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equator, or his ring. The sun appears to be but about one ninetieth part as large to Saturn as to us, and his light and heat are in the same small proportion. However this seems to be wisely compensated by seven moons revolving round him nearly in the plane of his ring, at different distances, and in different periods. Besides these, he is surrounded with a thin broad ring at the distance of about twenty-one thousand miles from him, and nearly of the same breadth, which answers the purpose of a continuation of moons quite round the planet. This appears to his inhabitants like a broad luminous arch in the heavens, as if it did not belong to him, and may be represented by the horizon of an artificial globe, removed to its breadth from the surface of the globe. This ring is inclined to the eclip'tic in an angle of 30 degrees, and intersects it in 19° 45' of Virgo and Pisces, so that when Saturn is in these points, the plane of the ring passes through the earth, the edge of it being turned directly towards us. At this time the ring disappears, and the body of Saturn is seen divested of the ring; as the edge of it is so thin, as to become invisible even with our best telescopes. This will happen twice in every revolution of Saturn: and when Saturn is in the middle between these points, that is about the 20th degrees of Gemini and Sagittarius, it appears most open to us, the eye being elevated about 30 degrees above the plane of the ring, when its apparent diameters are to each other as 9: 4. As Saturn goes round the sun, his obliquely-posited ring continues parallel to itself, like the axis of our earth, and therefore twice in every Saturnian year, or once in every fifteen years, its edge is turned towards the sun; and being very thin it then becomes invisible to the earth, wheresoever the earth may be. As the sun shines for half a year on the north pole of our earth

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and then disappears from it for the same time; so the sun, in like manner, shines for fifteen years on the north side of the ring, and then disappears from it for 15 years more, while he is shining on its south side. In 1789, the ring will disappear and reappear twice, viz. disappear 5th May, and reappear 24th August; disappear 15th October, and reappear 30th January 1790. There is some reason to believe that the ring of Saturn turns round on an axis; because, when it is almost edgewise to us, it appears to be somewhat thicker on one side than on the other, and the thickest side has been seen on different sides of the planet, at different times. Herschel has discovered lately, that it revolves in 10 32′ 15′′. When the ring is viewed with a very good telescope, it appears double, as if it were composed of two rings, one surrounding the other, and separated by a dark black list or line. The outer ring, as there are really two of them, appears to be but about half as broad as the inner one. Saturn is flattened towards the poles, his diameters being to each other as

11: 12.

HERSCHEL.

HERSCHEL is the outermost planet of the solar system, which we have yet discovered. It was late, ly discovered by Mr. Herschel, of Bath, in England, who accidentally met with it, in his telescope, as he was viewing the fixed stars. Upon considering its motion and comparing its present place, with a new star mentioned by Mr. Meyer, of Gottingen, as discovered about thirty years before, in a certain place of the heavens where no star is now to be found, the astronomers of Europe have concluded, that what Meyer had then seen, was the same star that Herschel has

lately discovered. Upon this supposition, they have endeavoured to ascertain the elements of its motion to be as follows.

It performs its revolution round the sun, like the other planets, from west to east, in about eighty-three years and four months, at the distance of near nineteen hundred millions of miles from the sun, in an orbit inclined to the ecliptic in an angle of 48′ 10", cutting the ecliptic in 13° 30′ of Gemini and Sagittarius. Its diameter is about 34217 miles, being to that of our earth as 4.31769 to 1, and its eccentricity 82034. It seems to have passed its perihelion on the 7th September 1779, moving at the rate of 4° 21′ 1′′.3 per annum. Its bulk is 80.49256 times that of the earth, though its quantity of matter is but 17.740612 as much. There is but little more known of the motions of this planet; as it has not yet performed half its periodical revolution, since the time it was first discovered. It is attended by two moons already discovered; the first revolves in 8d 17h 1′ 19′′ and the other in 13d 11h 5' 1.5".*

ASPECTS OF THE SUPERIOR PLANETS.

ANY of these superior planets, Mars, Jupiter, Saturn, or Herschel, may be found in opposition to the sun, as well as in conjunction with him. They appear in opposition to the sun, when they are seen in opposite parts of the heavens; and as they revolve in orbits which include the orbit of the earth, whenever they are on the same side of the heavens with the sun, they are said to be in conjunction with him, and must then appear of a less diameter, because of their greater distance from

* Since the writing of this lecture, four additional moons have been discovered.

Ed.

the earth; but when they are on the opposite side of the heavens, they are consequently nearer to the earth, and must appear of a greater diameter. Thus the diameters of the superior planets, in opposition and conjunction, are to each other, in the following ratios; of Mars, as 25:5; of Jupiter, as 62: 42; of Saturn, as 105: 85; -of Herschel is not known but by computation.

As all the superior planets move in the same direction with the earth, and with less velocity, the appearances which they exhibit to us will be the same, if we suppose them to be at rest, and the earth to move with the difference between their respective velocities and that of the earth. Now, while the earth is moving through that part of her orbit, which is nearest to the superior planet, the planet will appear to move from east to west, or retrograde, among the stars; first increasing in its velocity as it approaches to the point of its opposition, where its velocity is greatest, and then gradually decreasing in velocity, until it come to that point where a right line from the planet to the earth will be a tangent to the orbit of the earth, where the superior planet will appear to be stationary for a small time, as the earth is then going directly from it. From thence the direct motion of the planet will commence, and its velocity will gradually increase till the conjunction of the sun and planet, when it will be again greatest, and then it will gradually decrease until it become stationary; after which its retrograde motion will again commence. So that when a superior planet is passing from one station to another through its opposition, its geocentric motion is retrograde; but while it is passing from one station to another through its conjunction, its geocentric motion is direct.

Half the arc of regress of a superior planet is equal

to the angle under which the semi-diameter of the earth's orbit would be viewed from the planet; and as this semi-diameter must appear under a less angle, the farther the eye is removed from it, it follows, that Mars's arc of regress is greater than Jupiter's, Jupiter's greater than Saturn's, and Saturn's greater than Herschel's. It follows also, that as the sine of half the arc of regress of a superior planet is to radius, so is the radius of the earth's orbit to the radius of the planet's orbit; so that the proportion of the distance of the earth from the sun, to the distance of any of the superior planets from the sun, may be known by observing the arc of their regress; and if the real distance of the earth be known in any determinate measure, the distance of the rest may be found.

In explaining the progressions, stations, and retrogressions of the superior planets, we have supposed that the planet was at rest, and that the earth moved with the difference of their velocities; and these will be the same that they would be upon the supposition of the proper motion of both; save only, that the stations, and the retrograde and direct motions of the planets, will happen in different portions of the heavens, and not always in the same parts.

The heliocentric latitude of a planet is greatest when it is at 90 degrees distance from either of its nodes: but the geocentric latitude is directly as the heliocentric latitude, and inversely as the distance of the planet from the earth. When a superior planet is in conjunction, its heliocentric is greater than its geocentric latitude, but less, when it is in opposition. The method of determining the mean times of the conjunctions or oppositions of the planets is, by first considering their mean motions as viewed from the

, and thereby determining how far any one of the

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