The movement 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 23½°. 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 from the ecliptic as celestial latitude, and along the ecliptic from one of the points where it cuts the celestial equator as celestial longitude. The equator, then, cuts the ecliptic at two points: one of these is chosen for the start-point of measurement along both the equator and the ecliptic, and 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 the sun or 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 anomalistic 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 at the present time something like 23½°, is called _the obliquity of the ecliptic_. This obliquity is subject to a slight change, to which I shall refer in a subsequent chapter. In order to give a concrete idea of the most important points in the yearly path of the earth round the sun, let us imagine four globes arranged on a circle representing the earth at different points of its orbit, with another globe in the centre representing the sun, marking the two practically opposite points of the earth's orbit, in which 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, and the two opposite and intermediate points at which the north pole of the axis is most inclined towards and away from the sun.