Of Saturn, of its Rings and its Satellites.

1. Saturn, the tenth planet in order of distance from the Sun, performs its sidereal revolution in 10,758 days, 23 hours, and 16 minutes, 34 seconds, which is the length of its year. The rotation of this planet on its axis, from west to east, is completed in 10 hours, 16 minutes, and 49 seconds, which is the length of its day.

Saturn shines with a very feeble light, compared with that of Jupiter, partly on account of his great distance from the Sun, and partly from his dull red colour.

2. The diameter of Saturn is 77,680 miles; his shape is an oblate spheroid, like that of Jupiter, but still more elliptical, the equatorial diameter being to the polar as 12 to 11. The apparent mean diameter of this planet is about 17"; and its relative mean distance from the Sun is 95, that of the Earth being considered as 10.

Saturn, next to Jupiter, is the largest planet in the solar system: its relative size is 974.78; its mass 93.271; and its density .095684; the size, mass, and density of the Earth being respectively considered as unity, or 1. And a body weighing 1 pound on the surface of the Earth, would weigh a little more than 14 pounds on the surface of Saturn.

3. The inclination of the orbit of Saturn to the plane of the ecliptic is 2° 30′ 18"; and the inclination of this planet's axis to its orbit is probably 60 degrees.

The inclination of Saturn's orbit is subject to a small secular diminution of 154 seconds.

4. The apparent motion of Saturn in its orbit is subject to irregularities, similar to those of Jupiter

and Mars. It commences and finishes its retrograde motion when the planet before and after its opposition is about 108° 54' distant from the Sun. The arc of retrogradation is about 6° 18', and its duration is nearly 131 days. The synodic revolution of this planet, or the time from opposition to opposition, is 378 days, 2 hours, and 1 minute.

TABLE,

Showing the mean distance of Saturn from the Sun, &c.

The sidereal secular motion of the apsides is 32' 17", but their tropical motion is 1° 55′ 47′′ in consequentia. The secular increase of the greatest equation is 1' 50".

5. Soon after the invention of telescopes, a remarkable appearance was observed about Saturn. After a considerable interval of time, Huygens having much improved them, discovered, by careful observations, a phenomenon unique, as far as we know, in the solar system. He found that Saturn is encompassed with a broad thin ring, inclined by a constant angle of about 30° to the plane of Saturn's orbit; and therefore at nearly the same angle to our ecliptic, and so always appearing to us obliquely.

When the edge of the ring is turned towards us, it is invisible, on account of its thinness not reflecting light enough to be visible, except in the very best telescopes. When the

plane of the ring passes between the Earth and Sun, it is also invisible, because its enlightened part is turned from us; and when it passes through the Sun it is also invisible, the edge only being illuminated: so that it may have, in the same year, two disappearances and reappearances. This takes place when Saturn is near the nodes of the ring.

The ring is a very beautiful object, seen in a good telescope, when in its most open state. It then appears elliptical, its breadth being about half its length. Through the space between the ring and the body, fixed stars have sometimes been seen. The surface of the ring appears more brilliant than that of Saturn.

Among the numerous discoveries of Dr. Herschel, those he has made with respect to Saturn and his ring are not the least. He has ascertained that the ring, which heretofore had generally been supposed single, consists of two, exactly in the same plane, and that they both revolve on their axis in the same time as Saturn, and in the plane of Saturn's equator. This is, however, doubted by Harding and Schroeter. See Dr. Brinkley's Astronomy.

Dr. Herschel also saw the ring when it has disappeared to other observers, either from the reflection of the edge, or from the dark side enlightened by the reflection of Saturn, as we see the whole Moon near new Moon. He observes that the ring is very thin, compared with its width, its thickness being about 1000 miles.

The outside diameter of the larger ring is 200,000 miles. Its width

6,700

2,800

180,000

19,000

At the mean distance of Saturn, the apparent diameter of the largest ring is about 47 seconds.

6. Saturn has also certain obscure zones, or belts, appearing at times across his disc, like those of Jupiter.

These zones, or belts, are supposed to be obscurations in his atmosphere, which Dr. Herschel has observed to be of considerable density.

7. Saturn has seven satellites revolving about him in orbits nearly circular; of which the sixth is seen without much difficulty, and was called the Huygenian satellite, from having been discovered by Huygens.

The 3d, 4th, 5th, and 7th were afterwards discovered. Dr. Herschel discovered the 1st and 2d.

It has long been supposed that the 7th (formerly the 5th) satellite revolved on its axis in the time of its revolution round Saturn. This has been confirmed by the observations of Dr. Herschel.

These satellites, except the 6th, require a very good telescope to render them visible. On which account they have been much less attended to than the satellites of Jupiter.

The periodical revolutions and distances of these satellites from the body of Saturn, expressed in semi-diameters of that planet, as well as in miles, are exhibited in the following table :

In what time does Saturn revolve on its axis ?

What is the shape of Saturn ?

In what angle is Saturn's orbit inclined to the

plane of the ecliptic?

Is the apparent motion of Saturn subject to any

irregularities?

What is the inclination of Saturn's ring to the

plane of the planet's orbit?

How many satellites has Saturn?

CHAPTER XII.

Of Uranus H, and his Satellites.

1. Uranus, the most remote planet yet known in the solar system, performs its sidereal revolution in 30,688 days, 17 hours, 6 minutes, and 2 seconds, or nearly 84 years, which is the length of the planet's year: the inclination of its orbit to the plane of the ecliptic is about 46'; and the time of rotation on its axis, or the length of its day, has not been yet ascertained.

The planet Uranus had escaped the observation of ancient astronomers, its distance from the earth being so great, that it cannot be seen by the naked eye, except when the atmosphere is very clear, and then it appears like a star of the 6th magnitude. Its light is of a bluish white colour; it shows no disc but with a very great magnifying power.

Flamstead, at the end of the last century, and Mayer and Le Moniuer, in this, had observed Uranus as a small star. But it was not till 1781 that Dr. Herschel discovered its motion, and soon after, by following this star carefully, it has been ascertained to be a true planet. Like Mars, Jupiter, and Saturn, the apparent motion of Uranus is subject to irregularities. Its motion, which is nearly in the plane of the ecliptic, begins to be retrograde when, previous to the opposition, the planet is 1030 30 distant from the sun. ceases to be retrograde when, after the opposition, the planet in its approach to the sun is only 1030 30 distant from him. The duration of its retrogradation is about 151 days, and its arc of retrogradation about 30 36". The synodic revolution is completed in 369 days, 15 hours, 44 minutes, and 38 seconds.

It

2. The diameter of Uranus is 35,000 miles; its apparent diameter is very small, amounting not quite to 4 seconds; and its relative mean distance from the sun is 192, that of the earth being considered as 10.

Uranus is much less than Jupiter, or Saturn: the relative size of this planet is 81.26, its mass 1.6904, and its density