Some of the planets have satellites or moons. There is also another order, called comets, with long luminous trains, which pursue very eccentric

courses.

The names of the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, and Herschel; with four small ones, called Asteroids, named Ceres, Pallas, Juno, and Vesta.

These are called primaries, and there are also eighteen satellites or moons, called secondaries. The Earth has one; Jupiter, four; Saturn, seven; and Herschel, six.

THE SUN.

THE SUN is the source of light and heat, and the centre of our Solar or Planetary System. Its form is nearly that of a sphere or globe. Its diameter is about 886 thousand miles, and its circumference about 2 million 700 thousand miles.

Some authorities make the Sun's diameter more, others less. Its distance from the Earth is 95 millions of miles. Its magnitude is more than a million of times larger than our own planet. The Sun has generally been thought a globe of real fire, but some suppose it to be an opaque body, surrounded by a luminous atmosphere.

A number of dark spots may be seen, by means of a telescope, on different parts of the Sun's surface. These consist of a nucleus, which is much

What are satellites? What are comets? What are the names of the planets and the asteroids? How are planets and satellites distinguished? How many secondaries are there? and how are they distributed?

What is the centre of our system? What are its dimensions? What its distance from the Earth? What its proportionate magnitude? What were the opinions of the ancients respecting it? What are those of some modern philosophers? What is usually seen on it through a telescope ?

darker than the rest, surrounded by a mist or smoke; and they are so changeable as frequently to vary during the time of observation. Some of the largest of them exceed in square miles the surface of the whole Earth, and are often seen for months together. Very rarely, no spots are visible.

Dr. Herschel supposes that the Sun is surrounded by an atmosphere of a phosphoric nature, composed of various, transparent and elastic fluids, by the decomposition of which, light is produced, and lucid appearances formed, of different degrees of intensity. Openings in these fluids are the spots.

The density of the Sun being less than one fourth that of the Earth, it will be obvious, that with one million 400 thousand times the bulk of this planet, it has about 330 thousand times as much matter, and its attraction at its surface is 27 times as powerful, as the attraction of the Earth. For light, color, and heat, we are indebted to the Sun's influence.

The Sun has THREE motions. One motion is round the common centre of gravity of all the bodies, belonging to our system. Another motion is its rotation on its axis. And a sublimer motion still, is that towards the constellation Hercules.

We infer, that it has the first motion from the known laws of gravitation. That it has the second motion, in a period of 25 days, we infer from an observation of its spots, passing across its disc. That it moves towards the constellation Hercules, is inferred from the divergency of stars in that direction, and the convergency of stars in the opposite direction.

What description can be given of these spots?

What is Herschel's opinion of the Sun ?

What is the density of the Sun? How much more matter than the Earth? What is its comparative attraction? For what are we indebted to the Sun's influence? How many motions has it?

What evidence that it has the first? What the second ? What the third?

The apparent mean diameter of the Sun, as seen from the Earth, is 32 minutes and 1 second; as seen from a planet revolving round a fixed star, it would have no perceptible disc.

CHAPTER II.

MERCURY.

THE nearest planet to the Sun, is called Mercury. Its diameter is 3200 miles; its density 9 times the density of water, and the amount of light enjoyed, 7 times as great as at the Earth. Its mean distance from the Sun, is 37 millions of miles. It turns on its axis in 24 hours, and revolves round the Sun in 88 days; at the rate of more than 100 thousand miles the hour, or 30 miles the second.

Though small, this planet has a very bright appearance, slightly tinged with blue; but as it never appears 30 degrees distant from the Sun, it is rarely seen.

Both Mercury and Venus have varying phases, like the Moon, proving them to be opaque bodies, shining with light borrowed from the Sun; and also demonstrating, that they revolve in orbits of inferior magnitude to the orbit of the Earth. Hence they are called inferior planets.

What is the apparent mean diameter of the Sun? If distant as a fixed star how would it appear?

What planet is nearest to the Sun? What are its dimensions? What its density? What its amount of light? What is its distance from the Sun? What is the period of its rotation? What of its revolution round the Sun, and at what rate?

What is its appearance?

What proves that Mercury and Venus are opaque bodies and nearer to the Sun than the Earth? What are they called ?

If the orbit of Mercury exactly coincided with the orbit of the Earth, and that planet was always moving in the plane of the Ecliptic, it would several times each year, appear crossing the disc of the Sun. These would be transits. But as the orbit of Mercury inclines 7 degrees to the plane of the Ecliptic, transits can occur only at the nodes; making from 12 to 14 such transits in each century. The last occurred in November, 1822. Others may be predicted for May, 1832, and Novem ber, 1835. Mercury's disc appears 10 seconds broad. Wallot supposes it has an atmosphere, and Schroeter calls its mountains ten miles high.

VENUS.

VENUS is the second planet in distance from the Sun; and much exceeds all the others in splendor. Its diameter, according to Herschel, is more than 8000 miles; its density is six times that of water; and its distance from the Sun, 68 millions of miles. The time of its rotation on its axis is 23 hours; of its revolution round the Sun, 224 days, at the rate of about 84 thousand miles the hour. The inclination of its orbit to the Ecliptic 3°, and of its axis to the plane of its orbit, is supposed to be 75°, making its torrid zone extend within 15°, of either pole. The light of the Sun is twice as great as at the Earth, and falls on a similar atmosphere to our own, and on mountains, by some astronomers, considered four times as high, as the highest on our globe.

What is a transit? About how many transits of Mercury occur in a century? When was the last? When will there be others? What is the apparent diameter of this planet? In what respects is it supposed to resemble the Earth?

What planet is the second from the Sun? What is its magnitude? What its density? What is its distance from the Sun? What is the period of its rotation? What of its revolution? At what rate the hour? What inclination has its orbit to the Ecliptic? What its comparative light? What are the supposed elevations of its mountains?

Venus is morning star when westward of the Sun, and evening star when eastward; and will continue morning or evening star, a considerable time more than half the revolution of the planet, because the Earth is moving in nearly a parallel line. Its least apparent diameter is 17 seconds, and its greatest diameter 57 seconds: proving its range of distance from the Earth, to vary from 27 to 163 millions of miles. About 35 days before or after its inferior conjunction, its light is such as to cast a shadow by night, and to be visible at noonday.

Two transits of Venus like those of Mercury, occur at the nodes, once in 120 years. The two last were in 1761, and 1769, and were of great utility in determining, with accuracy, the distance of the planets from the Sun. The next tránsits will be in 1874 and 1882.

This subject will be resumed in another chapter.

The causes why the inferior planets sometimes appear stationary, and sometimes direct or retrograde in their motions, will be easily understood by an attentive examination of Plate ii. Figure 1.

CHAPTER III.

THE EARTH.

THE Earth is next to Venus, as we recede from the Sun. It is 7912 miles in its polar, and 7938 in its equatorial diameter. Its density is 5 times the density of water; its mean distance from the Sun,

When is Venus morning star? When evening star? Why each, more than half its orbit? What variations in its apparent diameter? What in its distance? When does it give most light?

How often are there transits of Venus? When were the last? When will there be others?

How will you account for the retrograde and stationary appearance of the planets?

What are the dimensions of the Earth? What is its density?