are called secondary planets, moons, or satellites. The number of comets is unknown. The sun is the centre of the system, and the eleven primary planets, at different distances, and in different times, move round him, from west to east, in the following order, Mercury, Venus, the Earth, Mars, Vesta, Juno, Pallas, Ceres, Jupiter, Saturn, and Herschel or Uranus. The Earth has one moon, Jupiter four, Saturn seven, and Uranus six. Venus and Mercury being nearer to the sun than our earth, are called inferior planets, and all the rest, which are without the earth's orbit, are called superior planets; some astronomers distinguish them by the terms interior and exterior, which seem preferable. The planets are retained in their orbits by the united operation of the centripetal force, by which a body is attracted to the centre of gravity, and the centrifugal force, by which it endeavours to persevere in a straight line. These two powers, mutually balancing each other, compel them to make their respective revolutions. The time of performing their revolutions round the sun is called their year, and the time of performing their revolution on their axis, their day. The axis of a planet is an imaginary line conceived to be drawn through its centre, about which it revolves, and the extremities of this line, terminating on opposite points of the planet's surface, are called its poles.

The first material step in improving the science of astronomy was the establishment of the present arrangement of the sun and planets by Copernicus, who died in the year 1543. This doctrine, it is true, was held by Pythag'oras, but it was now presented in a new and stronger light, with its leading features more fully and distinctly unfolded. It is remarkable, that in so many instances, it should have exposed its authors and defenders to persecution. Pythagoras, we are told, made it known only to a select few; but one of his disciples, who had the courage to teach it publicly, was obliged to flee in order to escape the odium it excited. Copernicus meditated upon the subject for many years, before he undertook to give his thoughts to the world, and scarcely surviving the publication of his work, he left to others to receive the shock that awaited those who espoused it. Galileo could not resist the accumulated evidence, that presented itself to his enlarged and philosophic mind, in favour of this refined scheme, and was accordingly destined

to bear the whole weight of indignation that was ready to burst upon the disturbers of a prejudice so old and so deeply rooted. He was arrested, and seven cardinals clothed with the authority of the church sat in judgment upon him, and sentenced him to the prison of the Inquisition for opinions, which they pronounced false in philosophy, heretical, and contrary to the word of God. After a year's confinement he was liberated, but continuing his discoveries, and apparently persevering in his opinions, he was imprisoned a second time. After being made to abjure what were deemed his errors, and to do penance for his offences, he was again restored to liberty. Indignant at the cruelty of this treatment, and the bigotry and blindness of his persecutors, he yet continued his pursuits; but in silence and fear. His excessive application, and the constant use of his telescope, together with frequent exposure to the air by night, had such an effect upon him, that he lost his sight. He died in 1642, at the age of seventy-eight.

QUESTIONS.-1. Of what does the solar system consist? 2. What are primary planets?-secondary planets? 3. What is the order in which the eleven primary planets move round the sun? 4. What planets have moons, and how many have each? 5. What are interior and exterior planets? 6. How are the planets retained in their orbits? 7. Define year, day, axis, poles. 8. What was the first material step in the progress of astronomy? 9. What is remarkable with respect to the true doctrine of the solar system? 10. What course did Copernicus adopt? 11. What is said of Galileo? 12. Explain Engr. IV.

LESSON 42.

The Sun.

Spheroid, a body approaching to the form of a sphere, but not exactly round.

Elliptical, oval,-an ellipse is produced from the section of a
cone by a plane cutting both its sides, but not parallel to the
base. All the planets move round the sun in elliptical orbits,
and the sun itself is situated in one of the foci of each ellipse
that focus is called the lower focus. See the Earth's orbit in
fig. 40.

GREAT source of day! best image here below
Of thy Creator, ever pouring wide,

From world to world, the vital ocean round;

On nature, write with every beam, His praise.
Soul of surrounding worlds!

'Tis by thy secret, strong, attractive force,
As with a chain indissolubly bound,

Thy system rolls entire; far from the bourn
Of utmost Herschel, wheeling wide his round
Of eighty years; to Mercury whose disk
Can scarce be caught by philosophic eye,

Lost in the near effulgence of thy blaze. THOMSON.

