We have said that the inclination of the equator to the ecliptic amounts to 231⁄2°, and this angle should be memorised. Now, since both the ecliptic and the equator are great circles of the celestial sphere, _i. e._, circles whose planes cut through the centre of the sphere, they must intersect one another at two opposite points. In the experiment just described, these two points lie on opposite sides of the ball, where the equator cuts the level of the water. These points of intersection of the equator and the ecliptic on the celestial sphere are called the equinoxes, or equinoctial points, because when the sun appears at either of those points it is perpendicular over the equator, and when it is in that position day and night are of equal length all over the earth. (Equinox is from two Latin words meaning “equal night.”)