SCENES ON THE PLANETS. 339 



There is a charm about the great planet when he rides high in 

 a clear evening sky, lording it over the fixed stars with his serene, 

 unflickering luminousness, which no possessor of a telescope can 

 resist. You turn the glass upon him and he floats into the field 

 of view, with his cortege of satellites, like a yellow-and-red moon, 

 attended by four miniatures of itself. You instantly comjjrehend 

 Jupiter's mastery over his satellites — their allegiance is evident. 

 JSTo one would for an instant mistake them for stars accidentally 

 seen in the same field of view. Although it requires a very large 

 telescope to magnify their disks to measurable dimensions, yet the 

 smallest glass differentiates them at once from the fixed stars. 

 There is something almost startling in- their appearance of com- 

 panionship with the huge planet — this sudden verification to your 

 eyes of the laws of gravitation and of central forces. It is easy, 

 while looking at Jupiter amid his family, to understand the con- 

 sternation of the churchmen when Galileo's telescope revealed that 

 miniature of the solar system, and it is gratifying to gaze upon one 

 of the first battle grounds whereon science gained a decisive vic- 

 tory for truth. 



The swift changing of place among the satellites, as well as the 

 rapidity of Jupiter's axial rotation, give the attraction of visible 

 movement to the Jovian spectacle. The planet rotates in four 

 or five minutes less than ten hours — in other words, it makes two 

 turns and four tenths of a third turn while the earth is turning 

 once upon its axis. A point on Jupiter's equator moves about 

 twenty-seven thousand miles, or considerably more than the en- 

 tire circumference of the earth, in a single hour. The effect of 

 this motion is clearly perceptible to the observer with a telescope 

 on account of the diversified markings and colors of the moving 

 disk, and to watch it is one of the greatest pleasures that the tele- 

 scope affords. 



It would be possible, when the planet is favorably situated, to 

 witness an entire rotation of Jupiter in the course of one night, 

 but the beginning and end of the observation would be more or 

 less interfered with by the effects of low altitude, to say nothing 

 of the tedium of so long a vigil. But by looking at the planet 

 for an hour at a time in the course of a few nights every side of 

 it will have been presented to view. Suppose the first observation 

 is made between nine and ten o'clock on any night which may have 

 been selected. Then on the following night between ten and 

 eleven o'clock Jupiter will have made two and a half turns 

 upon his axis, and the side diametrically opposite to that seen 

 on the first night will be visible. On the third night between 

 eleven and twelve o'clock Jupiter will have performed five com- 



