mean time (in the Hven island, close to Copenhague, where Tycho Brahé constructed an observatory). In reality, the conjunction happened 9 hours later; the nearly grazing transit was observable only from Asia.

Hortensius 1633, Dissertatio [[ 21319-(001à095)]]

The Dutch Martinus Hortensius (1605-1639), also known as Maarten Van den Hove, was a student of Willebrord Snell (1580-1626). He corresponded with numerous scientists such as Galilée (1564-1642) and Gassendi (1592-1655). He writes here some “comments” (or Dissertatio) about Mercury transit observations performed by Gassendi in November 1631, who was not able to observe the Venus transit of December 1631 (predicted by Kepler, but not visible from Europe).

The text about Venus "not seen" (Venere invisa) can be found p. 78. In order to compare predictions from several astronomical tables (LANSBERGE (1561-1632), Kepler), Hortensius described other observations : some eclipses, a Saturn occultation as well as a very close conjunction (3') of Mercury and Venus on the morning of July 31st, 1632 (p. 90).

[21319] HORTENSIUS Martinus, Dissertatio di Mercurio in sole viso et Venere invisa, instituta cum clarissimo, ac doctissimo viro, D Petro Gassendo, 1633, in 4°, Latin text (96 p.)