Earth's Rotation

Earth rotates around its axis (in a counter-clockwise direction, when viewed looking down on the North Pole), producing sequencial periods of daylight and night.

The period of rotation defines the day, one of Earth's natural time scales.

Earth's rate of rotation is relatively rapid, that is, there are many rotations (days) in one revolution (year).

Earth's rotation axis is currently tilted at ~23.5° with respect to the ecliptic axis, the line drawn from the center of the Earth and perpendicular to the ecliptic plane. This tilt is called the obliquity of Earth's axis. As a large rotating mass, Earth acts like a gyroscope as it revolves around the Sun, that is, at every point along its orbit, its axis of rotation points toward the same fixed point in distant space. On the celestial sphere, that point is very near the star Polaris, commonly called the "North Star". The both the magnitude and the orientation of the tilt of the rotation axis change slowly over time. The change in orientation of the tilt is due to a wobble in Earth's rotation axis.


The tilt of Earth's rotation axis is the primary cause of the seasons.

The seasons modulate how much solar radiation is received at a point on Earth's surface through the course of a year.
  This modulation is strongest at high (polar) latitudes and less at low (tropical) latitudes.

Earth's rotation gives rise to ... ... a centrifugal force, which causes the bulge of the equator (and the attendant flattening of the polar regions), relative to a perfect sphere.

... a coriolis force, which is important in determining the flow of jet streams in the atmosphere and currents in the ocean.

"Wobbling" of Earth's Axis of Rotation (Axial Nutation and Axial Precession)

As noted above, Earth's rotation axis presently points toward the distant star called Polaris. However, the other bodies in the Solar System -- especially the Moon -- are constantly perturbing Earth's rotation through gravitational interactions.

Just like a spinning top that is perturbed, Earth wobbles slightly. To first order, over time the rotation axis ...

... bobs up and down (axial nutation) by 9.18" with a period of 18.6 years (this is a very small effect).

... rotates slowly clockwise about the ecliptic axis (axial precession), completing a circuit around a circle centered on the ecliptic axis every ~ 25,700 years (this is a large, albeit very slow effect).

This wobbling is independent of the Earth revolving around its orbit.

Axial precession determines where along the orbit the various seasons occur.

The seasons very slowly slip counter-clockwise along the orbit.

It is just by chance that humanity has reached the level of technology that is has when the Northern Hemisphere winter solstice is near apehelion.
If we look 13,000 years in the future (or equivalently, 13,000 years in the past), the rotation axis points toward the star a-Lyrae. All other factors being the same, Northern Hemisphere winter solstice now occurs near perihelion! This will likely make Northern Hemisphere winters more severe. For more details, see Milankovitch Cycles.