<SONS IN ASTRONOMY. 



: 







compasses, and any globular body an, for instance, a cannon- 

 ball, or a common round bottle. If we open the compasses 

 BO that the poinU just touch opposite sides of the body, 

 we shall find that the angle contained between the leg* 

 M but small, while a considerable portion of the surface is 

 included between the place touched by them. Now press the 

 hinge nearer and nearer, and it will be seen that the angle 

 becomes larger and larger, while at the same time the space 

 included becomes leas. When the hinge nearly touches, the two 

 limbs will bo almost in a straight lino. 



If now wo imagine the hinge to represent the position of the 

 observer and the limbs the visual rays, we shall understand 

 that when the observer is at all elevated above the earth's sur 

 face, the sensible horizon appears depressed below him, and this 

 angle of depression is called the dip of the horizon. Thus, in 

 the figure, if v a be the horizontal line passing through the point 

 A, as determined by a level, then the angle p A c or o A B (for 

 they are equal) will bo the dip. 



The globe we inhabit is completely surrounded by a layer of 

 invisible gases known as the air or atmosphere. We live, in 

 fact, at the bottom of an aerial ocean, the depth of which is 

 supposed to be about forty-five or fifty miles ; it diminishes, 

 however, very rapidly in density as we ascend. The physical 

 properties of this envelope will be referred to at length in the 

 Lessons on Pneumatics in another volume : we need not, there- 

 lore, inquire generally into them here ; we must, however, refer 

 to the influence of the air on our observations. At many times, 

 especially in our changeable climate, the amount of moisture in 

 the air is so great that it is almost impossible to employ a 

 powerful telescope. The higher powers of Lord Basse's large 

 instrument can, on this account, only be used at rare intervals, 

 sometimes only for a few times in the course of the year. The 

 main effect, however, which we must speak of, is that known as 

 refraction. If a ray of light in its course pass from any medium 

 into another differing from it in density, it becomes bent out of 

 its original path. The simplest plan of showing this is to put 

 a stick partly into a pond or a vessel of water, when it will 

 appear to be bent at the surface of the liquid, owing to the rays 

 from the lower part being refracted as they leave the water. 



Now our atmosphere, though very rarefied when compared 

 with bodies on the earth's surface, is much denser than the 

 fluid which is supposed to pervade all space. The lower layers 

 of the air likewise ore much denser than those more elevated, 

 and hence the rays of light which reach our eyes from the stars 

 are more or less bent, so that we do not see these objects in the 

 places in which they really are. Now if we are to ascertain the 

 true position they occupy and we must if we would calculate 

 their movements we must learn what the effect of refraction 

 really is, and what allowance has to be made for it. 



Fif. 9 in the next page will make the matter more clear. Let 

 the Ikie E c T represent a portion of the earth's surface, c being the 

 position of the observer ; the horizontal line H c o touching the 

 earth at this point will then represent his sensible horizon. 

 Also let M D and u p represent the limits of successive layers of 

 t.ie atmosphere, each being less dense than the one below it. 



s is the actual position of the sun or any bright star in the 

 heavens, and we should at first suppose that it would be seen by 

 a ray of light passing straight from c to s. This, however, is 

 not the case ; for as soon as this ray meets the atmosphere it 

 is bent downwards, and as it enters each successive stratum it 

 is more and more deflected, so that it reaches the earth at a 

 point situated some way to the right of c. The ray of light by 

 which the star is in reality seen, is one which, but for the air, 

 would pass on to L. It is, however, refracted at A to the direc- 

 tion A N, and again at B into the direction B c, each of these 

 refractions bringing it more nearly into the vertical direction. 



Now we always imagine an object to lie in the direction in 

 which a ray from it reaches the eye. Hence, if we now prolong 

 c B, we shall find the apparent place of the star, s', differing 

 considerably from its real place, s. We see thus that the effect 

 of refraction is to cause the heavenly bodies to appear more 

 elevated above the horizon than they really are. 



The amount of alteration thus produced is greater when a 

 body is near the horizon, and diminishes as it ascends, till at 

 the zenith, as at s", no effect at all is produced by it. All 

 observations are accordingly, as far as practicable, made when 

 the star or planet has attained its greatest altitude, and this is, 

 as has been explained, when it is on the meridian. 



