is an opaque body? Q. What are their names, beginning at the sun? A. Mercury, Venus, the Earth, Mars, (Fifty-seven Asteroids or small Planets,) Jupiter, Saturn, Uranus, and Neptune. Q. How many secondary planets or moons are there? Q. Which planets have moons? A. The Earth has 1, Jupiter 4, Saturn 8, Uranus 4, and Neptune 1. (note) LESSON VI. Question. How many revolutions has a primary planet? Answer. Two; one on its axis, and another around the sun. Q. What is the axis of a planet? A. It is a straight line, round which it turns. Q. What is the path called, in which a planet revolves around the sun? A. It is called its orbit. Q. What is the earth's orbit called? A. It is called the ecliptic. Q. Why is it so called? A. Because eclipses take place, only when the moon is in its plane. Q. How many revolutions has a secondary planet? A. Three. 1st, the revolution upon its axis; 2d, the revolution around its primary; 3d, the revolution with s a body which shines only by reflecting the its primary around the sun. luminous body. ch are the luminous bodies in the heavens ? sun' and fixed stars are luminous bodies. ch are the opaque bodies in the heavens? e moon, planets, and comets. y do the moon, planets, and comets appear luminous ? cause they reflect to us the light of the sun. at is the shape of the heavenly bodies? ey are round like a globe or ball. at do the sun, moon, planets, and comets constitute ? ey constitute the solar system. LESSON V. THE SOLAR SYSTEM. . How are the bodies constituting the solar system arranged? r. The sun is placed in the centre of the sysh the planets and comets revolving around it at distances. w many planets are there in the solar system? ney are divided into two classes, primary and ry. hat is a primary planet? is a planet which revolves around the sun only. hat is a secondary planet? is a planet which revolves around its primary, th it around the sun. hat are the secondary planets usually called? hey are called satellites or moons. ow many primary planets are there? large planets and 57 asteroids or small planets. Q. How are the planets divided, with respect to their distance from the sun? y are the two points around which the ellipse (FIG. 7.) e are these points situated? Q. What is the mean place of the earth, or a planet in its orbit? A. It is that point in its orbit where it would be if it moved in a circle, and with the same velocity at all times. (FIG. 8.) Q. What is the true place of the earth or a planet? A. It is that point in its orbit where it really is at any given time. (FIG. 8.) Q. What is the aphelion? A. It is that point in the orbit of the earth or planet farthest from the sun. (FIG. 8.) Q. When is the earth in the aphelion, or farthest from the sun? A. July 1st. (FIG. 8.) Q. What is the perihelion? A. It is that point in the orbit of the earth or planet nearest to the sun. (FIG. 8.) Q. When is the earth in the perihelion, or nearest to the sun? LESSON X Question. In what points of a planet's orbit do its mean and true places coincide? Answer. At the aphelion and perihelion. (SEE FIG. 8.) Q. What straight line connects these points, and påsses through the sun? A. The apsis line. Q. When is the true place of the earth or planet behind its mean place? A. While it is moving from the aphelion to the perihelion. (SEE FIG. 8.) he greater axis, at equal distances from the place? Q. When is the true place of the earth or planet before its mean A. While it is moving from the perihelion to the aphelion. (SEE FIG. 8.) Q. When does it move with the least velocity? A. When it is at its greatest distance from the sun. Q. Why does the motion increase from the aphelion to the perihelion? A. The centrifugal force at the aphelion is not sufficiently great to prevent its falling towards the sun. Q. When does the earth or planet move with the greatest velocity? A. When it is the nearest to the sun. Q. When is the motion of the earth or planet decreasing? A. While it is moving from the perihelion to the aphelion. Q. Why does the motion decrease from the perihelion to the aphelion? A. The centrifugal force at the perihelion is so great as to carry it farther from the sun. CENTRIPETAL AND CENTRIFUGAL FORCES. A body projected by any force would always move forward in a straight line, and with the same velocity, unless acted upon by some other force. A ball discharged from a gun or thrown from the hand soon loses its projectile force by the resistance of the atmosphere, and is brought to the ground by the attraction of the earth, or centripetal force. (FIG. 3.) These two forces can be well illustrated. (SEE FIG. 1, 2,) by tying a string to a ball and swinging it round, the centrifugal force imparted to the ball by the hand and by means of the string, causes the ball to move in a circle; but if the string should break, the centrifugal force would carry it off in a straight line, if the ball were not attracted by the earth. The string corresponds to the attraction of the on in our eclar system, which causes the planets to move in regular curves around the su trai ht lines. |