Opaque bodies are those which shine only by light received from some luminous body. The sun and stars are luminous bodies. The planets and comets are opaque bodies. A system, in astronomy, is a certain assemblage of heavenly bodies. The Solar System is that which consists of the sun, with the planets and comets revolving around it. Planets are opaque bodies, which revolve around the sun. They are of two kinds, primary and secondary. Primary planets are those which revolve around the sun only. Secondary planets are those which revolve around their primaries, and, with them, around the sun. Secondary planets are sometimes called Satellites. There are eight primary planets in the solar system, viz., Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Herschell or Uranus, and Neptune or Leverrier, besides the minor planets or Asteroids. There are twenty-one secondary planets, viz., the Earth has one, Jupiter four, Saturn eight, Uranus six, and Neptune two. Mercury and Venus are called inferior planets, because their orbits are within that of the earth. Mars, Jupiter, Saturn, Uranus, and Neptune, are called What are opaque bodies? What bodies are luminous? What bodies are opaque? What is a system? What is the solar system? What are planets? Of how many kinds are planets? What are primary planets? What are secondary planets? What are they sometimes called? How many primary planets are there? How many secondary? Which are the inferior planets? Which are the superior? superior planets, because their orbits are outside that of the earth. The Asteroids are small planets which revolve around the sun, between the orbits of Mars and Jupiter. They are fifteen in number, viz., Vesta, Juno, Ceres, Pallas, Astraea, Iris, Hebe, Flora, Metis, Hygeia, Parthenope, Clio, Egeria, Irene, and Eunomia. Comets are bodies which revolve around the sun in very elongated orbits, and are generally accompanied by a long train of light. The Stars are those bodies which appear never to change their positions with respect to each other. They are, for this reason, called Fixed Stars. The Orbit of a planet, is the path in which it revolves around the sun or central body. The plane of the earth's orbit is called the Ecliptic. The Elongation of any body, is its angular distance from the sun. Fig. 9. Thus, if E represent the earth in its orbit, and V, or V', Venus in its orbit, the angle VES, or V'ES, will represent the elongation of Venus at either of the two points. At I and O, it will be seen the elongation is nothing, while at V it is greatest. A body is said to be in Conjunction with the sun, when it appears in the same part of the heavens. Conjunction is of two kinds, Inferior and Superior. What are the asteroids? How many are there? Name them. What are comets? What are the stars? a planet? What is elongation? What are they called? What is the orbit of Inferior conjunction is when the body is between the earth and the sun. Superior conjunction is when the body is on the opposite side of the sun from the earth. A body is said to be in Opposition, when the sun and body are on opposite sides of the earth. A body is said to be in Quadrature with the sun, when it is 90 degrees distant from it. The Axis of a body, is an imaginary line around which it revolves. Diurnal rotation is the revolution of a body on its axis, and is called its day. Annual revolution is the revolution of a body around the sun, and is called its year. The Mass of any body, is the quantity of matter which it contains. What is inferior conjunction? What is superior conjunction? position? What is quadrature? What is the disc of a body? digit? What is the axis of a body? What is diurnal rotation? nual revolution? What is the mass of a body? What is op What is a What is an The Density of any body, is the degree of compactness of its substance. The Angular diameter of a body, is its apparent diameter, expressed in minutes and seconds, as seen from the earth. The Linear diameter is its actual diameter. The heavenly bodies are, in general, oblate spheroids. QUESTIONS FOR EXERCISE. What is the elongation of a body in quadrature? What bodies can be in inferior conjunction? What bodies can be in superior conjunction? What bodies can be in quadrature? Can the elongation of Venus exceed 90 degrees? (Fig. 9.) Can that of Mercury? Can that of Jupiter? What is the greatest elongation of Mars-Saturn ?—Neptune? When Venus is in inferior conjunction, and Mars in opposition, what is the angular distance between the two bodies? What is the angular distance of one from the other, if Venus is in inferior conjunction, and Mars in superior conjunction ? What is their angular distance, when Venus is in superior conjunction, and Mars in quadrature? What is the density? What is the angular diameter? What is the linear diameter? What is the shape of the heavenly bodies? CHAPTER III. PLANETARY MOTIONS. ALL the planets move in their orbits from west to east. A planet's revolution in its orbit, is sustained by the united action of two forces, viz., the Centripetal and Centrifugal forces. The Centripetal force is that which draws a body towards the centre, or body around which it is revolving. It is also called the attraction of gravitation, being, in fact, the power which all bodies possess, of mutually attracting each other. The Centrifugal force is that by which a body tends to fly off from the orbit in which it is revolving. It increases with the velocity of the body. Laws, in astronomy, are general and invariable facts respecting the motions of the heavenly bodies. The following are the three great laws of planetary motion, discovered by Kepler, and hence called Kepler's laws: 1. The planets' orbits are ellipses, having the sun or central body in one of the foci. 2. The radius-vector of a planet's orbit, passes over equal spaces in equal times. In what direction do the planets move in their orbits? How is a planet's revolution in its orbit sustained? What is the centripetal force? What is this force also called? What is the centrifugal force? How does it increase? What are laws in astronomy? Recite Kepler's laws. |