The sun is a fountain of light that illuminates the world; it is the cause of that heat which maintains the productive power of nature, and makes the earth a fit habitation for man. The figure of the sun is a spheroid, higher under the equator than about the poles; and his diameter is computed to be nearly nine hundred thousand miles. His solid bulk is more than a million of times larger than that of the earth. The sun has two motions; the one is a periodical motion, in an elliptical or very nearly circular direction, round the common centre of all the planetary motions; the other is a revolution upon its axis, which is completed in about twenty-six days. That the sun has a rotation round his axis is made evident by the spots seen on his surface. Some of these spots have made their first appearance near the edge or margin of the sun, and have been seen some time after on the opposite edge; whence, after a stay of more than thirteen days, they have re-appeared in their first place, and taken the same course over again. These spots were entirely unknown before the invention of telescopes, though they are sometimes of sufficient magnitude to be discerned by the naked eye. Some have been so large, as by computation to be capable of covering the continents of Asia and Africa, the whole surface of the earth, or even five times its surface. The sun has commonly been considered a globe of fire; but this has been doubted by modern astronomers. The celebrated Herschel considers the sun as a most magnificent habitable globe, surrounded by a very extensive atmosphere, which consists of elastic fluids that are more or less lucid and transparent; and of which the lucid ones furnish us with light. The appearances, called spots in the sun, he considers as real openings in the luminous clouds of the solar atmosphere.

The sun is accompanied by a phenomenon called the zodiacal light. It is a beam of light of a triangular form, visible a little after sunset and before sunrise, with the base towards the sun. It is most clear about the beginning of March in the evening, and in September in the morning, but in the torrid zone it is constantly seen. It is generally supposed to proceed from the sun's atmosphere.

QUESTIONS.-1. What is the figure of the sun? 2. Describe the motions of the sun. 3. How is it made evident that the sun has a rotation round his axis? 4. What is said of the spots that have been seen in the sun? 5. What does Dr. Herschel consider the sun to be? -The spots? 6. Describe the zodiacal light. 7. In what proportion do the planets receive light and heat from the sun? (see Appendix.) 8. What rule is given? 9. What is said of the attraction of bodies? 10. What is the rule for finding the distances of the planets from the sun? 11. What was ascertained by Kepler? 12. What is the rule for finding how many times one planet is greater than another? [NOTE. When any body, revolving round the sun, is nearest to him, it is said to be in its perihelion; and when it is most distant, in its aphe'lion (pron. ǎf-e'le-un.) The common centre about which the sun revolves in its periodical motion is always found to be exceedingly near the sun, and most commonly within it: it may, therefore, without any material error, be regarded as the centre of the planetary system.]

LESSON 43.

Mercury and Venus.

Elonga/tion, a planet's elongation, or its angular distance from the sun, is an angle formed at the earth by two lines, one drawn from the earth to the sun, and one from the earth to the planet. Disk, the face of the sun and moon, as it appears to us on the earth.

MERCURY is seldom visible to the inhabitants of the earth, for its greatest apparent distance from the sun, or its greatest elongation, is not more than twenty-eight degrees, and its reflected light is absorbed in the more powerful rays of the sun. He always appears on the same side of the heavens with the sun; of course, he can be seen in the east, only in the morning a little before sunrise, and in the west in the evening a little after sunset. When viewed with a telescope of high magnifying power, he exhibits nearly the same phases as the moon, and they are to be accounted for in the same manner. Mercury revolves round the sun at nearly

the mean distance of thirty seven millions of miles, and completes his revolution in about three months. According to Sir Isaac Newton, the heat and light of the sun on the surface of Mercury, are almost seven times as intense as on the surface of the earth in the middle of summer; which, as he found by experiments made for that purpose with a thermometer, is sufficient to make water fly off in steam and vapour. Such a degree of heat, therefore, must render Mercury uninhabitable to creatures of our constitution; and if bodies on its surface be not inflamed and set on fire, it must be because their degree of density is proportionably greater than that of such bodies is with us. When Mercury passes over the sun's face, or is between us and the sun, this is called his transit, and the planet appears like a black spot in the sun's disk. The light emitted by Mercury is a very bright white.

Fair Venus next fulfils her larger round,

With softer beams, and milder glory crowned;
Friend to mankind, she glitters from afar,
Now the bright evening, now the morning star.
BAKER.

Venus is computed to be sixty-eight millions of miles from the sun, and completes her annual rotation in about seven and a half months, turning on her axis in a little less than twenty four hours. The light, which this planet reflects, is very brilliant, and often renders her visible to the naked eye in the day-time. When Venus is to the west of the sun, she rises before the sun, and is called the morning star; when she appears to the east of the sun, she shines in the evening, and is then called the evening star. She is in each situation alternately, for about two hundred and ninety days; and, during the whole of her revolution, she appears, through a telescope, to have all the various shapes and appearances of the moon. As the orbit of Venus is within that of the earth, like Mercury, she sometimes passes over the sun's face, and her transits have been applied to one of the most important problems in astronomy,-that of determining the true distances of the planets from the sun. The atmosphere of Venus has been calculated to be fifty miles high; this has been learned from observing her transits, when her atmosphere was seen to throw a shade on the sun's disk