336 KANSAS CITY REVIEW OF SCIENCE. 



day. The scientific history of past transits was then noticed at length. When 

 the Ptolemaic theory of the solar system was in vogue, astronomers correctly be- 

 lieved Venus and Mercury to be situated between the earth and Sun, but as these 

 planets were supposed to shine by their own light, there was no reason to antici- 

 pate that they would be visible during a transit, if indeed a transit should occur. 

 Yet singularly enough, so far back as 8 07,. Mercury is recorded to have been seen 

 as a dark spot upon the face of the Sun. 



We now know that it is much too small to be visible to the naked eye in that 

 position, and the object observed could have been nothing less than a large Sun- 

 spot. Upon the establishment of the Copernican theory it was immediately per- 

 ceived that transits of the inferior planets across the face of the Sun must occur, 

 and the recognition of the value of transits of Venus for determining the solar 

 parallax was not long in following. The idea of utilizing such transits for this 

 purpose seems to have been vaguely conceived by James Gregory or perhaps 

 even by Horrocks, but Halley was first to work it out completely, and to him is 

 usually assigned the honor of the invention. His paper published in 17 16 was 

 mainly instrumental in inducing the Governments of Europe to undertake the 

 observations of the transit of Venus in 1761 and 1769, from which our first accu- 

 rate knowledge of the Sun's distance was obtained. When Kepler had finished 

 his Rudolphine tables, they furnished the means of predicting the places of the 

 planets with some approach to accuracy, and in 1627 he announced that Mercury 

 would cross the face of the Sun on November 7, 1631, and Venus on December 

 6th of the same year. The intense interest with which Gassendi prepared to ob- 

 serve these transits can be imagined when it is remembered that hitherto no s ich 

 phenomena had ever greeted mortal eye. He was destitute of what would now 

 be regarded as the commonest instruments. The invention of telescopes was 

 only twenty years old and a reasonably good clock had never been constructed. 

 His observatory was situated in Paris and his appliances were of the most primi- 

 tive kind. By admitting the solar rays into a darkened room through a small 

 round hole, an image of the Sun nine or ten inches in diameter was obtained up- 

 on a white screen. For the measurement of position angles a carefully divided 

 circle was traced upon this screen and the whole was so arranged that the circle 

 could be made to coincide accurately with the Sun. To determine the times of 

 ingress and egress, an assistant was stationed outside with a large quadrant, and 

 he was instructed to observe the attitude of the Sun whenever Gassendi stamped 

 upon the floor. Modern astronomical predictions can be trusted within a minute 

 or two, but so great did the uncertainty of Kepler's tables seem to Gassendi, that 

 he began to watch for the expected transit of Mercury two whole days before the 

 time set for its occurrence. On the 5th of November it rained, and on the 6th 

 clouds covered the sky almost all day. On the 7th toward nine o'clock the Sun 

 because distinctly visible, and turning to its image on the screen the astronomer ob- 

 served a small black spot upon it. He at first took it to be a Sun-spot, and shortly 

 he was surprised to see it had suddenly disappeared. After continued observation 

 Gassendi concluded, as he saw the spot on the Sun move, that it was really the 



