NA TV RE 



\_May 29, 1884 



THE MOVEMENTS OF THE EARTH 1 

 IV. — The Earth's Revolution 

 TT will be clear from what has gone before that the daily 

 *■ movement of the stars is an apparent one due to the real 

 movement of the earth in an exactly opposite direction, and 

 that the stars in the heavens appear to rise in the east 

 and set in the west, because the earth rotates from west to east. 

 And now comes this question : The period of twenty-four hours 

 which is so familiar, and which is divided roughly into day and 

 night, has apparently two perfectly different sides to it; for a 

 certain period the stars are not seen at all in consequence of a 

 body, which we call the sun, flooding the earth's atmosphere 

 with its own tremendous light. Why should this be? In 

 giving an answer to this question it is enough to say that 

 the sun is a star so close to us, and so entirely outshining 

 the other and more distant stars which are seen in the skies, that 

 they seem to be things of a different order altogether. But they 

 are not things of a different order, they are very much like our 

 sun, and the different appearance is simply the result of the fact 

 that the one is a star very near to us, whilst the others are suns 

 inconceivably remote. In considering this apparent daily u ove- 

 ment of the stars, and taking the sun into consideration, the fact 

 is soon arrived at that the stars have another apparent move- 

 ment differing somewhat from that one with which tip to the 

 present time we have alone been engaged. It has been said, 

 and it is so obvious that it might almost have been left unpaid, 

 that as a rule the stars are not seen when the sun is visible, so 

 that the question whether the sun moves or appears to move 

 among the stars must be attacked in a rather indirect manner. 

 An observer on that part of the earth's surface directly under the 

 sun sees it as at midday. Under the^e conditions the stars are 

 of course net seen by him, but if he waited twelve sidereal hours, 

 until that portion of the earth which he inhal ited was opposite 

 the sun's place, the star; would then be vi-ible, and by noticing 

 whether those seen by him each night were the same, he would 

 be able to determine whether or not the sun moved or appeared 

 to move among them. In one position of the sun it occupies 

 that constellation of stars known as the Bull. These stars 

 cannot then be seen, beciuse the intense brilliancy of the sun 

 puts them out, but with the sun in this position the group of 

 stars known as the Scorpion is seen opposite at midnight. Then 

 at a later period the sun gets into the constellation called the 

 Crab, and we see at midnight no longer the Scorpion group but 

 the group which is called the Goat. In this way it can be 

 determined that the sun has an apparent movement among the 

 stars, which is completed in a prriod which we call a year, at 

 the end of which time the sun occupies the same position that it 

 did a year previously, and the same group of stars is seen again 

 in the south at midnight. 



Not only, then, do the stars appear to make a conn lete revo- 

 lution once a day, in consequence, as wt have seen, of the earth's 

 rotation, but once a year they also gradually change their 

 apparent places, so that at the same hour each night different 

 stars appear due south, thus indicating a move nent of the sun 

 among them. 



The same difficulty that was met with before is again en- 

 countered here ; is this movement of the sun among the stars a 

 real or an apparent one? It is a question, however, which has 

 been long since answered ; and it can he very definitely stated, 

 not only that the earth rotates on its axis in a period of 

 twenty-four sidereal hours, but that it moves or revolves 

 round the sun in a period which we call a year, and that 

 it is this real movement which causes the apparent one of 

 the sun among the stars. Let the reader tate a top and 

 ■ pin it. Perhaps the top has a movement of progression as well 

 as a movement of rotation, and it is in thit way quite easy to see 

 that the earth may rotate on its axis and revolve about the sun 

 at one and the same time. And with a top of special construc- 

 tion its axis of rotation might be inclined so that its plane of 

 rotation ceased to coincide with the plane of its motion of pro- 

 gre-sion ; still the t*o movements would go on, and in whatever 

 po-ition the top might be placed, its axis might be made to 

 remain practically parallel to itself during its movements. 



