The triumphant establishment of the Copernican doctrine dates from the invention of the telescope.  Soon there was not to be found in all Europe an astronomer who had not accepted the heliocentric theory with its essential postulate, the double motion of the earth—­ movement of rotation on her axis, and a movement of revolution round the sun.  If additional proof of the latter were needed, it was furnished by Bradley’s great discovery of the aberration of the fixed stars, an aberration depending partly on the progressive motion of light, and partly on the revolution of the earth.  Bradley’s discovery ranked in importance with that of the precession of the equinoxes.  Roemer’s discovery of the progressive motion of light, though denounced by Fontenelle as a seductive error, and not admitted by Cassini, at length forced its way to universal acceptance.

Next it was necessary to obtain correct ideas of the dimensions of the solar system, or, putting the problem under a more limited form, to determine the distance of the earth from the sun.

In the time of Copernicus it was supposed that the sun’s distance could not exceed five million miles, and indeed there were many who thought that estimate very extravagant.  From a review of the observations of Tycho Brahe, Kepler, however, concluded that the error was actually in the opposite direction, and that the estimate must be raised to at least thirteen million.  In 1670 Cassini showed that these numbers were altogether inconsistent with the facts, and gave as his conclusion eighty-five million.

The transit of Venus over the face of the sun, June 3, 1769, had been foreseen, and its great value in the solution of this fundamental problem in astronomy appreciated.  With commendable alacrity various governments contributed their assistance in making observations, so that in Europe there were fifty stations, in Asia six, in America seventeen.  It was for this purpose that the English Government dispatched Captain Cook on his celebrated first voyage.  He went to Otaheite.  His voyage was crowned with success.  The sun rose without a cloud, and the sky continued equally clear throughout the day.  The transit at Cook’s station lasted from about half-past nine in the morning until about half-past three in the afternoon, and all the observations were made in a satisfactory manner.

But, on the discussion of the observations made at the different stations, it was found that there was not the accordance that could have been desired—­the result varying from eighty-eight to one hundred and nine million.  The celebrated mathematician, Encke, therefore reviewed them in 1822-’24, and came to the conclusion that the sun’s horizontal parallax, that is, the angle under which the semi-diameter of the earth is seen from the sun, is 8 576/1000 seconds; this gave as the distance 95,274,000 miles.  Subsequently the observations were reconsidered by Hansen, who gave as their result 91,659,000 miles.  Still later,