238 



THE MAJOR PLANETS. 



The globe of Jupiter is therefore about as heavy as if it was composed of 

 water from its surface to its centre. 



There is nothing connected with the motion of the planets more surprising 

 than their enormous velocities, which, to our observation, are nevertheless 

 scarcely perceptible, owing to the fact that their distances from us are propor- 

 tionally great. Jupiter, when nearest to us, is at a distance of four hundred 

 millions of miles. A cannon-ball which moves at the rate of five hundred 

 miles an hour, would require nearly a hundred years to come from Jupiter to 

 us, and if a steam-engine on a railway, moving at twenty miles an hour, were 

 to take its departure for Jupiter, it would not arrive at its destination until the 

 expiration of two thousand three hundred years. 



Taking the diameter of Jupiter's orbit at a thousand millions of miles, its 

 circumference is more than three thousand millions of miles, which is traversed 

 in less than twelve years. The space moved over annually by Jupiter is, then, 

 two hundred and fifty millions of miles ; and the space moved over monthly 

 about twenty millions of miles ; and the space moved over daily about seven 

 hundred thousand miles ; and the space moved over hourly about thirty thou- 

 sand miles ; being at the rate of about five hundred miles a minute ; a velocity 

 sixty times greater than that of a cannon-ball. 



DIURNAL ROTATION OF JUPITER. 



Although the varieties of light and shade which characterize the disk of 

 Jupiter are uh)ect to variations which show, as will be seen hereafter, that 

 they are principally produced by clouds in his atmosphere, yet permanent 

 marks weie discovered upon it at an early epoch, by which the fact was estab- 

 < lished that the planet has a diurnal rotation. In the years 1664-'5, Hook and 

 Cassmi observed a spot on one of the belts which was permanent in its pt>si- 

 uon, and was observed to move across the disk of the planet. It contracted 

 in us breadth as it approached the edge of the disk ; a circumstance which ob- 

 viously arose from its being fore-shortened by the position in which it was 

 there presented to the eye, that portion of the surface of the planet being seen 

 very obliquely, the spot disappeared at one side, and after being invisible for 

 a time reappeared at the other. This spot continued to be seen for more than 

 a year, and fully proved the fact that -Jupiter completes his rotation on an axis 

 very slightly inclined to his orbit in nine hours and fifty-six minutes. 



The alternations of light and darkness on Jupiter are therefore regulated by 

 intervals much shorter than those which govern the days and nights of the 

 minor planets, and we shall presently see that this is a character which prob- 

 ably prevails among all the major planets. The average interval of the days 

 and nights must be a little under five terrestrial hours. 



This rapid motion, considered with reference to the great magnitude of Ju- 

 piter, leads to the inference that the velocity of that part of his surface which 

 is near his equator must be exceedingly great. The circumference of Jupiter 

 at his equator must be about two hundred and seventy thousand miles, and as 

 this revolves in ten hours, the motion of any point upon it must be at the 

 enormous rate of twenty-seven thousand miles an hour, or a little less than five 

 hundred miles a minute. Thus it appears that the velocity which the equa- 

 torial regions have, in virtue of the diurnal motion, is very little less than the 

 orbitual motion of the planet round the sun. 



This rapid diurnal rotation would produce a considerable variation in the 

 weights of bodies at different latitudes on the surface of Jupiter, since the cen- 

 trifugal force near the equator would counteract the weight in a very sensible 

 manner, while toward the poles its effects would cease to be perceptible. 



