JOVIAN ECLIPSES. 



consequently the shadow of the planet, are sufficiently removed 

 from the plane of the planet's equator to enable this moon to 

 clear the boundary of the shadow, and to pass through oppo- 

 sition without entering it. This is the only case in which any of 

 the moons can ever pass through opposition without also passing 

 through the shadow of the planet, and consequently the only 

 times the Jovians ever enjoy the spectacle of a full moon. 



When these circumstances are combined with the rapid revo- 

 lution of the moons, it will be easily understood that the celestial 

 phenomena of the Jovians must offer great variety, and that 

 their chronology must be curiously complicated. A total lunar 

 eclipse of the first or nearest moon must take place every forty- 

 two terrestrial hours, that is, every fourth Jovian day ; and for 

 a long interval before and after the equinoxes a total or partial 

 solar eclipse must take place at like intervals, being alternated 

 with the lunar eclipses, and separated from them by intervals of 

 only twenty-one terrestrial hours, or two Jovian days. 



The same phenomena exactly take place with relation to the 

 second satellite, at intervals of 3^ terrestrial, or about 8^ Jovian 

 days ; to the third at intervals of 7 terrestrial, or 17 Jovian days ; 

 and to the fourth at intervals of 16^ terrestrial, or 40 Jovian days, 

 subject, nevertheless, with respect to the last, to an interruption at 

 the Jovian summer and winter, from the cause already explained. 



17. The appearance which the satellites of Jupiter present 

 when viewed with a telescope of moderate power, is that of 

 minute stars ranged in the direction of a line drawn through the 

 centre of the planet's disc, nearly parallel to the direction of the 

 belts, and therefore coinciding with that of the planet's equator. 



The entire system is comprised within a visual area of about 

 two-thirds of the apparent diameter of the moon. If, therefore, 

 we conceive the moon's disc to be centrically superposed on that 

 of Jupiter, not only would all the satellites be covered by 

 it, but that which elongates itself most from the planet would 

 not approach nearer to the moon's edge than one-sixth of its 

 apparent diameter. 



If all the satellites were at the same time at their greatest 

 apparent distances from the planet, they would, relatively to 



Fig. 7. 



the apparent diameter of the planet, present the appearance 

 represented in fig. 7. 



