NA TURE 



[Sept. 



contented ourselves simply with sidereal time, whieh would have 

 at least the advantage of being constant, so that it is most neces- 

 sary if any benefit is to be derived from this mean sun of ours 

 that it should not differ very much from the true sun. The longi- 

 tude of our mean sun is therefore made equal to the mean longi- 

 tude of the true sun. This having been premised, the terms "mean 

 time " and "mean noon" will now be clear without any ex- 

 planation. "Greenwich mean time" of course means time 

 referred to the meridian of Greenwich. 



We thus finally discard our sidereal time, and replace it by 

 mean solar time so arranged that the maximum departure of 

 this from true solar time shall be fifteen minutes in the month 

 of February and fifteen minutes at the beginning of November. 

 We have seen that the sidereal day is shorter than a solar day. 

 and that consequently the hours, minutes, and seconds which 



Fig. 53.— Diag 



make up the sidereal day must be shorter than those whieh form 

 the solar day. The relation between the seconds of solar and 

 sidereal time may be thus shown. 



idereal second = '9973 of a mean-time second. 

 One mean-time second = i - 0027 of a sidereal second. 



We have now got the results of the earth's revolution com- 

 bined with its rotation, so far as day and night, considered in 

 their more general aspects, are concerned ; but we have not 

 done with day and night yet. When we were considering the 

 question of the inclination of the earth's axis, we went so far as 

 to say that it was inclined 23A" to the plane of the ecliptic, 

 and that it always remained practically parallel to itself. Now 

 ;uppose we arrange four globes in a circle, to represent the 

 earth in different parts of its orbit, and we have in the centre an 

 electric lamp to represent the sun. Then, if the earth's axis is 

 thus inclined to its path round the sun, and always remains 

 parallel to itself, it will be seen that at one position the north 

 pole will be all in the light of the electric lamp (which repn 

 the sun) during the entire revolution of the earth on its axis. 



At the point opposite this the reverse happens, for during the 

 entire rotation of the earth the north pole will be in the dark. 

 At the two remaining points the pole will be just on the bound- 

 ary of light and darkness. We need not consider the case of 

 the south pole ; there exactly the reverse will happen to what 

 occurs at the north pole — when the north pole is always in the 

 light, the south pole will be always in the dark, and vice versa, 

 as may be seen by looking at the globes. Now it should be 

 clear that the fact of the earth's axis being inclined to its path 

 causes different lengths of day and night throughout the year. 

 It is simply that, and nothing else. At the poles, which, as we 

 have seen, are sometimes entirely in the light and sometimes 

 ! altogether in the dark, there will be six months of this light and 

 six months of darkness. At the equator it will be readily under- 

 stood the days and nights will be of twelve hours' duration at 

 ! whatever part of her orbit the earth may be. If you take those 

 positions of the earth where the boundary of light and darkness 

 passes through both the poles, it is perfectly clear that the days 

 and nights are of equal length all over the world, and a line 

 drawn from those points through the sun is therefore called 

 the "line of equinoxes." These points are respectively at the 

 ascending and descending nodes of the orbit. The two other 

 points where the North Pole is most in the light or in the dark 

 during the whole of a rotation are known as the solstices, be- 

 c ause it is at these times that the sun for some days appears to 

 attain the same height at noon. 



To sum up then, it will be seen that the earth's rotation 

 and the earth's revolution, in conjunction with the important 

 fact of the non-coincidence of the planes in which they 

 take place, give us not only our days and our nights, but cause 

 the lengths of them in different latitudes to vary throughout 

 the year. We have in this inclination of these planes to each 

 other, too, the cause of the seasons, because when the northern 

 hemisphere of the earth has been for a long time in that posi- 

 tion with the sun longest above the horizon, the temperature will 

 be very different to what it is when the earth is in the other 

 position. In the forme] position we have summer in the 

 northern hemisphere, in the latter winter. The conditions of 

 life at two such points in the orbit will l> -rent. At 



the equator, where the days and nights are always of equal 

 length, the course of nature will be very uniform. As the 

 equator is receded from and the poles are approa hed, this uni- 

 formity begins to disappear until, as has been said, at the pole 

 six months of perpetual daylight alternate with six months when 

 there is no sun. 



But even now when we have g A our day and our year, we 

 have not got all. 



It must next be pointed out that, whilst the axis of the earth 

 may lie said to remain practically parallel to itself, yet that it 

 does 11- >t absolutely remain so. 



As a result of this and of the earth's movement round the sun, 

 we gel a very important outcome. Although the consideration 

 of the dimensions of tile earth has scarcely come within our sub- 



•-€> 



Fig. 54. — The 



ject, yet the earth's rotation may be used to bring in the dimen- 

 sions of the body on which we dwell in just the same way as the 

 velocity of light was used to refer to the dimensions of its 

 orbit. 



We need not, however, consider the question in detail, but 

 we may state that the earth is a globe of something like 8000 

 miles in diameter, the equatorial diameter being longer than the 

 diameter from pole to pole by some twenty-six miles, so that we 

 have, as it were, round the equator a ring of matter some thir- 

 teen miles thick and eight thousand in diameter. Now this ring 

 of matter, this equatorial protuberance, is presented to the sun 

 at an angle to the line joining the centres of the sun and earth, 

 as shown in Fig. 54, and the sun's attraction upon it can be 

 resolved into two forces, one parallel to the line joining the 

 centres of the sun and earth, and the other at right angles to this 

 direction ; and if we consider what will be the effect of this 



the earth's equatorial protuberance. 



latter force upon such a ring, we can easily understand that 

 it will result in an alteration of the inclination of the ring. 

 In an arrangement for showing the effect of this attraction, the 

 ring of matter on which the sun acts may be represented by 

 an iron ring attached to a spinning top, and the resolved portion 

 of the sun's pull may be imitated by the attraction of a magnet 

 held in a nearly vertical position. As the ring rotates, the 

 attraction of the magnet draws the ring out of the horizontal, 

 and the poles revolves in a circle. This is what takes place 

 with the earth's axis ; hence it is not true to say that it 

 always remains parallel to itself. This revolution is always 

 slowly going on, being completed in a period of about 25,000 

 of our years. In consequence of this motion, what happens 

 is this : the line of equinoxes which is at right angles to 

 the line of solstices is constantly changing its position along 

 the earth's orbit, producing what is called the precession of 



