206 THE POPULAR SCIENCE MONTHLY. 



members of his little family of four moons may occasionally be caught 

 sight of. With an opera-glass not more than one or two of these can 

 be seen as excessively minute dots of light half-hidden in the glare of 

 the planet. If you succeed under favorable circumstances in seeing 



one of these moons with your glass, 

 you will be all the more astonished 

 to learn that there are several appar- 

 ently well-authenticated instances of 

 one of the moons of Jupiter having 

 been seen with the naked eye. 



With a field-glass, however, you 

 will have no difficulty in seeing all 

 of the moons when they are properly 

 situated. If you miss one or more 

 of them, you may know that it is 

 either between you and the planet, 

 or behind the planet, or buried in 



Jttpitkr and his Moons. (Seen with a the p l ane t's shadow, Or else SO close 



field-glasa ; power, six diameters.) r ' w "^ ^wv, 



to the planet as to be concealed by 

 its radiance. For the convenience of the beginner, I will indicate the 

 positions of Jupiter's moons for several evenings in June, based upon 

 the " Nautical Almanac " predictions ; the time of observation is sup- 

 posed to be 10.30 o'clock p. m. : 



On June 1st all four satellites will be visible with a good field- 

 glass, one being on the eastern and the other three on the western 

 side of the planet, two of the latter appearing close together like a 

 double star. 



On June 2d the satellites will all be on the western side, two of 

 them being pretty close to the planet and also close together, while 

 the others are more distant and wide apart. 



On June 6th all four will be on the eastern side of Jupiter, and 

 three of them, besides being near the planet, will be close together in a 

 slanting row. It will require close watching with a fine glass to see 

 them all, but the spectacle will be well worth some painstaking on the 

 part of the observer. 



On June 8th two will be on one side and two on the other, the two 

 on the west being close together, while those on the east are widely 

 separated. 



On June 14th two will be close together on the east, while one is 

 visible on the west, and the fourth, the outermost of the system, is 

 passing just above the north pole of the planet, and so close that it 

 will probably be beyond the ability even of a powerful field-glass to 

 detect it. 



On June 17th there will be a very good opportunity for the inex- 

 perienced observer to see all four of the satellites, as two will be on 

 each side of the planet and all will be well separated. 



