Page:Popular Science Monthly Volume 31.djvu/218

206 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 apparently 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 the planet's shadow, Or else so close 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 supposed to be 10.30 o'clock :

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 inexperienced observer to see all four of the satellites, as two will be on each side of the planet and all will be well separated.