On earth with wonder all night long beheld,
Moon above moon, his fair attendants dance.”
To a spectator placed on the surface of Jupiter, each of the satellites would put on the phases of the moon; but as the distance of any of them from Jupiter is but small, when compared with the distance of that planet from the sun, the satellites are therefore illuminated by the sun very nearly in the same manner with the primary itself; hence they appear to us always round, having constantly the greatest part of their enlightened half turned towards the earth: and indeed they are so small, that were they to put on the phases of the moon, these phases could scarcely be discerned through the best telescopes. When the satellites pass through their inferior semicircles, they may cast a shadow upon their primary, and thus cause an eclipse of the sun to his inhabitants; and in some situations this shadow may be observed going before or following the satellite. On the other hand, in passing through their superior semicircles, the satellites may be eclipsed in the same manner as our moon by passing through the shadow of Jupiter: and this is actually the case with the first, second, and third; but the fourth, by reason of the extent of its orbit, passes sometimes above or below the shadow, as is the case with our moon.
These satellites were first discovered on the 7th of January, 1610, by the celebrated Galileo, who called them Medician Stars, in honor of the family of the Medici, dukes of Tuscany, his patrons. These satellites, revolving about Jupiter at different distances, from west to east, when viewed through a telescope, make a beautiful appearance. As our moon revolves round the earth, enlightening the nights, by reflecting the light she receives from the sun; so these satellites, revolving round Jupiter, may also be supposed to enlighten the nights of that planet.
Saturn is a very conspicuous planet, though he shines with a pale and feeble light, very unlike that of Jupiter and the other planets. He was called by the Greeks φαινων. “From the account given by Diodorus Siculus,” says Costard, “it seems as if the Chaldeans called this planet by some name not widely different from this of the Greeks. In the language of Chaldea, the verb פנא phana, or פנה phanah, signifies convertere se, divertere se, declinare. And whatever vanishes, or disappears, very properly declines, or turns aside, from our view. This planet, therefore, was most probably called פן _phen_, or פין phain, and, with a Greek termination, φαινων, on account of his withdrawing himself, by reason of his distance. And this conjecture is yet further confirmed from his name in another dialect, or among another people. For from סתר sater, latuit, abscondit se, with the paragogic ן nun which is not unusual in the formation of Eastern words, comes the word סתרן Saturn, and with the Latin termination us, Saturnus.”
His mean distance from the sun is 900,000,000 miles, consequently his motion in his orbit is proportionably slow; and his annual revolution round the sun, from west to east, being so much longer likewise than that of the other planets, he takes 29 years, 164 days, 7 hours, 21 minutes, 50 seconds, which is almost thirty of our years, to accomplish it, in his orbit travelling with a velocity of 22,000 miles an hour. His diameter is 79,000 miles; and his magnitude is about 1,000 times that of the earth. The time of rotation upon his axis is 10 hours, 17 minutes.
“Still further off, scarce warm’d by Phœbus’ ray,
Through his wide orbit, Saturn wheels away;
How great the change, could we be wafted there!
How slow the seasons! and how long the year!”