480 



NA TURE 



[Sept. ii, 1884 



THE MOVEMENTS OF THE EARTH 1 

 VI. 

 VXT'E have now to consider some of the results of these Move- 

 ments of the Earth — first round its own axis, its rotation ; 

 then round the sun, its revolution — which we have been con- 

 sidering, results to which of course a general interest attaches, 

 and which there will be no difficulty in showing are of very great 

 importance to us. Occasion was taken to point out that the 

 different appearance presented by the sun and the stars was 

 simply due to the fact that the sun was very near to us whilst the 

 stars were very distant, the one, a sun which happens to be near 

 to us, the others, also suns, but happening to be very far removed 

 from us. Now suppose we have a globe in which we have an 

 electric light, to represent the sun, and a little globe to represent 

 the earth, then it will be obvious that that part of the earth which 

 is turned towards the lamp will be bathed in light, while that 

 half which is turned from it will be in darkness, being, so to 

 speak, only under the light of the distant stars. This shows us 

 the reason for that great difference which we call day and night, 

 and we can quite understand how it is that we get the apparent 

 rise of the sun which occurs when the part of the globe on which 

 we live is carried from the darkness into the light, and sunset 

 which of course occurs when the globe is being carried by its 

 rotation from the light into the darkness. This phenomenon 

 of day and night is thus one of the most obvious results of the 

 rotatory movement of the earth, and one which might have 

 been dismissed in two words had we so chosen, but we will dwell 



earth and star as it did at I, for in consequence of the earth's 

 revolution round the sun we shall get a gradually increasing 

 angle as the earth in its orbital course gets farther and farther 

 from its initial position at I. Now it is obvious if we are going 

 to have our time regulated by the sun instead of by the stars — 

 and that is what we must do for the purposes of civil life — we 

 shall have to arrange our clock so that when we pass from 1 to 2 

 it must, if it showed 12 o'clock when the sun was due south in 

 the former position, show 12 o'clock also when the sun is due 

 south in the latter position. If this be so, and we have this 

 angle made by the line joining sun and earth and star, we shall 

 have to make our sun-clock go more slowly than our sidereal 

 clock, for the reason that the sidereal clock will have gone round 

 once in less time than the earth will have got round to the same 

 place with regard to the sun. But if we choose, and we do 

 choose, to say that we will have twenty-four hours from sun- 

 southing to sun-southing, then these twenty-four hours and neces- 

 sarily also their minutes and seconds, will be longer than the 

 hours, minutes, and seconds of sidereal time. Let us take 

 another illustration. Consider the case of the earth in three 

 different positions, represented by three globes round a central 

 lamp. Then suppose that in each of these globes a wire is put 

 to represent the direction in which the transit instrument points 

 at Greenwich when the same star is observed at three consecutive 

 intervals of twenty-four hours of sidereal time. These three 

 wires should therefore be placed parallel to each other. Now 

 let us take the electric lamp to represent the sun, then we shall 

 find that, when the transit instrument on each of the earths 



the lengths of the 



upon it forjja few moments, because this fundamental difference 

 between day and night furnishes us with a reason why we 

 should discard that sidereal time to which up to the present 

 reference has alone been made. 



Fig. 40 will show how it is that under the circumstances in 

 which we thus find ourselves, a new kind of time must 

 take the place of sidereal time. In this diagram we have 

 the earth represented at two positions in its orbit, 1 and 2. It 

 travtls in this orbit in the direction of the arrows, rotating on its 

 axis the while in the direction also indicated by arrows. Now 

 lei 11 consider the start-point 1, and suppose that when the earth 

 occupies this position a particular star is on the meridian at mid- 

 night. The earth it will be remembered rotates in twenty-four 

 sidereal hours ; it will therefore take twelve hours to turn half 

 round, so that if we consider the sun to lie directly opposite the 

 star which is south at midnight it is obvious that they are twelve 

 hours apart. Now consider the earth at position 2. Then re- 

 membering this fundamental fact, that the distance of the stars is 

 so enormous that a string stretched from the observer to the star 

 at one point of the earth's orbit would be practically parallel to 

 a string stretched to the same star from any other part of 

 the orbit ; it is obvious that the star will have the same right 

 ascension in both positions of the earth, and the line pointing 

 to the star will be practically in the same direction. But the 

 sun will no longer lie along the prolongation of the line joining 

 1 Continued from p. 256. 



showing how the sun's apparent motion along the inclined 

 lines representing the ecliptic in the direction indicated by the arrow- 

 heads is represented by a smaller amount when referred to the earth's 

 equator (the horizontal lines in the figure) at the spring (T) and autumn 

 (=£5=) equinoxes. 



is brought round to point at the sun, the three wires which 

 represent the instruments will not be parallel to each other but 

 at si .me angle. At first sight it might seem that we could easily 

 get a sun-time to replace the star-time, but unfortunately when 

 we go a little deeper into it we find, as we often do in other cases, 

 that it is not quite so easy — and for two reasons. We found, it 

 will be remembered, when we came to consider the form of the 

 earth's orbit, that it was not quite circular, that it was in fact what 

 is called an ellipse, and that the radius vector, i.e. the imaginary 

 line joining the centres of the sun and earth did not sweep through 

 equal arcs in equal times but through equal areas, so that, 

 if we want to invent a clock which will show twenty-four hours 

 from the time of sun-southing one day to the time of sun-southing 

 the next, that clock will require to be regulated differently for 

 every day in the year, because the greater or less part of its orbit 

 moved over by the earth will cause the greater or less angle 

 between the lines joining sun, earth, and star. 



That I hope is clear. Thus then there is good reason why 

 this arrangement of having a sun-time from noon to noon will 

 not work. We should have to regulate our clock for every day 

 in the year, or rather for every two opposite days. But there is 

 another matter. We are now in full presence of the fact that 

 the equator of the earth is inclined at an angle of about 23J to 

 the plane of the ecliptic. Fig. 50 will perhaps enable us to 

 understand this matter more easily. Let the horizontal lines 

 represent the plane of the equator and the inclined lines the 



