May 



NATURE 



depended upon a very definite law. We now, of course, 

 more fortunate than the early Egyptians, know exactly 

 what this law is. We saw in the last lecture that not 

 many years ago Foucault gave us a means of demon- 

 strating the fact that the earth rotates on its axis. We 

 have also a perfect method of demonstrating that the 

 earth not only rotates on its axis once a day, but that it 

 moves round the sun once a year, an idea which was 

 undreamt of by the ancients. As a pendulum shows us 

 the rotation, so the determination of the aberration of 

 light demonstrates for us the revolution of the earth round 

 the sun. 



We have, then, the earth endowed with these two move- 

 ments—a rotation on its axis in a day, and a revolution 

 round the sun in a year. To see the full bearing of this 

 on our present inquiry, we must for a time return to the 

 globe or model of the earth. 



To determine the position of any place on the earth's 

 surface we say that it is so many degrees distant from 

 the equator, and also so many degrees distant from the 

 longitude of Greenwich : we have two rectangular co- 

 ordinates, latitude and longitude. When we conceive 

 the earth's equator extended to the heavens, we have a 

 means of determining the positions of stars in the heavens 

 exactly similar to the means we have of determining the 

 position of any place on the earth. We have already 

 defined distance from the equator as north or south 

 declination in the case of a star, as we have north lati- 

 tude or south latitude in case of a place on the earth. 

 With regard to the other co-ordinate, we can also say it 

 is at a certain distance from our first point of measure- 

 ment, whatever that may be, along the celestial equator ; 

 speaking of the stars we call this distance right ascension, 

 as speaking of matters earthy we measure from the 

 meridian of Greenwich and call this distance longitude. 



The moveinent of the earth round the sun is in a 

 plane which is called the plane of the ecliptic, and the 

 axis of rotation of the earth is inclined to that plane at 

 an angle of something like 23^^. We can if we choose use 

 the plane of the ecliptic to define the positions of the 

 stars as we use the plane of the earth's equator. In that 

 case we talk of distance above the ecliptic as celestial 

 latitude, and along the ecliptic as celestial longitude. 

 The equator, then, cuts the ecliptic at two points : one of 

 these is chosen for the start-point of measurement along 

 either the equator or the ecliptic. It is called the first 

 point of Aries. 



We have, then, two systems of co-ordinates, by each 

 of which we can define the position of a star in the 

 heavens : equatorial co-ordinates dealing with the earth's 

 equator, ecliptic co-ordinates dealing with the earth's 

 orbit. Knowing that the earth moves round the sun 

 once a year, the year to us moderns is defined with the 

 most absolute accuracy. In fact, we have three years : 

 we have a sidereal year — that is, the time taken by the 

 earth to go through exactly 360" of longitude ; we have 

 what is called the tropical year, which indicates the time 

 taken by the earth to go through not quite 360", to go 

 from the first point of Aries till she meets it again ; and 

 since the equinoctial point advances to meet the earth, 

 we talk about the precession of the equinoxes ; this 

 year is the sidereal year minus twenty minutes ; then 

 there is also another year called the anomahstic year, 

 which depends upon the movement of the point in the 

 earth's orbit where the earth is nearest to the sun ; this is 

 running away, so to speak, from the first point of Aries, 

 instead of advancing to meet it, so that in this case we 

 get the sidereal year plus nearly five minutes. 



The angle of the inclination of the earth's plane of 

 rotation to the plane of its revolution round the sun, 

 which, as I have said, is something like 23^'', is called 

 the obliquity of the ecliptic. This obliquity is subject to a 

 slight change ; 6000 years ago it was over 24.^ 



In order to give a concrete idea of the most important 



NO. I I 23, VOL. 44] 



points in the yearly path of the sun round the earth, I 

 have here four globes representing the earth, with another 

 globe in the middle representing the sun, showing the 

 four practically opposite points of the earth's orbit, in 

 which the north pole of the axis is most inclined to the 

 sun ; the north pole of the axis is most inclined away 

 from the sun ; and the two opposite and intermediate points 

 where the axis is not inclined to or from the sun, but is 

 at right angles to the line joining the earth in these two 

 positions. 



A diagram (Fig. 6) shows what will happen under these 

 conditions. If we take the two points at which the axis, 

 instead of being inclined towards the sun, is inclined at 

 right angles to it, it is perfectly obvious that we shall 

 get a condition of things in which the movement of the 

 earth on its axis will cause the dark side of the earth 



¥- 



e 



'4 



Fig. 6. — Diagram showing the equality of the sun's zenith distance at the 

 two equinoxes. N, north pole of the earth ; s, south pole ; z, zenith of 

 Greenwich. 



and also the light side represented by the side nearest to 

 the sun both being of equal areas, to extend from pole 

 to pole ; so that any place on the earth rotating under 

 those conditions will be brought for half a period of rota- 

 tion into the sunlight, and be carried for half a period 

 of the rotation out of the sunlight ; the day, therefore, 

 will be of the same length as the night, and the days and 

 nights will therefore be equal all over the world. 



We call that the period of the equinoxes ; the nights 

 are of the same length as the day in both these positions 

 of the earth with regard to the sun. 



But in Fig. 7 we have a" very different condition. Here 

 the north pole is inclined at the greatest angle of 23^^ 

 towards, and away from, the sun. If I take a point 

 very near the north pole, that point will not, in summer, 

 be carried by the earth's rotation out of the light, 



'i< 



Z ^ 



WINTER SUMMER 



Fig. 7. — Diagram showing the variation of the sun's zenith distance from 

 solstice to solstice, n, north pole of the earth ; s, south pole ; z, zenith 

 of Greenivich. 



and a part equally near the south pole will not be able 

 to get into it. These are the conditions at and near 

 two other points called the solstices. 



In each of these globes I have placed a wire to represent 

 the overhead direction fromjermyn Street, London, and if 

 I observe the angle between this direction of the zenith to 

 the sun in winter I get a considerable one ; but if I take 

 the opposite six-monthly condition and take the same 

 zenith point, I get a very small angle. In other words, 

 under the first condition the sun will be far from the 

 zenith of Jermyn Street, we shall have winter ; and in the 

 other condition the sun will be as near as it can be to 

 the zenith of Jermyn Street, we shall have summer. 

 These two points represent the two points in the earth's 

 orbit at which the sun has the highest declination north 

 or south. With the greatest north declination the sun will 

 come up high, appear stationary for a day or two, as it 



