68 



NA TURE 



{Nov. 15, i; 



preci e relation of the dot which re.ire^ents the time at which 

 the observation was made to the other d )ts which represent the 

 various seconds dotted out by the clock, and not only the exact 

 distance of the observation pricli from the nearest second, 

 whether it be J, or i/ioth, or i/iooth of the distance between 

 that second and the next, but the omission of the record of the 

 first second in the minute will give the relation that observation 

 has to the nearest mimtte. 



For the sake of simplicity the ca'e of one observer making 

 one ob-ervation has al me been considered ; but if the work be 

 properly arranged, then not only one electromagnet, but two, 

 or three, or four, may be at work upon the same cylinder at the 

 same time, each making its record, and that is how such work is 

 being done at the Greenwich Observatory. 



Ohservinr Conditions 

 This power of measuring and dividing time then having been 

 obtained, we seem to have reached our subject, " The Movements 

 of the Earth." Yet even m\v there .are one or two other 

 matters which require to be discussed before we consider the 

 movements themselves. The first of these is the important fact 

 that the earth is spherical in its fjrm. There have been many 

 views held at different times as ti the real shape of the earth, 

 but the only view we need consider is that stated. In goin<T 

 down a river in a steamboat, or, better still, in standing upon 

 the sea-shore at some place, such as Kamsgate, r«here there 

 are cliffs, and where, consequently, one may get from the sea- 

 level to some height above it, it is observed that when any ship 

 disappears from our view by reason of its distance it seems to 

 disappear as if it were passing over a gentle hill. 



It does this in whatever direction it goes. This familiar 

 fact IS a clear proof that the earth is a sphere, and is so obvious 

 that It may seem unnecessary to mention it, but it was as well to 

 do so for a reason which will appear shortly. Besides this argu- 

 ment in favour of the spherical shape of the earth there is the 

 argument from analogy : the m wn is round, the sun is round, 

 all the known planets are round. The stars are so infinitely 

 removed from us that it cannot be determined whether they also 



FlG. 23.— Model 1 



: p.^ralla 



are spherical, but doubtless they are .as round as the earth. 

 This point of the tremendous distance of the stars is an 

 important one to bear in mind. Their distance cannot be 

 conveniently stated by thousands, nor even by millions of 

 miles, it is something far greater than that. It may be asked 

 why it is that such a statement can be thus positively made. 

 For this reason : the stars have been observed now for many ages, 

 and the historical records of ancient times show that the chief 

 constellations, the chief clusters of stars visible in the heavens 

 now, were seen then. In the Book of Job, for instance, there 

 IS a reference to the well known constellation of Orion, and 

 there is very little doubt that for thousands and thousands of 

 years that constellation has preserved the familiar appear.ance of 

 its main features. The constellation called Charles's Wain, or 

 the Great Bear, was also known to the ancients. If the stars 

 were very near to the earth this could not be. If they were 

 close to us the smallest motion either of earth or star would at 

 once change their apparent position, and would prevent this 

 fixity of appearance, and the skies would be filled, not with the 



constellations with which we are .so familiar, bat with new and 

 ever-changing clusters of stars. This constancy of the con tella- 

 tions, not only from century to century, but from era to era, 

 clearly proves then that the stars of which they are made up must 

 be at an infinite distance from the earth. 



Let us consider the question of distance a little further. If 

 two pieces of wood (see Fig. 23) joined together by a cross-piece 

 be taken, a moment's thought will make it obvious that the 

 angles w hich A li and c B make with the cross-piece A c, will 

 vary with the distance of the body, which can be seen first by 

 looking along A B and then by looking along c B. If these 

 pointers be directed to a very near object in the room, they must 

 be greatly inclined (as in i). If something more aistant be 

 taken, there is less inclinalijii, and if it were possible to sight 

 St. Paul's by looking first along A B and then along c B, there 

 would be still less. And if something at a still greater distance 

 were sighted, say St. Giles's at Edinburgh, the inclination of A B 

 and c B would be still smaller than it was in the case of St. Paul's, 

 because St. Giles's is at a much greater distance. It follows then 

 that in sighting an object so infinitely rem ived from us as a star, 

 the light trom it will be in a condition of parallelism, and A B 

 and c B consequently be placed quite parallel in viewing it (see 

 2). That is another reason for saying that the stars are at 

 this infinite distance from the earth. Why it is so important to 

 insist on this point will appear very clearly by and bye. 



Now suppose that in the centre of this lecture-theatre a little 

 globe were hung to represent the earth, the wills of the theatre 

 and the people in it representing the heavens surrounding the 

 earth. Now in such a case it is clear that the appearances pre- 

 sented would be the same whether the heavens moved round the 

 earth or the earth itself were endowed with motion. Let us, 

 without making the assertion, a sume that the earth does move. 

 It is perfectly obvious, since the apparent mo ions of the heavens 

 are so regular, that if that be so she must move with 

 wonderful constancy and regularity ; she does not first move iu 

 one direction and at one inclination, and then at another ; that 

 would be very serious. 



If she rotates she must rotate round some imaginary line called 

 an axis. This introduces an important consideration because, 

 whether the e.arth itself rotates on an axis or the heavens move 

 rou id the earth — and in the latter case the heavens must also 

 move round an axis — in either case the motion must be an equable 

 one ; so that if the matter is thus limited to a constant axial 

 rotation or a constant revolution, as it would be called in the 

 case of the stars, several things will happen. Let us take the 

 former case, in which the earth itself moves. Then the motion 

 of the surface of the earth will be least at those points which 

 are nearest the ends of the axis on which it turns. Take the 

 case of an observer at such a point, he will be carried a very 

 little distance round during each rotation ; similarly, if the stars 

 move, a star near the ends of the axis on which the stars move 

 will be carried a very little distance round during each revolu- 

 tion of the celestial sphere. 



Change the position of the man on the earth from the pole to 

 the equator. Then he will be carried a very considerable 

 distance round in each rotation of the earth : similarly with the 

 stars ; if they move, a star in the celestial equator will be carried 

 round a very great distance during a revolution. That is the 

 first point. Another point is that if we assume the earth to 

 rotate we must carefully consider the varying conditions which 

 are brought about by the different positions of an inhabitant of the 

 earth under those circumstances. For instance take the case of 

 a man at the equator, he looks at things from an equatorial point 

 of view, and in the rotation of the earth he plunges straight 

 up and stra'ght down. Similarly, if the stars' daily revolution 

 belongs not to the earth but to the stars, to an observer at the 

 equator of the earth they would appear to move straight up and 

 straight down ; and now in dealing with this question and en- 

 deavouring to ascertain whether it be the earth or the stars which 

 move it is most necessary to consider the relation of the move- 

 ments or apparent movements of the star^ to the place from which 

 they are observed, and in so doing it is found that there is an 

 immense difference between the conditions which obtain at the 

 poles and at the equitor with reference to the phenomena which 

 are observable in each case. 



Let us take a globe to represent the earth, and let London 

 be considered the central point for our observations. Now 

 at all places on the earth, in whatever direction we look, we see 

 an apparent meeting of earth and sky ; and supposing our obser- 

 vation to be made on an extended plain or at sea, the surface of 