In our figure wo only drew two layers of air, bat in reality 

 the density diminishes very gradually, so that there are MI 

 infinite number ; the only difference, however, that this rns<ie, 

 U to make the ray curve evenly itmtead of by a series of brads. 

 The amount of refraction when the object is situated near the 

 horizon is about 33', or rather more than the apparent diameter 

 of the son, so that iU lower edge ju*t appears to touch the 

 horizon when in reality iU upper edge in entirely below it. 

 Another effect of refraction is somewhat to distort the form of 

 the sun or moon when they are rising or setting. In this posi- 

 tion they will frequently appear elliptical in form, as if they 

 were a little flattened. The real cause of this u the rapid rate* 

 at which refraction increases as we approach the horizon. The 

 lower portion of the sun's disc is, accordingly, more elevated In 

 this way than the upper side, and thus the vertical diameter 

 appears less than it otherwise would. The lateral diameter u, 

 of course, quite unaffected by this cause. The increased size of 

 the sun or moon when near the horizon does not arise from the 

 effects of refraction. It is merely an illusion arising from the 

 fact that we see these bodies by the side of terrestrial objects, and 

 thus compare their size* with known object*. If we measure 

 carefully their apparent diameters, we shall find they are just 

 the same as when higher up in the sky. 



We have not yet referred to the effect of refraction which i 

 of the greatest importance to us in our every-day life, namely, 

 its influence in producing twilight. As we have seen, the ran 

 is visible for a short time after it is in reality below the horizon, 

 but after it has disappeared many of its rays continue for a 

 considerable period to reach us. They cannot, of course, come 

 direct to us, being prevented by the curvature of the earth ; but 

 they pass through the upper layers of the air, and in so doing 

 get bent and reflected so as to reach the earth, and thus, for 

 some time after the sun has set, the whole sky is brilliantly 

 lighted. The floating particles of vapour in the atmosphere, 

 and, it may be, even the particles of the air itself, reflect many 

 of these rays, just as when a ray of sunlight shines through an 

 aperture into a dusty room, the fragments of dust diffuse enough 

 light feebly to illuminate the apartment. 



Were it not for this effect of the air, the sun would set, and 

 immediately the deepest darkness would overshadow and con- 

 ceal everything, so that the period of sunset would be one of 

 great danger. As it is, however, twilight continues until the 

 sun is 18 below the horizon, the light of day only gradually 

 giving way to the darkness of night, and thus we have the 

 benefit of the sun's light for a much longer period than we 

 should otherwise have. In our latitude, as a reference to the 

 globe will show, the sun never descends 18 below the horizon 

 from the end of May to the middle of July, so that during all 

 that period we have no real night ; and in Arctic regions the 

 twilight in some places lasts several weeks, so that it nicely 

 relieves the monotony of the long dark night. The diffused 

 light of day is also owing to the air, and this adds in no small 

 degree to our comfort, causing the beautiful gradations of light 

 and shade in place of the dazzling light and black darkness 

 which we should otherwise have. 



We mnsj now master a few more terms and definitions which 

 we shall constantly be meeting with in the course of our study. 

 In order to mark out the positions of any placec on a globe, it 

 is necessary for us to have some fixed points and lines to refer 

 them to. Thus, if we wanted to describe the exact position of 

 the point F in Fig. 10, and the only fixed lines we had to 

 refer to it were A B and c D at right angles to one another, wo 

 could easily fix its place by drawing the lines r a and r H per- 

 pendicular to A B and c D. We should then say its distance 

 above A B waa equal to F o, and its distance to the right of c u 

 was equal to F H or a E, and by taese distances we could at any 

 time fix again on the exact spot. Or we might join F E, and 

 then we could fix the position of F, by giving the length of E F. 

 and the angle F a o. 



Now if we attempt to draw a straight line on the surface of 

 a globe, it will pass completely round it and form a circle. I : 

 is clear, then, that we must have some fixed circles on the eart'u 

 or in the sky if we are to ascertain the position of the various 

 heavenly bodies ; and if we refer to an ordinary terrestrial globe 

 we shall find that there are several circles drawn upon it, but 

 we shall also observe that these are of different sizes, the 

 parallels of latitude near the poles, and the polar circles, being 

 much smaller than those nearer the equator. These circles aza 



