Q. In what manner do the planets move round the sun? A. They all revolve round the sun, from west to east ; but not in the same orbit, nor in the same time.

Q. What is meant by the orbit of a planet?

A. The orbit of a planet means its path or circle round the

sun.

Q. What is the motion of the planets, round the sun, called?

A. Their annual motion.

Q. Have the planets any other motion?

A. Yes; they all revolve round their own axes, from west to east, which is called their diurnal motion.

Q. What is meant by the axis of a planet?

A. The axis of a planet is an imaginary line drawn from north to south through its centre; the extreme points of which are called its poles.

Q. Are the orbits of the planets perfect circles?

A. No; they are all elliptical, or in an oval form. And the sun is placed in the lower focus of the orbits of all the planets. Q. What is meant by the focus of a planet's orbit? A. The foci or focusses of an ellipsis, are two opposite points at an equal distance from its centre.

Q. What is meant by the perihelion and aphelion of a planet?

A. When a planet is in that part of its orbit nearest the sun, it is said to be in its perihelion; (from the Greek peri, about, and helios, the sun,) and when it is in that part of its orbit farthest from the sun, it is said to be in its aphelion, (from the Greek apo, from, and helios, the sun.)

Q. Are the orbits of all the planets coincident, or in the same plane?

A. No; they all intersect or cross the earth's orbit in different parts of it.

Q. What is meant by the nodes of a planet ?

A. The nodes of a planet, (from the Latin nodus, a knot,) are the intersections of its orbit with the orbit of the earth. That part of the earth's orbit where a planet crosses, and ascends northward, is called the ascending node of that planet; and the opposite intersection is called its descending node. Q. What is meant by the ecliptick?

A. If we imagine the earth's orbit to be extended to the starry heavens, it will describe a circle quite round the heavens, which circle is called the ecliptick. It is so called

from a Greek word (ekleipo) which signifies to eclipse; because no eclipse can happen unless the moon be in or near the plane of the ecliptick.

Q. What is meant by the zodiack?

A. If we imagine two circles to be drawn parallel to the ecliptick, at the distance of eight degrees on each side of it, they will include 12 constellations, or collections of stars. This broad belt, of 16 degrees, is called the zodiack, and the twelve constellations are called the twelve signs of the zodiack. These signs are counted from west to east, beginning at the vernal equinox. Each sign is divided into 30 equal parts, called degrees. The earth, in its annual revolution round the sun, passes from west to east, through one of these signs (or rather between the sun and one of these signs) every month. The ancients imagined, that these constellations resembled certain animals, and they named them accordingly. Hence, the word zodiack is from a Greek word (zodion) which signifies an animal. The zodiack includes the orbits of all the planets, and the orbit of the moon.

SECTION II.

OF THE PLANETS.

Q. WHAT is the magnitude, distance, and periodical revolution of Mercury?

A. Mercury's diameter is 3222 miles, his distance from the sun is 37 millions of miles, and his revolution round him is performed in 87 days 23 hours. Mercury is seldom seen, as he is so near the sun.

Q. In what time does Mercury revolve on its own axis? A. It is not known.

Q. What is the magnitude, distance, and revolution of Venus?

A. The diameter of Venus is 7687 miles, her distance from the sun is 68 millions of miles, and she performs her revolution in 224 days, 16 hours. When Venus appears west of the sun, she rises before him in the morning, and is called the Morning Star; when she appears east of the sun, she shines in the evening, after he is set, and is called the Evening Star; being each in its turn 290 days.

Q. Why does Venus appear longer on the east or west of the sun, than her whole period round him?

A. Because the earth is all the while going round the sun the same way, though not so fast, as Venus.

Q. In what time does Venus revolve on her axis?
A. In about 24 hours.

Q. How is it known that the orbit of Venus includes that of Mercury?

A. Because her greatest elongation, or distance from the sun, is almost double that of Mercury's greatest elongation. Q. What is the magnitude, distance, and revolution of the Earth?

A. The Earth is 7964 miles in diameter, its distance from the sun is 95 millions of miles and its revolution round him is performed in 365 days, 6 hours nearly.

Q. In what time does the Earth revolve on its axis?
A. Once in 24 hours.

Q. What is the diameter of the Earth's orbit?

A. About 190 millions of miles.

Q. How fast must the Earth move in its orbit in order to go round the sun once in a year?

A. About 68 thousand miles in an hour; which motion is 140 times greater than that of a cannon ball.

Q. What is the inclination of the Earth's axis to the axis of its orbit?

A. About 23 degrees. It is this oblique position of the Earth, while moving round the sun, which causes the variety of seasons.

