Earth

Earth in the Solar System

It takes the Earth 23 hours, 56 minutes and 4.091 seconds (1 sidereal day) to rotate around the axis connecting the north pole and the south pole. From Earth, the main apparent motion of celestial bodies in the sky (except meteors which are within the atmosphere and low-orbiting satellites) is the movement to the west at a rate of 15 °/h = 15'/min, i.e., a Sun or Moon diameter every two minutes.

Related Topics:
1 sidereal day - Axis - North pole - South pole - Meteor

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Earth orbits the Sun every 365.2564 mean solar days (1 sidereal year). From Earth, this gives an apparent movement of the Sun with respect to the stars at a rate of ca. 1 °/day, i.e., a Sun or Moon diameter every 12 hours eastward.

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The orbital speed of the Earth averages about 30 km/s, which is enough to cover one Earth diameter (~12,700 km) in 7 minutes, and one distance to the Moon (384,000 km) in 4 hours.

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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.

Related Topics:
Natural satellite - Moon - 27 1/3 days

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Viewed from Earth's north pole, the motion of Earth, its moon and their axial rotations are all counterclockwise.

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The orbital and axial planes are not precisely aligned: Earth's axis is tilted 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).

Related Topics:
Axis is tilted - Season

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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) comfortably orbits.

Related Topics:
Hill sphere - Moon

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In an inertial reference frame, the Earth's axis undergoes a slow precessional motion with a period of some 25,800 years, as well as a 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. Also, the rotational velocity varies, a phenomenon known as length of day variation.

Related Topics:
Precession - Nutation - Polar motion - Chandler wobble - Length of day

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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.

Related Topics:
Perihelion - January 3 - Aphelion - July 4 - Precession - Milankovitch cycles

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The Moon

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The Moon, sometimes called 'Luna', is a relatively large terrestrial planet-like satellite, whose diameter is about one-quarter of the Earth's. With the exception of Pluto's Charon, it is the largest moon in the Solar system relative to the size of its planet. The natural satellites orbiting other planets are called "moons", after Earth's Moon.

Related Topics:
Pluto's - Charon - Natural satellite

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The gravitational attraction between the Earth and Moon cause the tides on Earth. The same effect on the Moon has led to its tidal locking: Its rotation period is the same as the time it takes to orbit the Earth. As a result, it always presents the same face to the planet.

Related Topics:
Tides - Tidal locking

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As the Moon orbits Earth, different parts of its face are illuminated by the Sun, leading to the lunar phases: The dark part of the face is separated from the light part by the solar terminator.

Related Topics:
Lunar phase - Solar terminator

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The Moon may dramatically affect the development of life by moderating the weather. Paleontological evidence and computer simulations show that Earth's axial tilt is stabilised by tidal interactions with the Moon. Some theorists believe that, without this stabilization against the torques applied by the Sun and planets to the Earth's equatorial bulge, the rotational axis might be chaotically unstable, as it appears to be with Mars. If Earth's axis of rotation were to approach the plane of the ecliptic, extremely severe weather could result, as this would make seasonal differences extreme. One pole would be pointed directly toward the Sun during summer and directly away during winter. Planetary scientists who have studied the effect claim that this might kill all large animal and higher plant life. This remains a controversial subject, however, and further studies of Mars—which shares Earth's rotation period and axial tilt, but not its large moon or liquid core—may provide additional insight.

Related Topics:
Axial tilt - Torque - Mars - Plane of the ecliptic - Weather - Planetary scientists - Rotation period

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The Moon is just far enough away to have, when seen from Earth, very nearly the same apparent angular size as the Sun (the Sun is 400 times larger, but the Moon is 400 times closer). This allows total eclipses and annular eclipses to occur on Earth. Here is a diagram showing the relative sizes of the Earth and the Moon and the distance between the two (click to enlarge):

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The most widely accepted theory of the Moon's origin states that it was formed from the collision of a Mars-size protoplanet with the early Earth. This hypothesis explains (among other things) the Moon's relative lack of iron and volatile elements, and the fact that its composition is nearly identical to that of the Earth's crust. See giant impact theory.

Related Topics:
Protoplanet - Giant impact theory

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Earth also has at least one known co-orbital asteroid, 3753 Cruithne.

Related Topics:
Asteroid - 3753 Cruithne

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