Mercury exhibits the same difference of phases with the moon; being sometimes horned, sometimes gibbous, and sometimes shining almost full. Like the moon, the crescent is always turned towards the sun. The distance of this planet from the sun is computed at thirty-two millions of miles, and his diameter at 2,600 miles. In his course round the sun, he moves at the rate of 95,000 miles every hour. The light and heat which he receives from the sun, are almost seven times as great as the quantity which the earth receives; and, to his inhabitants, the sun must appear nearly seven times as large as he does to us. It is evident that Mercury revolves round the sun in an orbit within that of the earth; because he is never seen opposite to the sun, nor distant above fifty-six times the sun's breadth, from his centre.

The planet nearest to the sun, after Mercury, is Venus, the most beautiful of the stars, known by the name of the morning and evening star; because, when she appears west of the sun, she rises before him in the morning; and when in the east, she shines in the evening, after he is set. She presents to view the same phenomena as Mercury; but her different phases are far more apparent.

Venus is computed to be 59,000,000 miles from the sun; and moving at the rate of 69,000 miles every hour, she revolves round the sun in 224 days, 17 hours, of our time. Her diurnal motion is performed in twenty-three hours and twenty minutes, according to one astronomer, Cassini; but according to the opi

nion of another, Bianchini, Venus takes twentyfour days, eight hours, for each revolution round her axis. Her diameter is 7,906 miles; and by her diurnal rotation, her equator moves at the rate of forty-three miles every hour. Her orbit includes that of Mercury; but is within that of the earth. Several astronomers imagined that they discovered a moon attending upon Venus; but this was, it seems, an optical deception, and no satellite belonging to this planet has, as yet, been seen. She receives twice as much light and heat from the sun as does the earth. According to the measurement of Schroeter, the mountains in Venus are of a still more astonishing height than that he measured in Mercury. A luminous margin round this planet, gives reason to suppose that it has an atmosphere of considerable

extent.

The planet Venus has been considered by most astronomers to be about 220 miles less in diameter than the earth; but from the measurements of Herschel, it appears that her apparent mean diameter is 18'.79, or 8,648 miles; while that of the earth is 17''.2, or 7,912 miles.

Mercury and Venus are called inferior planets, because their orbits are included within that of the earth. They are frequently seen between the earth and the sun; and consequently they must be nearer to that luminary than to the earth. These planets appear, at times, to pass over the sun's disk; and such passages are called transits of Mercury and Venus.

QUESTIONS.

What is astronomy? Is astronomy an ancient science, and what nations of antiquity cultivated it? Who were the most illustrious astronomers of ancient and modern times? How does astronomy divide the heavenly bodies? How far distant from the earth, is the nearest fixed star? What are constellations? What was the ancient, and what is the modern number of the constellations? What is the galaxy or milky way? What are nebulæ ? Of what bodies does the mundane, or solar system, consist? What were the planets known to ancient astronomy, and in what order are they situated with regard to the sun? What new planets have modern astronomers discovered, and how are they situated with respect to the sun? What are comets? What opinions are formed concerning the nature of the sun? What appearances does the sun's disk present? What motions has the sun? What is the zodiacal light? In what time does Mercury revolve round the sun, and at what distance? What is the diameter of Mercury? Is Mercury easily observed ? At what rate does Mercury move round and what quantity of light and heat does he receive from that luminary? At what distance is Venus from the

the sun,

sun,

and in what time does she revolve round him? Why is Venus called the evening and morning star? What is the diameter of Venus, and what quantity of light and heat does she receive from the sun? What is said concerning the mountains of Mercury and Venus? What peculiar appearances do Mercury and Venus present? What common name is given to Mercury and Venus? Why are the orbits of Mercury and Venus supposed to be included within that of the earth? What are transits of Mercury and Venus?

CHAP. XVIII.

ASTRONOMY - continued,

THE earth is placed next after Mercury and Venus, as to distance, from the sun; round whom

it revolves from tropic to tropic, in 365 days 5 hours, 48 minutes, 57 seconds. Its form is an oblate spheroid; that is, round, but flattened a little at the poles, and swelling out at the equator.

Its spherical figure is shown by its shadow being round, as seen in eclipses of the moon; by navigators having actually sailed round it; by the circumstance of the masts of vessels sailing from land remaining in sight longer than their hulks; and by that of the peaks of mountains, and steeples of towers, being first visible to those who are approaching the shore from sea.

The earth revolves about its axis in 23 hours, 56 minutes, 4 seconds. This is called its diurnal motion; by which all parts of the Earth's surface, are successively turned towards and from, the sun; and consequently, pass successively through light and darkness.

The distance of the earth from the sun is, between 95 and 96 millions of miles; and its diameter is about 8000 miles. It revolves round the sun in 365 days, 5 hours, and 49 minutes; moving at the average rate of 58,000 miles every hour; but this motion of the earth in its orbit is irregu lar.

The movement of the earth round its own axis from west to east, occasions the apparent diurnal motion of the heavenly bodies from east to west. By this rapid movement, the inhabitants about its equator are carried at the rate of 1,042 miles an hour; and those on the parallel of London, about 580 miles. The earth's motion in its orbit, causes the apparent annual movement of the sun; and the plane of its orbit being inclined to that of the equator, which is the plane

of its diurnal motion, causes the difference of seasons and climates; and the inequality in the length of the days and nights. Besides its diurnal and annual revolutions, the earth has another motion called the precession of the equinoxes. This is a rotatory motion in the axis of the earth round the axis of the ecliptic. If the inclination of these axes remain the same, their revolution is accomplished in 27,000 years.

The seas and unexplored parts of the earth contain 160,522,026 square miles: the inhabited parts contain 38,390,569 square miles; Europe, 4,456,065; Asia, 10,768,823; Africa, 9,654,807 ; America, 14,110,874. In all, 199,512,595; the number of square miles on the whole surface of the globe.

The moon is a secondary planet attendant upon the earth, as her centre of motion, and near whom she is constantly found; so that, if viewed from the sun, she would never appear to depart from the earth, by an angle greater than 10." The moon revolves round the earth in twenty-nine days, twelve hours, and forty-four minutes, at the distance of about 240,000 miles. The orbit

in which she performs this movement is an ellipsis. Her rate of motion, is about 2290 miles every hour. She revolves round her own axis in the same time that she goes round the earth; and, consequently, she has but one day and one night in the course of one of our months. The moon shines by reflecting the light of the sun; and sometimes a large portion of her disk is faintly illuminated by light reflected to her from the