598 



NA TURE 



[Oct. 1 8, 1883 



THE MOVEMENTS OF THE EARTH 1 

 I. — Measurement of Space 



IN proceeding to deal with the application of the various 

 branches of physical science to the investigation of those 

 phenomena which lie beyond the earth, there is a very large field 

 fr m which to make choice of a subject which will show, now 

 the application of one branch of science, and now the applica- 

 tion of another, and bring us, in this way, somewhat nearer to 

 the truths and the beauties which lie in the most distant realms 

 of space for all who will take the trouble to look for them. But 

 perhaps it may be more desirable to select that part of the sub- 

 ject which, so to speak, lies nearer home, and endeavour to 

 point out how, by means of the application of principles, and 

 methods, and instruments which are generally familiar, and 

 which at all events are of daily use, the various movements 

 with which our planet is endowed may be studied, not only with 

 reference to the phenomena themselves, but with reference also 

 to the causes which lie at the bottom of them. 



The various branches of knowledge which will have to be 

 drawn upon in furni-hing the materials necessary for this inquiry 

 were really started long before it w as imagined that the earth 

 had any movements at all ; but still, on the whole, the growth 

 of the knowledge of its movements has been so beautifully con- 

 tinuous, that we cannot do better now than consider historically 

 the way in which those sciences have grown up, which enable us 

 to make certain measurement' , and to get out coirectly certain 

 quantities, which must nece-sirily lie at the bottom of any sound 

 knowledge. 



What particular things do we want to measure ? It has been 

 already said that when the sciences to which attention will have 

 to be called later on were founded, very few people on this 

 planet knew that it moved at all, but it is now generally known 

 that the earth does move. It will be obvious however that, whether 

 the earth moves or not (and that may be considered still a moot 

 question), if we wish to form a basis for our judgment in any 

 direction, we must be able to measure time and space. It has 

 been well said that "time and space are the moulds in 

 which phenomena are cat ; " for when it is desired to gain any 

 useful knowledge concerning any fact, the relation which it 

 bears to the things around it, and the time of its occurrence must 

 be known, and that is the only thing an astronomer tries to do 

 when he is investigating that portion of his subject to which we 

 must first turn our attention. We will begin then by considering 

 those measurements of space which are of the fir.-t importance 

 to the astronomer. I do not here refer to the ordinary familiar 

 measurement of inches, yards, and miles, but to the measurement 

 of angles, and it will be w ell to get a good no ion of this angular 

 measurement as so in as possil le. 



There is no special necessity for dividing the circle into 

 360 parts, but the greatest number of people have made that 

 division, and it is still continued to be done. When the 

 Chinese begin to make circles they divided them, not into 

 360 parts, but into 365!. Now there was a great advantage, 

 and a great disadvantage about that. The advantage was that 

 this number of divisions in the Chinese circle was the same as 

 the number of days in the year ; the disadvantage was lhat they 

 were not dealing with whole number^, and their 365! was not 

 such a convenient number to halve and quarter, and so on, as is 

 360. In quite recent times it has been suggested that 400 parts 

 should be taken instead of 360, but that is a suggestion which 

 up to the present time has not been acted upon. 



We have then an angle defined as the inclination of two straight 

 lines starting from a centre; if we get one of these lines tra- 

 versing an entire circumference, the other remaining at rest, the 

 travelling line will have traversed 360° ; we have what is called 

 a right angle when one of the lines has been separated from the 

 other through a quarter of a circumference — that is, 90 . This is 

 the fundamental idea of angular measurement, the only measure- 

 ment of space with which we shall have to deal at present. 



For instance, if a little ivory rule be opened, its two 

 parts become inclined to each other, and inclose what is 

 known as an angle. That angle may be made large or 

 small by opening and closing the two parts, A and B (see 

 Fig. 1) of the rule. Suppose the rule to be shut, the point on 

 which it turns being in the centre of the circle, C D E F, and 

 that, whilst A remains at rest, B is made to travel successively 



1 Report of Lectures to Working Men given at the Royal School of Mines 

 by J. Norman Lockyer, F.R.S. 



through B and B 1 to B*. It will then have travelled half th.3 cir- 

 cumference of the circle c D E F, but civilised people, in order 

 to get perfectly clear notions about this measurement, and to be 

 able to tell each other what particular measurement they have 

 made in this way, instead of talking of a circumference merely, 



Fig : —Use cf a two-foot rule to explain angular measurement. With the 

 part a at rest, the movement of the other to B, b' and b= gives us 45 . 

 90% and i8o\ 



and of certain rough divisions of it, have divided all circles into 360 

 parts called degrees, and say that the travelling part, B, of the 

 rule has travelled through not a quarter, or a half circumference, 

 but through 90 and 180 degrees respectively. 



Why are these measurements of space required ? For the 

 reascn that when we are dealing with the heavenly bodies and 

 seeking to define the position of any object, two facts at least 

 are required to be known before its exact position can be deter- 

 mined. An observer going out at night upon an extended plain 

 would see some celestial bodies near where the earth meets the 

 sky all round, which is called the circle of the horizon, and he 

 might happen to see another body exactly overhead, in what is 

 called the zenith. In passing from this zenith to the horizon it 

 will be obvious that a quarter of a circumference is traversed (see 

 Fig. 2). That distance may therefore be divided into 90". 



W.H. 



E.H. 



Fig. 2. — Measurement of altitudes. 



Similarly in passing from the eastern horizon to the western 

 horizon half a circumference is travelled over. This distance 

 therefore is divided into 180 of angular measurement in the 

 same way that the half of the circumference traversed by the 

 travelling rule was divided into 180°. 



Now if it can be ascertained of any body that it is exactly in 

 the zenith, the position of that one body has been definitely 

 stated for the particular time at which the observation is made. 

 But consider the case of another body not in the zenith. 

 Suppose that the lines, the one A B (see Fig. 2), passing from 

 the observer to the object, and the other, AC, passing from 

 the observer to the horizon, inclose an angle of 45 . This 

 angle is called the star's altitude. But to say simply that the 

 altitude of a star is 45° does not sufficiently define its position. 

 Let the leader imagine himself to be standing in the Albert 

 Hall. He knows that he may look up and see rows of panes of 

 glass and ornamented work running around the hall at different 

 heights above the floor. He may also notice, let us say, various 

 series of ornamentation arranged vertically from floor to roof. 

 Now suppose it were desired to define the position of any one 

 pane of glass or piece of ornamentation in any one of these hori- 

 zontal or vertical rows. It is obvious that to say of any pane of 

 glass at one level that it is at a certain height above the floor will 

 not suffice, for all the panes of glass in that row are at the same 



