|
Earth In Solar System
The orbital speed of the Earth
averages about 30 km/s, which is adequate to cover
one Earth diameter (~12,700 km) in 7 minutes,
and distance to the Moon (384,000 km) in 4 hours.
Earth has one natural satellite, the Moon, which
orbits around Earth every 27 1/3 days. From Earth
this gives an apparent movement of the Moon with
respect to the Sun and the stars at a rate of
roughly 12 °/day, i.e., a Moon diameter every
hour eastward.
Viewed
from Earth's North Pole, the motion of Earth, its moon
and their axial rotations are all counterclockwise.
The orbital and axial planes are not exactly aligned:
Earth's axis is at an angle of some 23.5 degrees against
the Earth-Sun plane (which causes the seasons); and
the Earth-Moon plane is tilted about 5 degrees against
the Earth-Sun plane (otherwise there would be an eclipse
every month). |
|
The Hill sphere (sphere of influence)
of the Earth is about 1.5 Gm (930 thousand miles)
in radius, within which one natural satellite
(the Moon) at ease orbits.
In an inertial reference frame, the Earth's axis
undergoes a slow precessional motion with a period
of some 25,800 years, also nutation with a main
period of 18.6 years. These motions are caused
by the differential attraction of Sun and Moon
on the equatorial bulge due to the Earth's oblateness.
In a reference frame attached to the solid body
of the Earth, its rotation is also slightly irregular
due to polar motion. The polar motion is quasi-periodic,
containing an annual component and a component
with a 14-month period called the Chandler wobble.
As well as the rotational velocity varies, a phenomenon
known as length of day variation.
In modern times, Earth's perihelion is always
about January 3, and aphelion is about July 4.
For other eras, see precession and Milankovitch
cycles.
Also see
v
Physical characteristics
v
Earths Moon
|
