SATELLITES OJ JUPITER — ^NICHOLSON 137 



these objects may be obtained by comparing their brightness with that 

 of a candle. If the light of a candle were not absorbed by the earth's 

 atmosphere it would have to be viewed from a distance of 3,000 

 miles in order to appear as faint as they are. They are so small 

 and so far from the planet that an observer on Jupiter itself would 

 require a 6-inch telescope to see them. Photographs of an hour's 

 exposure with the 100-inch telescope are capable of showing much 

 fainter objects than those that were found, and the fact that fainter 

 satellites were not found indicates that, if Jupiter has more undis- 

 covered satellites, they are probably much fainter than those now 

 known. 



The size of the new satellites, although not directly measurable, 

 can be inferred from the measured brightness and an assumed value 

 of the surface brightness.^" Satellite X is fainter than XI and there- 

 fore probably smaller. Unless their surfaces are extremely dark, 

 their diameters must be less than 15 and 19 miles, respectively. The 

 diameters of the other faint satellites of Jupiter, likewise inferred 

 from their brightness, are: V and VI, 90 miles; VII and VIII, 25 

 miles; IX, 19 miles. The satellites discovered by Galileo are huge 

 in comparison. The smallest is 2,000 miles in diameter, almost the 

 same size as our Moon; the largest, 3,000 miles in diameter, is as 

 big as the planet Mercury. 



Satellite X, like VI and VII, revolves around Jupiter in a period of 

 about 260 days, and in the same direction in which they move. The 

 orbit of satellite XI is still not accurately known but the preliminary 

 calculations show that it revolves in the same direction as VIII and 

 IX, in a period of about TOO days. 



The five inner satellites form a family group at an average distance 

 of a little less than 1 million miles from the planet, all revolving 

 nearly in the plane of Jupiter's equator. Satellites VI, VII, and X 

 form another family at a distance of 7 million miles, while VIII, 

 IX, and XI form still a third, 15 million miles from the planet. 

 The characteristics of each group are so closely accordant that they 

 cannot be the result of chance. Whether the small outer satellites 

 have been captured by Jupiter is a debated question, but the fact 

 that they exist in families seems to point toward a common origin 

 for each group unless it can be shown that satellites of Jupiter have 

 much more stable orbits at distances of 7 and 15 milHon miles than 

 at other distances. 



Many have asked what the new satellites are to be named. They 

 will be known only by the numbers X and XI, written in roman 



"The diameter of a satellite of Jupiter may be computpd by the formula 

 log d = 4.4G — 0.2 m — log ^p 

 where d is the diameter in miles, m is the photographic magnitude at mean opposition, 

 and p is a factor, closely related to the albedo, which for the darker satellites and asteroids 

 has a value of about 0.1. 



