The term WHAT THE EARTH SPINS ON is solved by the four letter word AXIS. The Earth spins on its AXIS, which is an imaginary line that runs through the center of the Earth from the North Pole to the South Pole. This AXIS is what causes day and night, as the Earth rotates around it. The AXIS also determines the seasons, as the Earth's tilt causes different parts of the planet to receive more or less sunlight throughout the year.