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Various theories regarding the origin of the moon have been considered as follows:<ref name="PSI">Planetary Science Institute: [http://www.psi.edu/projects/moon/moon.html The Origin of the Moon]</ref>

Various scientific theories regarding the origin of the moon have been considered as follows:<ref name="PSI">Planetary Science Institute: [http://www.psi.edu/projects/moon/moon.html The Origin of the Moon]</ref>

# One theory was that the Moon formed in an orbit around Earth, at the same time that Earth formed. But the lack of iron on the Moon, in contrast with the abundance of iron on Earth, speaks against this.

#The Moon formed in an orbit around Earth at the same time that Earth formed.

# Another theory was that the Moon formed in a part of the solar system that lacked iron, and was later captured by the Earth's orbit. This failed upon examination of lunar rocks, which exhibit the same isotope composition as the Earth.

##This idea was discarded when it was discovered that the moon has proportionally far less iron in it than the Earth.

# Yet another theory was that the Earth spun off the Moon. This solves the isotopic similarity, but an analysis of the total angular momentum and energy involved shows that this is impossible in light of the current orbit.

#The Moon formed in a part of the solar system that was low in iron, and was later captured into an orbit arount the Earth.

# The final theory is that that impact with a large planetesimal (asteroid-like bodies) released a great quantity of debris into orbit, a portion of which collected under the influence of gravity. This theory is the currently accepted explanation of the origin of the Moon, and several things speak for it. Among them are explanations how the theory is compatible with the findings that speak against the other theories:

##This failed upon examination of lunar rocks, which exhibit the same oxygen isotope composition as the Earth, unlike rocks known to come from other sources in the solar system, e.g. Mars.

##The theory states that the impact of the planetesimal happened relatively late, which would explain (among other things) the lack of iron: By the time of the impact, the Earth's iron had already sunk deeper towards the core. Thus, the debris that eventually formed the Moon came from the iron-depleted mantle.

#The Earth was originally spinning so fast that it spun off a fragment of lower density (iron-poor) material which became the Moon.

##This solves the isotopic similarity and the relative lack of iron, but an analysis of the total angular momentum and energy involved ruled out this possibility.

#An impact with a large planetesimal (asteroid-like body) released a great quantity of debris into orbit, a portion of which collected under the influence of gravity. This theory is the currently accepted scientific explanation of the origin of the Moon as it is able to explain the facts which contradicted earlier ideas:

##The impact of the planetesimal happened relatively late in the earth's formation, which would explain (among other things) the lack of iron: By the time of the impact, the Earth's iron had already sunk deeper towards the core. Thus, the debris that eventually formed the Moon came from the iron-depleted mantle.  Most of the planetesimal itself would have sunk into the earth's core.

##Since the Moon was created from debris blown out of the Earth, the Earth and the Moon have the same oxygen isotope composition.

##Since the Moon was created from debris blown out of the Earth, the Earth and the Moon have the same oxygen isotope composition.

== Reference ==

== Reference ==

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