The museum of science & art / edited by Dionysius Lardner.

Date:
1856-1860
Catalogue details

Licence: Public Domain Mark

Credit: The museum of science & art / edited by Dionysius Lardner. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

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    By comparing their real diameters with their distances, the apparent diameters of the several satellites, as seen from Jupiter, may be easily ascertained. 18. The first satellite has an apparent diameter equal to that of the moon ; the second and third are nearly equal and about half that diameter; and the apparent diameter of the other satellite is about the fourth part of that of the moon. It may be easily imagined what various and interesting nocturnal phenomena are witnessed by the inhabitants of Jupiter, when the various magnitudes of these four moons are combined with the quick succession of their phases and the rapid apparent motions of the first and second. The motions of the first three satellites are so related that they never can be at the same time on the same side of Jupiter ; so that whenever any one of them is absent from the Jovian firmament at night, one at least of the others must be present. The nights are, therefore, always moonlit, except during eclipses, and often enlightened at once by three moons of different apparent magnitudes, and seen under different phases. 19. Of all the planets either of this or the terrestrial group, that which presents to the astronomical observer the most astonishing spectacle is Saturn—a stupendous globe, nearly 900 times greater in volume than the earth, surrounded by two, at least, and probably by several thin flat rings of solid matter, outside which revolve a group of eight moons; this entire system moving with a common motion so exactly maintained that no one part falls upon, overtakes, or is overtaken by another in their course around the sun. Such is the Saturnian system, the central body of which was known as a planet to the ancients, the annular appendages and satellites being the discovery of modern times. The distance of Saturn from the sun is so enormous that if the whole earth's orbit, measuring nearly 200,000000 of miles in diameter, were filled with a sun, that sun seen from Saturn would be only about 24 times greater in its apparent diameter than is the actual sun seen from the earth. A cannon-ball moving at 500 miles an hour would take about 200 years ; and a railway train moving 50 miles an hour woidd take ahout 2000 years to move from Saturn to the sun. Light, which moves at the rate of nearly 200000 miles per second, takes 1 hour 15 minutes to move over the same distance. Tet to this distance solar gravi- tation transmits its mandates, and is obeyed with the utmost promptitude and the most unerring precision. Taking the diameter of Saturn's orbit at 1S00,000000 of miles, its circumference is 5650,000000 of miles, over which it
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