Q. What is the magnitude, distance, and revolution of Mars?

A. The diameter of Mars is 4189 miles; his distance from the sun is 144 millions of miles; and he performs his revolution in about 1 year, 321 days.

Q. What is the magnitude, distance, and revolution of Jupiter?

A. Jupiter's diameter is 89,170 miles; his distance from the sun is 490 millions of miles; and his revolution is performed in 11 years, 314 days. Jupiter is about one thousand times greater than our earth. He appears to be surrounded by faint substances, called belts, which are supposed to be clouds.

Q. In what time does Jupiter revolve on his axis?

A. In 9 hours 56 minutes.

Q. What is the magnitude, distance, and revolution of Saturn?

A. Saturn's diameter is 79,042 miles; its distance from the sun is 900 millions of miles; and its revolution is performed in 29 years, 167 days.

Saturn is surrounded by a thin broad ring, as an artificial globe is by a horizon. This ring is 21 thousand miles in breadth, which is equal to its distance from Saturn on all sides. Q. In what time does Saturn revolve on its own axis? A. In 10 hours 16 minutes.

Q. What is the magnitude, distance, and revolution of Herschel ?

A. Herschel's diameter is 34,217 miles; its distance from the sun is 18 hundred millions of miles; and its revolution is performed in 83 years, 140 days.

Q. In what time does Herschel revolve on its axis?
A. It is not known.

Q. What are those planets called which move within the orbit of the earth?

A. The inferior or interior planets.

Q. What are those planets called which move without the orbit of the earth?

A. The superior or exterior planets.

Q. What retains the planets in their orbits, and prevents them from flying off from the sun?

A. The power of attraction or gravitation. All bodies attract in proportion to the quantity of matter contained in them; and as the sun is much larger than all the planets added together, his attractive power on them is much greater than theirs on him.

Q. Which of the planets move the fastest in their orbits? A. Those nearest to the sun; and a planet moves faster when approaching its perihelion, than when it is approaching its aphelion.

2. What is the cause of this?

A. When a planet is approaching its perihelion, the projectile force and gravitating power act in conjunction, and tend to accelerate its motion; but when a planet is approaching its aphelion, the gravitating power acts in opposition to the projectile force, and retards its motion.

Q. What is meant by the projectile force of the planets? A. The projectile force (from the Latin projicio, I throw forward,) is that impulse or impetus given to them at their creation, which would have forever driven them forward in straight lines, had they not been drawn off into curves by

the attraction of the sun; and these powers are so balanced as to cause them to circulate in the manner they do.

Q. What are meant by the centrifugal and centripetal motions of the planets?

A. That tendency in the planets to fly off from the sun, is called their centrifugal force; (from the Latin centrum, the centre, and fugio, I fly,) and that attractive power which solicits them towards the sun, is called their centripetal motion; (from the Lata centrum, the centre, and peto, I seek.)

OF THE SATELLITES AND COMETS.

Q. WHAT are the satellites, or secondary planets? A. They are orbs which circulate round the primary planets, and are carried, with them, round the sun. They are called satellites, from a Latin word (satelles) which signifies an attendant.

Q. How many satellites are there?

A. Eighteen. One of which, called the moon, revolves round our earth, four revolve round Jupiter, seven round Saturn, and six round Herschel. These are all dark bodies, like our earth, and shine only by reflecting the sun's rays. Q. How large is the moon?

A. The moon's diameter is 2326 miles, it being about one fortieth part as large as the earth.

Q. How far distant is the moon from the earth?

A. About 240 thousand miles.

Q. In what time does the moon revolve round the earth?
A. In about 29 days, 12 hours, 44 minutes.

Q. In what time does the moon revolve on her axis?

A. The moon revolves on her own axis in the same time that she revolves in her orbit, which is the cause of our always seeing the same side.

Q. Is the moon's orbit a perfect circle?

A. No, it is elliptical, like the orbits of the planets. And when she is in that part of her orbit nearest the earth, she is said to be in perigee; (from the Greek peri, about, and gē, the earth,) and when she is in that part of her orbit farthest from the earth, she is said to be in her apogee, (from apo, from, and gē, the earth.)

Q. What is an eclipse?