equant

The big circle centred on the equant point is called a deferent.

The Earth was still regarded as the central object in the Universe, but everything else revolved around the equant points, not around the Earth itself.

Furthermore, the system was often criticized as being unduly complicated, while the need for equant points made many thinkers uneasy—it raised doubts about whether the Earth ought really to be regarded as the centre of the Universe.

Copernicus proposed to abolish the equant, and to eliminate an epicycle for each planet further from the sun than the Earth by showing how the movement of the Earth created an apparent movement in the sky equivalent to an epicycle.

The equant was a device for speeding up and slowing down the movement of a body in the heavens by measuring its movement not from the centre of a circle but from another point.