506 SCIENTIFIC RECREATIONS. 



When we say, for example, that the longitude of a place is 30, it may lie 

 on any point whatever of the line, N L s, on the whole hemisphere (fig. 5 54). 

 This point must therefore be determined more accurately, and hence the first 

 meridian is divided into 90 equal parts north and south of the equator 

 towards the poles. These are called degrees of latitude, and the lines 

 drawn through these round the globe, parallel to the equator, are called 

 circles or parallels of latitude, and diminish as they approach the poles. 



Hence, by the latitude of a place we mean its distance from the 

 equator towards the poles, and we speak of north and south latitude 

 according as the place is situated in the northern or southern hemisphere. 



So, for example, the point L (fig. 554), which has 30 longitude and 

 60 N. latitude is in Sweden. 



The latitude is also observable by ascertaining the altitude of the polar 

 star above the horizon when in the northern hemisphere. The longitude is 

 found by the chronometer ; for if we know the time at Greenwich we can 

 calculate how far we are east or west of it by seeing whether the local 

 time be an hour (say) earlier or later, and that difference shows we are i 5 

 to the east or the west as the case may be. 



The earth's rotation, according to sidereal time, is less than solar time, 

 as we have seen, so we have 365 solar days and 366 sidereal days; so a 

 person going round the world gains or loses a day as he travels east or west 

 according to his reckoning, as compared with the reckoning of his friends at 

 home. We can easily ascertain the earth's motion by watching the stars 

 rise and set. Now the path in which the earth moves is called an ellipse, 

 very nearly a circle, but it does not always move at the same rate exactly. 

 We will now look at the relations of the sun and the earth. 



Let us take an example. Suppose we have a rod, at each end of which 



we fix a ball (see diagram), and let one ball be three 



,.-"*" ">M, times as large as the other, the common centre of 



gravity will be at c, at one quarter of the distance 



/ /"" - x \ between the centres, and there the bodies will be in 



c(O-^ 1___) equilibrium. If these masses be set spinning into space 



\ \ / ../' /J they will revolve at that distance from each other, 



the attraction of gravitation and the force in opposi- 

 tion to it equalizing each other. 



The earth, as we know, proceeds with a tre- 



Fig. 555. Earth and Sun. ... 



mendous force around the sun, not in a circle, 



remember, but in an ellipse or oval track, from which it never moves 

 year by year in any appreciable degree. Now what prevents this earth 

 of ours from rushing off by itself into space? Why should not the 

 earth fly away in a direct line ? The reason is because the sun holds it 

 back. The force of the sun's gravitation is just sufficient, or we may say 

 so enormously great, that it suffices to retain our globe and all the other 

 planets in their various orbits at the very same distance, and to counteract 

 the force which launches them through space. Therefore, as we have 



