Use your browser's Back button or this image map to return from where you came. Epicycle A geometric device invented to help explain retrograde motion in the geocentric model of the universe. Eccentric A geometric device used to help explain the non-uniform motion of the planets in the geocentric model of the universe. Equant A geometric device used to help explain the non-uniform motion of the planets in the geocentric model of the universe. The projection of the earth's equator onto the celestial sphere.
Declination is measured vertically from the equator and right ascension is measure along it.