We may now, then, make the following statements : — The earth 

 revolves round the sun, and throughout the r. vrftttun the axis of 

 rotation remains practically parallel to itself. With regard to the 

 latter part of this statement it may be added that if this were not 

 so — if the axis of the earth were subject to perpetual change of 

 1 t'nntinueJ from vol. xx'ix. p. 20s. 



direction — the declinations of the stars would also be tubject to 

 constant change. 



The demonstration of this movement of the earth round the 

 sun depends upon physical considerations in exactly the same 

 way as does the demonstration of the earth's movement of rota- 

 tion, and to these considerations attention must now be turned. 

 It will be found that we have now to do with an entirely different 

 branch of physics to that which we drew upon when seeking for 

 a proof of the rotation. The utilisation of its principles for 

 the purpo es of astronomy is due to Dr. Bradley, a former 

 Astronomer- Royal. In the year 1729 he made a series of ob- 

 servations of stars, expecting certain results to flow from them. 

 Instead, however, of gettn g the results for which he had looked, 

 his observations gave him some which differed entirely from his 

 predicted ones, and which he failed to understand. Kor :uch a 

 thing as this to happen is a piece of good fortune for the scien- 

 tific investiga'or ; it sets him thinking and working, and 

 frequently leads him to the discovery of some hitherto unknown 

 physical law. It set Dr. Bradley thinking and working. 

 Curious as it may seem, the observation which led him to a 

 complete understanding of this subject was what he observed one 

 day when a boat at anchor near the shore at Greenwich began to 

 get under weigh in a stimsh breeze. The little boat had one of 

 those short pennants on its mast, and Dr. Bradley noticed that, 

 as s ion as the boat began to move, the direction of the wind, as 

 indicated by the movements of this pennant, changed. Before 

 proceeding to consider the bearing which this fact, seemingly 

 remote from astronomy, has upon s'ar work, it may be advisable 



Fig. 35. — Model to illustra'e the aberration of light. A square tube, with 

 glass front and a slit along the centre of its upper side to allow the pas- 

 sage of a thread, is inclined at 45' and caused to run along a level track, 

 while a weight suspended from a thread passing round three pulleys and 

 attached at the other end to the front of the carriage is allowed to 

 descend. In this figure the weight is at the commencement of its fall. 



to take one or two simple illustrations which will show what 

 must have pas ed through Bradley's mind as the explanation of 

 the strange unexpected movemtns of the stars was slowly 

 growing within it. The first illu-tration is one due to Sir George 

 Airy. Suppose that a ve-sel is passing a fort, and that a shot 

 is fired from the fort at the moving vessel. The shot will 

 travel in a straight line ; but it i> evident that since the 

 ship is moving, if that shot really pierces both sides of the vessel, 

 then a line joining the spot where the ball pierced the one 

 side to the spot where it pierced th : other side will not 

 be square to the direction of the ship's motion. During the 

 short time taken by the shot to pass from one side of the ship 

 to the other, the vessel has moved through a certain small 

 distance, and if the line joining the two shot-holes were 

 alone considered, it might be inferred that the shot had come 

 from a direction in advance of the true one. That is one 

 illustration, the point of it being that the motion of the vessel 

 ^eems to have given a new direction tu the shot. Take another 

 illustration, more familiar, and perhaps almost as clear. In this 

 country frequent opportunities offer themselves of travelling in 

 cabs or railway trains, with the rain falling on their closed 

 windows. Every one must have noticed that at such times there 

 is always a very curious slant in the apparent direction of the 

 drops whilst the train or the cab is in motion ; the rain seems to 

 come from a point in front of us ; we always seem to meet the 

 rain. The fact is that a body in motion, and especially a body 

 with the velocity of an express train, does not receive the rain 

 under the same conditions as when it is at rest. The question 

 of its velocity has to be taken into consideration. An experi- 

 ment will show better what is meant. 



