78 lyte Astronomer Royal on tlie 



moon's orbit be in such a position that the moon, when ap- 

 proaching the state of conjunction 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 1833, July 17; 1842, 

 July 8 ; 1851, July 28 ; and 1860, July 18. For the ex- 

 planation of this curious circumstance it was necessary to 

 shew, 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, tl^e moon's path would 

 cross the sun's disk : secondly, how it happened that at in- 

 tervals 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 

 shewn that the moon revolves in an orbit whose plane is in- 

 clined to the plane of the ecliptic (the apparent orbit of the 

 sun round the earth), and that the inclination is nearly in- 

 variable, but that the position of the line in which the plane 

 of the moon's orbit intersects that of the ecliptic is con- 

 stantly changing, revolving steadily in the direction opposite 

 to the moon's motion, and performing a complete revolution 

 in something more than nineteen years. Therefore, if any 

 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 pos- 

 sible in July at the end of each period. But to shew 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 Therefore, if in 1833 the shorter end 

 of the ellipse were nearly turned to the July sun, in 1842 

 the axis of the ellipse would have completely revolved, so 

 that the shorter end of the ellipse would again be nearly 

 tunied to the July sun : and thus the eclipse which occu rred, 



