that great imaginary circle of our globe, every point of which is 90 degree from the poles. All places which are on it have invariably equal days and nights. Our earth is divided by it into the northern and southern hemispheres. From this circle is reckoned the latitude of places both north and south. There is also a corresponding celestial equator, in the plane of the terrestial, an imaginary great circle in the heavens, the plane of which is perpendicular to the axis of the earth. It is everywhere 90 degree distant from the celestial poles, which coincide with the extremities of the earth's axis, supposed to be produced to meet the heavens. During its apparent yearly course the sun is twice in the celestial equator and twice vertically over the terrestrial equator, at the beginning of spring and of autumn. For the magnetic equator, see ACLINIC LINE.