Moon

:For other moons in the solar system see natural satellite. For the astrological meaning of the Moon, see Solar system in astrology. For other uses see Moon (disambiguation).

Earth & Moon

The tides on Earth are generated by the Moon's gravitation (see tide and tidal force for a more detailed discussion). There are two tidal bulges, one in the direction of the Moon, and one in the opposite direction (figure 1). The buildup of these bulges and their movement around the earth causes an energy loss due to friction. The energy loss decreases the rotational energy of the Earth.

Related Topics:
Tide - Tidal force

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Since the Earth spins faster than the Moon moves around it, the tidal bulges are dragged along with the Earth's surface faster than the Moon moves, and move "in front of the Moon" (figure 2). Because of this, the Earth's gravitational pull on the Moon has a component in the Moon's "forward" direction with respect to it's orbit. This component of the gravitational forces between the two bodies acts like a torque on the Earth's rotation, and transfers angular momentum and rotatonal energy from the Earth's spin to the Moon's orbital movement.

Related Topics:
Torque - Angular momentum

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Because the Moon is accelerated in forward direction, it moves to a higher orbit. As a result, the distance between the Earth and Moon increases, and the Earth's spin slows down (figure 3). Measurements reveal that the Moon's distance to the Earth increases by 38 mm per year (lunar laser ranging experiments with laser reflectors are used to determine this). Atomic clocks also show that the Earth's spin slows down by 15 micro-seconds a day each year.

Related Topics:
Laser reflectors - Atomic clocks

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However, the formation of tidal bulges on Earth is irregular and not directly related to the frictional energy loss which accompanies the tides. For example, continents on Earth may cause an increase in frictional energy losses and hamper the buildup of tidal bulges (figure 4).

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The energy loss of the Earth's spin (loss of rotatonal energy of the Earth) is related to both the energy transfer to the Moon, which depends on the geometry of the mass distributions on Earth (causing a gravity component which pulls the Moon froward), and also to frictional losses, which depends on the properties of the material moving around within tides. The transfer of angular momentum to the Moon's orbit, in contrast, depends only on the geometry of the mass distribution. In general, the angular momentum transferred to the Moon will not correspond to an equivalent energy transfer. There will be a surplus or a deficit in the transfer of angular momentum to the Moon, compared to the energy transfer (figure 5).

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Since both angular momentum and energy are conserved, there must be a mechanism on earth to store a surplus or a deficit of angular momentum. Candidates for this mechanism are the Earth's magnetic field and internal material currents of the Earth (figure 6).

Related Topics:
Conserved - Magnetic field

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