HISTORY OF PHYSICAL ASTRONOMY.

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In his “ Supplement to Vitellionem,” Kepler remarked that from theposition of Mercury ’s nodes, it was possible that he might frequently passover the sun’s disk* * * § . With respect to Venus , he asserted that althoughshe might sometimes be interposed directly between the earth and sun,the instances of such an occurrence were very rare. He announced, infact, that a transit of this planet would not take place during the whole ofthe seventeenth century f.

The completion of the Budolphine Tables in the year 1627, havingenabled their illustrious author to calculate the motions of the inferiorplanets upon a more accurate basis than any hitherto employed by him,he now arrived at results of considerable precision relating to the times oftheir transits over the sun’s disk. In 1629 he published a small tract, inwhich he ventured to predict that both Mercury and Venus would passover the sun’s disk in the year 1631, the former on the 7th of November,and the latter on the 6th of December J. He announced at the same timethat there would not happen another transit of Venus before the year 1761.This, however, was a mistake, as we shall presently have occasion to showmore particularly.

The transit of Mercury, which Kepler had predicted, was actually wit-nessed by Gassendi , at Paris. He has given an interesting account ofhis observations in a letter to Shickhard, Professor of Mathematics in theUniversity of Tubingen . “The crafty god,” says he, “had sought todeceive astronomers by passing over the sun a little earlier than wasexpected, and had drawn a veil of dark clouds over the earth in order tomake his escape more effectual. But Apollo, acquainted with his knavishtricks from his infancy, would not allow him to pass altogether unnoticed.To be brief, I have been more fortunate than those hunters after Mercurywho have sought the cunning god in the sun. I found him out, and sawhim, where no one else had liitherto seen him.”§

Gassendi made preparations to observe the transit of the planet, byadmitting the solar light into a dark chamber through a small aperture inthe window, and receiving the image of the sun upon a white screen,having a circle described upon it adapted exactly to the magnitude of theimage. The diameter of the circle was three-fourths of a French foot inlength, and was divided into sixty parts, so that by supposing the apparentdiameter of the sun to be 30', each of the subdivisions would represent anapparent magnitude of 30". The circumference of the circle was alsodivided into 360 degrees. In order to obtain an accurate knowledge ofthe time, he placed a person in the room above him, with a quadrant oftwo feet radius, charging him to observe the altitude of the sun wheneverhe should hear him stamp with his foot. In order that the transit of theplanet might not escape him, he resolved to commence his observations aday or two earlier than that which Kepler had fixed for its occurrence.The 5th of November proved unsuitable for this purpose, being all dayrainy. The 6th turned out to be equally unfavourable, the sky beingovercast during almost the whole day. The 7th, which was the day ap-pointed for the transit of the planet, was very changeable in the morning,

* Ad Vitellionem Paralipomena, p. <306. f Ibid, p. 305.

t The following is the title of this small tract:—“ Admonitio ad Astronomosrerumque

celestium studiosos, de miris rarisque anni 1631 phsenomenis, Veneris puta et Mereuriiin solem incursu.” Lips., 1629.

§ Opera Omnia, tom. ii., p. 537.