Page:Popular Astronomy - Airy - 1881.djvu/132

118 to the axis round which the heavens appear to turn; but I said that it is a great circle; which amounts to the same as saying that the path is in a plane which passes through the centre of the globe; or, to express it otherwise, the path is in a plane which passes through the eye of the spectator. If we take a pair of compasses, and open them so that one leg is square to the other, or (as we usually express it) that their angle of inclination is 90 degrees; and if we hold one leg fixed in position, and make it serve as a spindle round which the compasses are to be turned; then the other leg will move in a plane: and if the eye of an observer be placed at the angle of the compasses, the plane which is described by that moving leg will be seen by him as a great circle of the celestial sphere; while by looking along the fixed leg he will see a point in the sphere which is the Pole of that great circle: (the word "Pole" being here used in a general sense, as related to any great circle). From this it appears that the angular distance of the Pole of a great circle, from any point of the great circle is 90 degrees. If we then suppose a circle to be traced through the heavens, of which every point is 90 degrees from the North Pole, that circle will be a great circle, or will be in a plane passing through the eye of the spectator. That great circle is the equator. Now, as I mentioned in the second lecture, if we trace (by the use of the transit instrument and mural circle) the annual path of the sun through the stars, (which is called the ecliptic), we find that the ecliptic is also a great circle; but it is not the same great circle as the equator. It is inclined to the equator, and crosses it at two points, which are called the first point of Aries and the first point of Libra. The first point of Aries is that crossing of the equator at which