Ave. lo, 1884.] 



• KNOWLEDGE ♦ 



133 



THE EARTH'S SHAPE AND MOTIO^^S. 



Bv EicuARD A. Proctor. 



(Continv/cd from page 109.) 



CHAPTER I.— THE DAILY IIOTIOX OF THE SUX. 



'^I'^HE first impression which the aspect of the earth and 

 .L sky gives to the observer is that the earth is a vast 

 plain over which the sky extends in the form of a dome. 

 £ propose in the present chapter to consider the lessons 

 =A'hich may be learned from the most easily recognised of 

 all the forms of motion which this dome presents to our 

 -contemplation. 



Each day we see the sun rise towards the east, pass to 

 the highest point of his path in the south, and then set 

 towards the west.* 



Xow the diurnal motion of the sun is far more instructive 

 ■than is commonly thought. To say merely that the sun 

 rises towards the east and sets towards the west, is to 

 mention nothing which can teach the real significance of 

 the motion. For we might see an object apparently rise 

 towards the east, culminate in the south, and then set in the 

 west, while the real character of its motion was totally 

 different from the sun's, and due to causes of another sort. 

 'It is the rate of the sun's motion and the figure of his 

 apparent path which are so instructive. I will describe 

 some simple observations which will show this. 



Here in England, when the sun rises nearly in the 

 east and sets nearly in the west — in other words, in spring 

 Rnd autumn — he attains in the south a height of about 

 38i deg. Suppose, then, that we fix a rod in such a posi- 

 tion that it will be square to a plane passing through the 

 east and west point, and also through the highest point of 

 the sun's path. In other words, let it have the position 

 indicated in the figure, where N S represents the north and 

 south line, E W the east and west line, O L (Fig. 1) the 

 direction of the sun at noon, in spring or autumn, and O P 

 the rod, which points towards the north, but is inclined 



Fig. 1. 



■>ih deg. to the horizon (51| being the angle, which with 

 •iS^ deg. makes up a right angle). 



Now to the rod P let the observer attach a cross-wire 

 L K N at right angles to the rod, and another cross wire, 

 K M, at right angles both to the rod and to L X ; and let 

 L K, K M, and K N be made equal. Then bend a strip of 

 card or paper into a semicircular shape, and attach it, as 

 •shown in the figure, to the ends of the wire. 



Set the rod in the sunlight in the position shown in Fig. 2, 

 ^nd so that the wire L K N may point east and west ; no 

 great exactness is required in this last respect. 



Now at any hour observe where the shadow of O P falls 



* I Bay towards the east and towards the west, for, as a rale, the 

 san does not rise exactly in the east nor set exactly in the we3t, and 

 ir is worth noticing in passing that in no place has the sun, since 

 the world began, ever risen exactly in the east and set exactly in 

 the west on one and the same day. 



on the strip of paper, as at R, and put a pencil-mark there. 

 Half an hour later repeat the observation, and set another 

 mark where the shadow now falls. Do this from half-hour 

 to lialfhour, or at shorter or longer intervals, as may b« 

 most convenient, only the successive intervals must be all 

 equal, and carefully timed. Finally, remove the strip of 

 paper, straighten it, and measure the spaces between suc- 

 cessive marks. These will be found exactly equal. 



Fig. 2. 



Fig. 3. 



In describing this experiment I have for convenience 

 spoken of a day in spring or autumn ; because this gave 

 the readiest way of showing how the position of O P was 

 to be determined. But this position once fixed (in other 

 words P set up in such a way as to point towards the 

 north at an angle of 51^ deg.) the same experiment can 

 be tried on any day of the year with prei:isely the same 

 result. 



We have no^ learned an important truth about the rate 

 of the sun's motion ; we know that if a plane were taken 

 always through O P and the sun, this plane would revolve 

 uniformly on O P as an axis. But we want to know 

 further what the shape of the curve traversed by the sun 

 may be. This is easily accomplished by the same simple 

 instrument. We have only to turn the rod O P on its axis, 

 keeping its direction unchanged, until the circular strip 

 has its centre towards the south, as shown in Fig. 3. 



Then note where the shadow of the strip falls on the rod, 

 as at S T ; i< tvill be found that the position of the shadow 

 remairis unchanged throughout the day. 



Now, what does this result teach us \ The following 

 illustration will show : — 



Suppose we attach to O P (Fig. 4) a straight rod A S, in 



Figs. 4 and 5. 



such a position that the point S coincides with the shadow 

 of the upper rim of the paper strip, whUe S A just touches 

 that rim. (The strip is removed from the figure for the 

 sake of distinctness.) Then, since the sun's rays towards 

 P, crossing the top of the paper strip, fall at S, we can, 

 by merely turning P round on its axis without changing 

 its position, make S A point exactly at the sun. And as 



