Polar axis and tilt

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 Earth rotates on its polar axis, it makes a slightly elliptical orbital revolution about the Sun in 365.26 days. The rates of rotation and revolution are functions of a planet's mass and orbital position.

Rotation about the polar axis results in a diurnal cycle of night and day, and causes the apparent motion of the Sun across the imaginary celestial sphere. The celestial sphere is an imaginary projection of the Sun, stars, planets, and all astronomical bodies upon an imaginary sphere surrounding Earth. The celestial sphere is a useful conceptual and tracking remnant of the geocentric theory of the ancient Greek astronomer Ptolemy .

During revolution about the Sun, Earth's polar axis exhibits parallelism to Polaris (also known as the North Star). Although observing parallelism, the orientation of Earth's polar axis exhibits precession—a circular wobbling exhibited by gyroscopes—that results in a 28,000-year-long precessional cycle. Currently, Earth's polar axis points roughly in the direction of Polaris (the North Star). As a result of precession, over the next 11,00 years, Earth's axis will precess or wobble so that it assumes an orientation toward the star Vega.

Precession also affects the dates of solstice. At the summer solstice (currently occurring about June 21), the north polar axis points in a direction 23.5 degrees from vertical— relative to the plane of the ecliptic—toward the Sun. At the winter solstice (currently occurring about December 21) the north polar axis points away from the Sun. At equinox neither pole is tilted toward the Sun but rather in a 23.5 degree tilt from vertical oriented at right angles to line between an imaginary line drawn between the Sun and Earth.

Annual changes in the orientation of the polar axis relative to the Sun result in the apparent movement of the path of the Sun (the ecliptic) across the celestial sphere. The maximum variation in the altitude of the ecliptic above the horizon is two times the polar axial tilt (i.e., 47 degrees).

Milankovitch cycles attempt to integrate and relate changes in Earth's orbital eccentricity, polar axial tilt, and polar axis precession to changes in climate (e.g., glaciation cycles).

The International Latitude Service was established in 1899 to collect data concerning polar axial motion. In 1962, the International Polar Motion Service assumed data collection. In 1988, the International Polar Motion Service, the Earth Rotation Division of the Bureau International de l'Heure, combined operations to form the International Earth Rotation Service. A number of geodetic measuring techniques—including VLBI (Very Long Baseline Interferometry), lunar laser ranging, and the current Global Positioning System (GPS ) contribute to accurate measurements concerning polar wobbling and rotational rates.

See also Astrolabe; Astronomy; Celestial sphere: The apparent movements of the Sun, Moon, planets, and stars; Revolution and rotation; Solar illumination: Seasonal and diurnal patterns