Definition - What does System Unit mean? A system unit is the part of a computer that houses the primary devices that perform operations and produce results for complex calculations. It includes the motherboard, CPU, RAM and other components, as well as the case in which these devices are housed. This unit performs the majority of the functions that a computer is required to do.
size constancy - the tendency to perceive the veridical size of a familiar object despite differences in their distance (and consequent differences in the size of the pattern projected on the retina of the eye)
Relative size means how big the planets are when compared to each other and the sun. Relative distance means how far apart the planets are when compared to each other and the sun. (AU) (150 million kilometers/93 million miles) unit of distance equal to the average distance between the Earth and the sun. width of a circle. large, spherical celestial body that regularly rotates around a star.