S.CT. V ECLIPSES OF PLANETS. 41 



A phenomenon altogether unprecedented occurred 

 during the total eclipse of the sun which happened on 

 the 8th of July, 1842. The moon was like a black 

 patch on the sky surrounded by a faint whitish light 

 about the eighth of the moon's diameter in breadth, in 

 which three red flames appeared in form like the teeth 

 of a saw ; from what cause they originated, or what 

 they were, is totally unknown. 



Planets sometimes eclipse one another. On the 17th 

 of May, 1737, Mercury was eclipsed by Venus near 

 their inferior conjunction ; Mars passed over Jupiter on 

 the 9th of January, 1591 ; and on the 30th of October, 

 1825, the moon eclipsed Saturn. These phenomena, 

 however, happen very seldom, because all the planets, 

 or even a part of them, are very rarely seen in con- 

 junction at once ; that is, in the same part of the heav- 

 ens at the same time. More than 2500 years before 

 our era, the five great planets were in conjunction. On 

 the 15th of September, 1186, a similar assemblage took 

 place between the constellations of Virgo and Libra; 

 and in 1801, the moon, Jupiter, Saturn, and Venus 

 were united in the heart of the Lion. These conjunc- 

 tions are so rare, that Lalande has computed that more 

 than seventeen millions of millions of years separate the 

 epochs of the contemporaneous conjunctions of the six 

 great planets. 



The motions of the moon have now become of more 

 importance to the navigator and geographer than those 

 of any other heavenly body, from the precision with 

 which terrestrial longitude is deter mined "by occultations 

 of stars, and by lunar distances. In consequence of the 

 retrograde motion of the nodes of the lunar orbit, at the 

 rate of 3' 10"-64 daily, these points make a tour of the 

 heavens in a little more than eighteen years and a half. 

 This causes the moon to move round the earth in a kind 

 of spiral, so that her disc at different times passes over 

 every point in a zone of the heavens extending rather 

 more than 5 9' on each side of the ecliptic. It is there- 

 fore evident, that at one time or other she must eclipse 

 every star and planet she meets with in this space. 

 Therefore the occultation of a star by the moon is a phe- 

 nomenon of frequent occurrence. The moon seems to 



