LKSSONS IN ASTlinNOMY. 



327 



LESSONS IN ASTRONOMY. VI 



THE SUN AND MOON MOTIONS OF TUB KAHTH IT8 FIGURE 

 H . M Tin: roLS8 PROOFS THAT THE I 



IB KOUND HOW SUPPORTED IN SPACE IIATIONAL AND 

 8EN8IHLE UOKIZONS. 



THE great apparent size of the aun and moon, as compared with 



t. of tho heavenly bodies, often leads us to overlook them 



when we think or speak of the store, and to regard them as 



belonging altogether to a different class. Henoe we usually 



look upon astronomy as a science to bo studied only by night, 



when in reality the star on which wo are most dependent, and 



whose movements with regard to us are of the greatest import- 



MI" sun whose presence causes our day. We use the 



t.-rin "star" here advisedly, for the sun is in reality to be 



classed as one of tho fixed stars, the reason of its great apparent 



1 brilliancy being merely that it is very much nearer to 



i any of the rest, its distance being reckoned by millions 



s only, while that of even the nearest fixed star requires 



billions to express it. 



The moon, though ranking next to the sun in its importance 

 to us, is in reality tho smallest of all our neighbours in space 

 which can bo detected by the unaided eye. It will, however, be 

 at once understood that both it and the sun will, on account of 

 their importance to us, claim a large share of our attention. 



Now, as we have already stated, the 

 sun, the moon, and all the stars appear 

 to be in constant revolution around us, 

 iiiul most of the phenomena we referred 

 to in our last lesson would be explained 

 by imagining all these bodies to bo fixed 

 to the inner surface of a hollow sphere 

 in the centre of which the earth was 

 situated, and then supposing this sphere 

 to be in constant rotation. One 

 end of the axis on which it 

 turned would in this case be 

 close against the pole-star, and 

 the other in the part of the sky 

 diametrically opposite to it. This 

 was, accordingly, for a long time 

 the received notion, and it is the one 

 wliich would naturally strike an observer 

 at first, for the earth seems to be at 

 rest, nor can we in any way by our 

 senses discover that this is not tho case. 

 The reason of this is that motion is to us 

 a relative idea, and if we ourselves and 



some this may seem a question pertaining rather to geograph? 

 than to astronomy. It is, however, intimately connucted with 

 both ncienoes ; and, as the earth is our stand-point of observa- 

 tion, it is of paramount importance to understand T***'^ 

 about it first of all. 



form of the earth, then, is almost spherical, bat not 

 absolutely HO; for it is somewhat flattened at the pole*, so that a 

 section passing through them would be slightly elliptical instead 

 of circular. This deviation from the spherical form is, however, 

 very slight ; so slight, indeed, that it would not be seen in anr 

 model that we could make. Suppose, for instance, we made a 

 globe with a diameter of thirty inches, the difference would only 

 be ^ of an inch, too small for even the keenest eye to detect. 

 The real dimensions are almost as follows : 



Greater or Equatorial Diameter 

 Lesser or Polar 



mi MBS* 



; 



tho objects around us are moving along together at exactly the 

 same rate, we do not observe the motion. If, for example, wo 

 are seated in the cabin of a boat when the water is perfectly calm 

 and the vessel is being propelled at a uniform rate, we are quite 

 unconscious of the movement. If, however, the motion be not 

 uniform, but consist of a succession of jerks and shakes, as when 

 travelling in a railway train, or if we look on the fixed and stationary 

 objects around, we shall soon become conscious of the fact that 

 we ourselves are being carried along. Now, as the motion of 

 the earth is perfectly uniform, and all terrestrial objects, includ- 

 ing the air, move with it, it is only by looking to the stars that 

 we become conscious of our movement. 



We cannot here go into all the reasons which prove that this 

 is the case, but we can easily see enough to satisfy any thought- 

 ful mind. We have only to look at the earth as a globe about 

 8,000 miles in diameter, and to remember that the diameter of 

 the sun is 111 times as great, and that all tho stars are largo 

 globes situated at enormous distances from us, and then ask 



showing a difference of a little over twenty-six miles. 



Several important effects arise from this. The surface of the 

 earth near the equator is, of course, further from the centre 

 than tho surface near tho poles ; and as the attraction of gravita- 

 tion diminishes with the in crease of distance, it is weaker at the 

 equator than it is further north or south. 



If a pendulum bo accurately adjusted so as to beat seconds in 

 ! the latitude of London, and then be moved further south, it 

 will, from this cause, beat more slowly. 

 So likewise, if a spiral spring be sus- 

 pended from a hook and a weight hung 

 from its lower end so as just to touch 

 the stand, it will be found, if we convey 

 it carefully, that as we approach the 

 equator the spring will be somewhat 

 shortened, and the weight will no longer 

 touch the stand as it did before; the 

 weight, in fact, appears to be less 

 than it was in the higher latitude. 

 A little consideration will soon 

 show us the cause of this flatten- 

 ing of the earth. If we suspend 

 a child's pail, filled with water, 

 by a piece of string, and then by 

 twisting the string cause it to torn 

 round rapidly, we shall at once see the 

 liquid leaving the centre and becoming 

 heaped up against the sides, and, if the 

 motion be sufficiently rapid, a portion 

 will be thrown off and scattered from 

 the edge. A similar but more conclu- 

 sive experiment may be tried with a common hoop, which for 

 this purpose should be mode as thin as possible. Let it have 

 two holes drilled through its sides opposite to one another, 

 and let it be placed vertically on a pivot in such a way that it 

 can be caused to rotate rapidly ; the upper side, however, 

 must not be fastened to the pivot. On spinning it rapidly, it 

 will at once be seen that the hoop becomes longer and flatter 

 from the action of centrifugal force upon it. 



The same reasoning applies to the earth. It is a body in rapid 

 rotation on its axis, and hence there is a tendency in all the 

 particles of matter composing it to become heaped up at the 

 equator, for the motion there is manifestly more rapid than at 

 the other parts of its surface. Careful calculation confirms this 

 theory, by showing that the difference between the two diameters 

 of the earth is just the amount that would be produced by this 

 cause. 



There are many familiar proofs of the rotundity of the earth. 

 One of the simplest of these is afforded by watching the depar- 



which is the more probable, that these mighty orbs should all I ture of a vessel from a seaport town. As a large object is 

 travel at an utterly incredible speed around this small globe on . always visible at a greater distance than a small one, we should 

 which wo live, or that the earth should itself turn round on its naturally expect that the masts and rigging would disappear first 

 axis ? If the former is true, the more remote stars must dash 

 through billions of miles in a single second of time, and all the 

 rest must be in rapid motion, round a body very much smaller 

 than themselves ; by the latter theory, all this is done away 

 with, and we simply see the earth rotating so that a spectator 



on its surface is turned in the course of a day and night towards 

 all parts of the sky. 



Having thus settled in our minds the foot of the motion of 

 t'.io earth on its axis, we must next inquire as to its shape. To 



100 N.K. 



in the distance, while tho hull would be the last thing to fade 

 from sight. Observation, however, soon shows us the contrary. 

 The whole ship remains visible till it has left the shore some 

 little distance, and then it seems to be gradually sinking ; the 

 lower port of the hull being the first to disappear, and then the 

 bulwarks, while the tops of the masts are the last things hid- 

 den, as seen in Fig. 5, which is, however, purposely exaggerated. 

 The reason of this evidently is that something is interposed 

 between the observer and the vessel, and that something is the- 



