Transits of Venus derive their astronomical celebrity from the belief entertained in former times that they afforded the best method of measuring the distance of the sun from the earth. The measures were made by the prin ciple of parallaxes. An observer, as far north as he could station himself to observe the tran sit, would see Venus pass over the sun's disc on a line apparently further toward the south than an observer who was in the Southern hemi sphere. The comparison of the observations made by observing the times which, at each station, it took Venus to complete its transit, afforded the means of calculating the parallax and the distance of the sun. The feasibility of doing this was pointed out by Halley about the middle of the 17th century, and, on his proposal, expeditions were sent by various na tions to points in the Northern and Southern hemispheres to observe the transits of 1761 and 1769. This was a period• of great unrest, and several of the expeditions became celebrated through the adventures to which they gave rise. In Mason and Dixon, the English astronomers, started on a ship of war for their station in the Southern hemisphere, but were at tacked by a French frigate and were compelled to return to nort after a severe battle. The king of Denmark sent Father Hell, a Jesuit astronomer of Vienna, to a point near the North Cape, where very successful observations were made. But doubts were thrown on the genuine ness of his record, which were not settled for more than a century. The transit of 1769 was visible in the Atlantic States and observations upon it were made under the auspices of the American Philosophical Society, held at Phila delphia. The most celebrated of the Phila delphia observers was David Rittenhouse.
When the observations of these transits were worked up, it was found that they would not give so certain a result as was anticipated. No
two observers seemed to agree as to the exact moment at which Venus had entered wholly upon the disc of the sun. The entrance was not seen in the sharp and precise way it should be seen, but seemed to be uncertain, through a dark haze forming on the two limbs at the moment when Venus was entering. The out come of the affair was that it was more than 60 years after the last transit before a result had been worked up from it which was sup posed to be quite satisfactory. This was done by Encke in 1822. The distance of the sun which he derived was, in round numbers, 95, 000,000 miles. This distance appeared in all astronomical textbooks and was almost uni formly accepted for 30 years.
Then it was found by Hansen and others that there was something wrong in this deter mination and it was claimed that the distance was more than 3,000,000 miles less than Encke had found it. The methods of making this determination are stated in ASTRONOMY, THEO RETICAL. It has since been shown very clearly that a great error did really exist in Enckc's determination, though it was not so great as had at first been supposed.
. Notwithstanding this failure of the method, it was supposed that with the greatly refined telescopes and better means of observation of recent times, the transits of 1874 and 1882 could he utilized advantageously in the same way. Accordingly, on each of these occasions, ex peditions, fitted out with the best instruments that science could provide, were sent by various nations to the best stations for observation in various parts of the world. But when the observations were worked up, the results were again found to be unsatisfactory and the obser vations turned out to be more useful for de termining the position of Venus, and the slight change from century to century of its node, than for determining the distance of the sun.