or Equator, in Astronomy and Geography, a great circle of the sphere, equally distant from the two poles of the world, or having the same poles with those of the world. It is called the equator, because when the sun is in it the days and nights are equal; whence also it is denominated the equinoctial, and, when drawn on maps and planispheres, the equinoctial line, or simply the line. Every point of the equator is distant a quadrant from the poles of the world; and hence it follows that the equator divides the sphere into two hemispheres, in one of which is the northern, and in the other the southern pole.