Perigee

Perigee is the point of the moon's orbit which is nearest to the earth. On account of the disturbing force of the sun and the planets, the perigee does not retain the same position, but makes a complete revolution in about nine years. This motion, however, is not regular, the true position of the perigee sometimes being as much as 12° away from its mean place. This irregularity was known in very early times, but Horrocks waa the first astronomer who deduced the law which determines its change.