SATELLITES OF JUPITER — NICHOLSON 138 



perceived that the stars which I saw had always been the same, for there 

 were no others either in front or behind, within a great distance, along the 

 Zodiac — at length, changing from doubt into surprise, I discovered that the 

 interchange of position which I saw belonged not to Jupiter, but to the stars 

 to which my attention had been drawn, and I thought therefore that they 

 ought to be observed henceforward with more attention and precision. 

 Accordingly, on January 11th I saw an arrangement of the following kind : 



B * * O ^ 



namely, only two stars to the east of Jupiter, the nearer of which was distant 

 from Jupiter three times as far as from the star further to the east; and the 

 star furthest to the east was nearly twice as large as the other one ; whereas 

 on the previous night they had appeared nearly of equal magnitude. I, there- 

 fore, concluded, and decided unhesitatingly, that there are three stars in the 

 heavens moving about Jupiter, as Venus and Mercury round the Sun; which 

 at length was established as clear as daylight by numerous other subsequent 

 observations. These observations also established that there are not only 

 three, but four, erratic sidereal bodies performing their revolutions roimd 



Jupiter 



Besides, we have a notable and splendid argument to remove the scruples 

 of those who can tolerate the revolution of the planets round the Sun in the 

 Copernican system, yet are so disturbed by the motion of one Moon about the 

 Earth, while both accomplish an orbit of a year's length about the Sun, that 

 they consider that this theory of the universe must be upset as impossible : for 

 now we have not one planet only revolving about another, while both traverse 

 a vast orbit about the Sun, but our sense of sight presents to us four satellites 

 circling about Jupiter, like the Moon about the Earth, while the whole system 

 travels over a mighty orbit about the Sun in the space of twelve years. 



In addition to demonstrating so clearly the nature of the solar 

 system, Jupiter's satellites were responsible for the discovery of 

 another very important fact of nature, namely, that time is required 

 for light to travel from one place to another. In 1675, the Danish 

 astronomer Roemer noticed that when Jupiter was far from the earth 

 the eclipses of its satellites occurred relatively later than when it was 

 near the earth. This delay could be explained if it took time for 

 light to come from Jupiter to the earth, 35 minutes at mean oppo- 

 sition. Since then the speed of light has been measured many times 

 by more exact methods but the result is essentially the same as that 

 obtained from Jupiter's satellites. 



After Galileo's time no more satellites of Jupiter were discovered 

 imtil September 9, 1892, when Professor Barnard, who had been 

 searching with the 36-inch telescope at the Lick Observatory one 

 night each week for 2 months, detected a faint satellite very close to 

 the planet's surface.* This fifth moon of Jupiter has the distinction 

 of being the last satellite in the solar system to be discovered visually ; 

 since then all such discoveries have been made by photography. 



*Astrophya. .Tourn.. vol. 12, p. 81, 1892. 



