ACCELERATION of a Planet. A planet is said to be accelerated in its motion when the real diurnal motion exceeds the mean diurnal motion. On the other hand, a planet is said to be retarded in its motion when the mean motion exceeds the real diurnal motion. This inequality arises from the change in the distance of the planet from the sun, which is continually varying; the planet moving always quicker in its orbit when nearer the sun, and slower when farther off.