Royal Imtitution. 509 



vi! Proceeding first to explain the simple causes of a solar eclipse, the 

 Lecturer remarked that the moon's distance from the earth is nearly 

 one four-hundredth part of the sun's distance, and that the moon's 

 diameter is very nearly one four-hundredth part of the sun's dia- 

 meter, and that therefore, on the average, the sun's apparent 

 diameter and the moon's apparent diameter are very nearly equal. 

 But in consequence of the elliptic forms of their orbits, the sun's 

 distance is liable to small variations, and the moon's distance to 

 very considerable variations : when the moon is at the most distant 

 part of her orbit, her apparent diameter is smaller than the sun's, 

 and if she happens at that time to be between a spectator and the 

 sun, she will be seen as a black disc covering the central part of 

 the sun and leaving a ring of light all round : when the moon is at 

 the nearest part of her orbit, her apparent diameter is larger than 

 the sun's, and she will, to a spectator in the proper locality, com- 

 pletely cover the sun, and produce a total eclipse. But neither of 

 these things can happen unless the plane of the moon's orbit be in 

 such a position that the moon, when approaching the state of con- 

 junction or new moon, is seen to pass not above the sun or below 

 the sun, but over the sun. 



The Lecturer then called attention to the circumstance that four 

 successive total eclipses occur in the month of July at intervals of nine 

 years, namely 1S33, July 17; 1842,July8; 1851, July28; and 1860, 

 July 18. For the explanation of this curious circumstance it was 

 necessary to show, first, how it happened that at intervals of nine 

 years the moon's orbit was in such a position that, for a nearly definite 

 apparent position of the sun, the moon's path would cross the sun's 

 disc ; secondly, how it happened that at intervals of nine years the 

 moon was at nearly her smallest distance from the earth, so that her 

 apparent diameter was larger than the sun's. In reference to the 

 former, it was shown that the moon revolves in au orbit whose plane 

 is inclined to the plane of the ecliptic (the apparent orbit of the sun 

 round the earth), and that the inclination is nearly invariable, but 

 that the position of the line in which the plane of the moon's orbit 

 intersects that of the ecliptic is constantly changing, revolving steadily 

 in the direction opposite to the moon's motion, and performing a 

 complete revolution in something more than nineteen years. There- 

 fore if one node or extremity of this line of intersection were directed 

 nearly to the July sun in 1833, the opposite node would be directed 

 nearly to the July sun in 1842, and so on for four successive periods 

 of nine years ; and eclipses would be possible in July at the end of 

 each period. But to show that they might be total eclipses, it was 

 necessary to remark that the moon revolves in an ellipse of which the 

 earth occupies one focus (a point much nearer to one end than to the 

 other), and that the position of this ellipse is constantly varying, 

 its long axis turning round in the same direction as the moon's 

 motion, and completing a revolution in nine years and a half. There- 

 fore if in 1833 the shorter end of the ellipse were nearly turned to 

 the July sun, in 1842 the axis of the ellijjse would have completely 

 revolved, 80 that the shorter end of the ellipse would again be nearly 



