SECTION II

Summary

In the year 1847, Dr. C. L. Gerling, a distinguished mathematician of the Marburgh University, suggested the importance of a new determination of the sun's parallax by observations upon Venus at and near her stationary periods. The determination of the dimensions of the solar system rests entirely upon the assumed value of the sun's parallax. The value now generally received, viz., 8′′.57, rests upon the observations of the transit of Venus in 1769. Transits of Venus over the sun's disc afford the best method of determining this parallax; but these phenomena are of very rare occurrence, there being not a single transit visible in any part of the world from 1769 to 1874. Now, although the observations of the transit of 1769 are believed to have afforded a very accurate value of the sun's parallax, yet it is much to be regretted that the results obtained by combining the observations at different stations two and two, differ among themselves by an entire second. It is therefore very desirable that this result should be verified by independent methods. Such methods are found in simultaneous observations of either Venus or Mars from two remote points of the globe. If an astronomer in a high northern latitude observes the position of one of these bodies when upon his meridian, and another astronomer in a high southern latitude does the same, a comparison of these two observations will give the parallax of the planet, from which we can compute its distance from the earth.