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rather more than 22,298 miles per hour. His appå. rent diameter, ata mean distance from the earth, is 16'.
It has not yet been ascertained with certainty by astronomical observation, whether Saturn revolves or not upon his axis. The sun's disc will appear ninety times less to an inhabitant of Saturn, than it does to us; but notwithstanding the sun appears so small to the inhabitants of the regions of Jupiter and Saturn, the light that he will afford them is much more than would be at first supposed; and calculations have been made, from which it is inferred, that the sun will afford 500 times as much light to Saturn, as the full moon to us; and 1600 times as much to Jupiter. To eyes like our's, unassisted by instruments, Jupiter and the Georgium Sidus would be the only planets seen from Saturn; to whom Jupiter would sometimes be a morning, sometimes an evening, star.
One of the first discoveries by the telescope, when brought to a tolerable degree of perfection, was, that Saturn did not appear like other planets. Galileo, in 1610, supposed it composed of three stars or globes, a larger in the middle, and a smaller on each side; and he continued his observations till the two lesser stars disappeared, and this planet looked like the others. Further observation shewed that what Galileo took for two stars, were parts of a ring. This singular and curious appendage to the planet Saturn, is a thin, broad, opake ring, encompassing the body of the planet, without touching it, like the horizon of an artificial globe; appearing double when
viewed through a good telescope. The space between the ring and the globe of Saturn is supposed to be rather more than the breadth of the ring, and the greatest diameter of the ring to be in proportion to that of the globe, as seven to three; the plane of the ring is inclined to the plane of the ecliptic, in an angle of 50', and is about 21,000 miles in breadth. It puts on different appearances to us, sometimes being seen quite open, at others only as
upon the equator. It is probable, that it will at times cast a shadow over vast regions of Saturn's body. The ring of Saturn, considered as a broad flat ring of solid matter, suspended round the body of the planet, and keeping its place without any connection with the body, is quite different from all other planetary phenomena with which we are ac-quainted. Of the nature of this ring, various and uncertain were the conjectures of the first observers, though not more perplexed than those of the latest. Of its use to the inhabitants of Saturn, we are as ignorant as of its nature; though there are reasons for supposing that it would appear to them as little more than a white or bright-coloured cloud. Some of the phenomena of Saturn's ring will be treated of more particularly in another part of this Essay.
Saturn is not only furnished with this beautiful ring, but has also seven attendant moons.
OF THE GEORGIUM SIDUS. H
From the time of Iluygens and Cassini, to the discovery of the Georgium Sidus by Dr. Herschel, though the intervening space was long, though the number of astronomers was increased, though assiduity in observing was assisted by accuracy and perfection in the instruments of observation, yet no new discovery was made in the heavens ; the boundaries of our system were not enlarged. The inquisitive mind naturally enquires, why, when the number of those that cultivated the science was increased, when the science itself was so much improved, in practical discoveries, it was so deficient? A small knowledge of the human mind will answer the question, and obviate the difficulty. The mind of man has a natural propensity to indolence; the ardour of its
pursuits, when they are unconnected with selfish views, are soon abated ; small difficulties discourage, little inconveniences fatigue it, and reason soon finds excuses to justify, and even applaud this weakness. In the present instance, the unmanageable length of the telescopes that were in use, and the continual exposure to the cold air of the night, were the difficulties that the astronomer had to encounter with; and he soon persuaded himself, that the same effects would be produced by shorter telescopes, with equal magnifying power; herein was his mistake; and hence the reason why so few discoveries have been made since the time of Cassini. A similar instance of the retrogradation of science occurs in the history of
the microscope, as I have shewn in my Essays on that instrument.
The Georgium Sidus was discovered by Dr. Herschel, in the year 1781: for this discovery he obtained from the Royal Society the honorary recompence of Sir Godfrey Copley's medal. He named the planet in honour of his Majesty King George III. the Patron of science, who has taken Dr. Herschel under his patronage, and granted him an annual salary. By this munificence he has given scope to a very uncommon genius, and enabled him to prosecute his favourite studies with unremitted ardour.
In so recent a discovery of a planet so distant, many particulars cannot be expected. Its year is supposed to be more than 80 siderial years ; its diameter 34,299 miles; the inclination of its orbit 43'' 35”; its diameter, compared to that of the earth, as 431,769 to 1 ; in bulk it is 8,049,256 times as large as the earth. Its light is of a bluish white colour, and its brilliancy between that of the Moon and Venus.
Though the Georgium Sidus was not known as a planet till the time of Dr. Herschel, yet there are many reasons to suppose it had been seen before, but had been considered as a fixed star. Dr. Herschel's attention was first engaged by the steadiness of its light; this induced him to apply higher magnifying powers to his telescope, which increased the diameter of it: in two days he observed that its place was changed; he then concluded it was a comet; but in a little time Dr. Maskelyne, himself, and others, determined that it was a planet, from its vicinity to
the ecliptic, the direction of its motion, being stationary in the time, and in such circumstances as to correspond with similar appearances in other planets.
With a telescope, which magnifies about 300 times, it appears to have a very well-defined visible disc; but with instruments of a smaller
it can hardly be distinguished from a fixed star, between the sixth and seventh magnitude. When the moon is absent, it may be seen by the naked eye.
Dr. Herschel has since discovered that it is attended by six satellites : a discovery which gave.
him considerable pleasure, as the little secondary planets seemed to give a dignity to the primary one, and raise it into a more conspicuous situation among the great bodies of our solar system.
As the distances of the planets, when marked in miles, are a burthen to the memory, astronomers often express their mean distances in a shorter way, by supposing the distance of the earth from the sun to be diyided into ten parts. Mercury nay then be estimated at four of such parts from the sun, Venus at seven, the Earth at ten, Mars at fifteen, Jupiter at fifty-two such parts, Saturn at ninety-five, and the Georgium Sidus 190 parts.
By comparing the periods of the planets, or the time they take to finish their revolutions, with their distance from the sun, they are found to observe a wonderful harmony and proportion to each other for the nearer any planet is to the sun, the sooner does he finish his revolution. And in this there is a