The polar axis is an imaginary line that extends through the north and south geographic poles. Earth rotates on its axis as it revolves around the Sun. Earth's axis is tilted approximately 23.5 degrees to the plane of the ecliptic (the plane of planetary orbits about the Sun or the apparent path of the Sun across in imaginary celestial sphere). The tilt of the polar axis is principally responsible for variations in solar illumination (insolation) that result in the cyclic progressions of the seasons.

Earth rotates about the polar axis at approximately 15 angular degrees per hour and makes a complete rotation in 23.9 hours. The length of day has changed throughout Earth's history and as rotation slows, the time to complete one rotation about the polar axis will continue to increase. Rate of rotation is a function of planet's mass and orbital position. As...