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
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
Axial precession determines where along the orbit the
various seasons occur.
The seasons very slowly slip counter-clockwise along the
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
For more details, see Milankovitch
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.