THE PLANETS. 



487 



other, is directly proportioned to their masses (not size), and inversely to the 

 square of their distances apart. 



This law operates amongst the heavenly bodies, and it is to the never- 

 changing action of gravitation that the planets are kept in their places ; Let 

 us see how this is effected. We have read of force, and motion, and rest. 

 Every body will remain at rest unless force compels it to change its position, 

 and it will then go on for ever in a straight path unless something stops it. 

 But if this body be acted on simultaneously by two forces in different 

 directions, it will go in the direction of the greater force. Two equal forces 

 will tend to give it an intermediate direction, and an equal opposing force 

 will stop it. The last axiom but one viz., the two equal forces in 

 different, not opposing directions, gives us the key to the curving line of 

 the planetary motions. Were it not for the attraction of the sun the 

 planets would fly off at a tangent ; while, on the other hand, were not 



the impelling force as great as it is, they would 

 fall into the sun. Thus they take an intermediate 

 line, and circle round the centre of the solar system 

 the SUN. 



The solar system consists of the sun and the 

 planets which revolve in space around him. These 

 stars are called planets because they move in the 

 heavens. We shall see that they have certain motions 

 going from east to west, from west to east, and 

 sometimes they appear to be quite motionless. 

 This change of place, appearing now at one side 

 of the sun and now at another, has given them their 

 title of " wanderers " (planets). Besides the planets 

 there are comets and meteors, asteroids and satel- 

 lites, all circling round the sun in more or less regular 

 orbits. And there must be families of comets, and 

 -whole systems of meteors that have not yet ap- 

 peared to us, and which make up the comets, as has been lately suggested. 



Five planets were known to the ancients, and were named after 

 gods and a goddess: MERCURY, VENUS, MARS, JUPITER, SATURN. In later 

 years a great number were discovered. We must, however, confine ourselves 

 to the consideration of the principal ones, eight in number, including our own 

 EARTH, URANUS and NEPTUNE completing the list. Of these VENUS and 

 MERCURY are the inferior, or interior planets moving between us and the 

 sun. MARS, JUPITER, SATURN, URANUS, and NEPTUNE are superior, or 

 exterior, and pass quite round the heavens. All the planets are spheroids, 

 and vary greatly in their magnitude, as will be seen by the illustration 

 (fig. 528), the largest body being the sun. Mercury, Mars, and Venus, are 

 not so large as the Earth. The other principal planets are considerably 

 larger than our globe. 



Mercury is the smallest of the planets, Venus being nearly as large as 



Fig. 527. The Moon. 



