as we reckon north and south from the equator. To this east and west reckoning the term longitude has been applied.

162. On the terrestrial globe we find what are termed parallels of latitude, and meridians of longitude, at every 10° or 15o. Besides these, at 231° on either side of the equator, are the Tropics: the north one the tropic of Cancer, the southern one the tropic of Capricorn; and at the same distance from either pole, we find the arctic and antarctic circles. These lines divide the Earth's surface into five zones-one torrid, two temperate, and two frigid zones.

163. The distance along the axis of rotation, from pole to pole, through the Earth's centre, is shorter than the distance through the Earth's centre, from any one point in the equator to the opposite one. In other words, the diameter from pole to pole (the polar diameter) is shorter than the one in the plane of the equator (the equatorial diameter), and their lengths are as follows:

Now turn these feet into miles: the difference after all is small; still it proves that the Earth is not a sphere, but what is called an oblate spheroid.

164. The Earth turns on its axis, or polar diameter, in 23h. 56m. In this time we get the succession of day and night, which succession is due therefore to the Earth's rotation. Before we discuss this further we must return to another of the Earth's movements. We know

also that it goes round the Sun, and the time in which that revolution is effected we call a year.

165. Let us now inquire into this movement round the Sun. We stated (Art. 135) that the planets travelled round the centre of the system in ellipses. We will here state the meaning of this. If the orbits were circular, the planet

would always be at the same distance from the Sun, as all the diameters of a circle are equal; but an ellipse is a kind of flattened circle, and some parts of it are nearer the centre than others.

166. In Fig. 14 the outermost ring is a circle, which can be easily constructed with a pair of compasses, or by

sticking a pin into paper, throwing a loop over it, keeping the loop tight by means of a pencil, and letting the pencil travel round. The two inner rings are ellipses. It is seen at once that one is very like the circle, and the other unlike it. The points D E and F G are called the foci of the two ellipses, and the shape of the ellipse depends upon the distance these points are apart. We can see this for ourselves if we stick two pins in a piece of paper, pass a loop of cotton over them, tighten the cotton by means of a pencil, and, still keeping the cotton tight, let the pencil mark the paper, as in the case of the circle. The pencil will draw an ellipse, the shape of which we may vary at pleasure (using the same loop) by altering the distance between the foci.

167. Now the Sun does not occupy the centre of the ellipse described by the Earth, but one of the foci. It results from this, that the Earth is nearer the Sun at one time than another. When these two bodies are nearest together, we say the Earth is in perihelion.* When they are furthest apart, we say it is in aphelion. † Let us now make a sketch of the orbit of the Earth as we should see it if we could get a bird's-eye view of it, and determine the points the Earth occupies at different times year, and how it is presented to the Sun.

the

168. Now refer back to Art. 106, in which we spoke of the position of the Sun's axis. We found that the Sun was not floating uprightly in our sea, the plane of the ecliptic; it was dipped down in a particular direction. So it is with our Earth. The Earth's axis is inclined in the same manner, but to a much greater extent. The direction of the inclination, as in the case of the Sun, is, roughly speaking, always the same.

169. We have then two completely distinct motionsone round the axis of rotation, which, roughly speaking, remains parallel to itself, performed in a day;-one Tepí, at or near to; Aios, the Sun. † ἀπό, from, and ἥλιος.

*

round the Sun, performed in a year. To the former motion we owe the succession of day and night; to the latter, combined with the inclination of the Earth's axis, we owe the seasons.

170. In Fig. 15 is given a bird's-eye view of the system. It shows the orbit of the Earth, and how the axis of the

Earth is inclined the direction of the dip being such that on the 21st of June the axis is directed towards the Sun, the inclination being 23. Now, if we bear in mind that the Earth is spinning round once in twenty-four hours, we shall immediately see how it is we get day and night. The Sun can only light up that half of

the Earth turned towards it; consequently, at any moment, one half of our planet is in sunshine, the other in shade; the rotation of the Earth bringing each part in succession from sunshine to shade.

In

171. But it will be asked, "How is it that the days and nights are not always equal?” For a simple reason. the first place, the days and nights are equal all over the world on the 22nd of March and the 22nd of September, which dates are called the vernal and autumnal equinoxes for that very reason-equinox being the Latin for equal night. But to make this clearer let us look at the small circle we have marked on the Earth-it is the arctic circle. Now let us suppose ourselves living in Greenland, just within that circle. What will happen? At the spring equinox (it will be most convenient to follow the order of the year) we find that circle half in light and half in shade. One half of the twenty-four hours (the time of one rotation), therefore, will be spent in sunshine, the other in shade; in other words, the day and night will be equal, as we before stated. Gradually, however, as we approach the summer solstice (going from left to right), we find the circle coming more and more into the light, in consequence of the inclination of the axis, until, when we arrive at the solstice, in spite of the Earth's rotation, we cannot get out of the light. At this time we see the midnight sun due north! The Sun, in fact, does not set. The solstice passed, we approach the autumnal equinox, when again we shall find the day and night equal, as we did at the vernal equinox. But when we come to the winter solstice, we get no more midnight suns; as shown in the figure, all the circle is situated in the shaded portion; hence, again in spite of the Earth's rotation, we cannot get out of the darkness, and we do not see the Sun even at noonday.

172. Now, these facts must be well thought of. If this be done there will be no difficulty in understanding how it is that at the poles (both north and south) the years