184 JOURNAL OF THE PLYMOUTH INSTITUTION. 
ASTRONOMICAL MEASUREMENTS 
WITH SPECIAL REFERENCE TO THE TRANSIT OF VENUS. 
ABSTRACT OF MR. F. G. LANDON’S PAPER. 
(Read October 29th, 1874.) 
Tue determination of the distances of the heavenly bodies is not 
a question of practical utility, as the application of astronomy to 
navigation and kindred pursuits depends only on the relative 
distances and positions of the heavenly bodies, and the calculations 
of the nautical almanac can be made with the minutest accuracy 
without the absolute distances appearing as an element in the 
problem. Still the human mind can set itself no higher task than 
to search out the mysteries of the universe, of which our earth 
forms so minute a part; to send out a plummet into the vast depths 
of the universe, which shall not only tell us how far we are from 
our sun, but will in time help us to know the mighty sweep of its 
path round its far distant centre. The first unit of measurement 
to be determined is the diameter of the earth, the next the diameter 
of the earth’s orbit, and the last the diameter of the sun’s orbit. 
The first may be considered known; the second is resolved with 
approximate accuracy, and it’is the object of the approaching 
observations of the transit of Venus to render the approximation 
closer. The third can only be determined in the course of ages. 
The sun’s distance depends upon its parallax, which is the angle 
between two lines drawn from the sun, one to the eye of the 
observer, and the other to the centre of the earth. By Kepler’s 
law, the parallax of the sun or any planet being known, that of the 
others can be found. The parallax of the planets Venus and Mars 
can be found, and hence the parallax of the sun and other planets 
can be deduced. By the transit of Venus in 1769 the parallax was 
found to be 8”.57, which gave the sun’s distance 95,378,000 miles. 
But more reeent observations have suggested 8’.93 as the true 
parallax, which gives a distance of 91,538,000 miles. The means 
used for finding the distance of the sun fail when applied to the 
