42 OCCULTATIONS. SECT. V. 



extensive atmosphere, at a certain height in which there is a 

 stratum of luminous clouds which constitutes the photosphere of 

 the sun ; above this rises his real atmosphere, so rare as to be 

 only visible as a white aureola or corona during total and annular 

 eclipses. M. Faye conceives that from the central mass gaseous 

 eruptions issue, which form the spots by dissipating and partly 

 extinguishing the luminous clouds ? and then rising into the rare 

 atmosphere above that they appear as rose-coloured protuberances 

 during annular eclipses. He estimates that the volume of these 

 vapours sometimes surpasses that of the earth a thousand or even 

 two thousand times. Sir William Herschel attributed the spots 

 to occasional openings in the luminous coating, which seems to 

 be always in motion ; but whatever the cause of the spots may 

 be, it is certainly periodical. The white corona and beads were 

 seen during the eclipse of the 15th March, 1858, but there were 

 no rose-coloured appearances, in England at least ; but the sky 

 was clouded, so that the eclipse was only visible at intervals. 



Planets sometimes eclipse one another. On the 17th of May, 

 1737, Mercury was eclipsed by Venus near their inferior con- 

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

 heavens 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 conjunctions 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 

 determined 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. 



