There had been varied theories put forward for how the moons actual existence came in being; one of these being the suggestion that it had been a captured body, formed elsewhere in the solar system and was slowed down by debris close to the earth and then finally captured by Earth's gravity. The suggestive evidence against  this theory is that the encounter would have occurred at a relatively high velocity, making a capture impossible. The Earth and Moon also have a marked difference in iron content, as well as this - and further supported by lunar rocks recovered by the Apollo missions shows that the Earth and the Moon have the same quantities of oxygen isotopes, suggesting a closer relationship. If the moon was captured, it would have had an extremely elliptic orbit, which it has not.

  Further to the information above, is the striking size of the moon compared to the actual Earth itself. No where else, apart for Pluto and her moon Charon is there a comparison to be made in relation to primary planet to the orbiting moon in their  respective sizes. Is Charon formed by accretion or was it, as suggested - a capture body from the Kuiper Belt ?  If it had been captured, was it possible for the Earth's moon to have been captured also. Perhaps our moons size alone may have precluded that, but perhaps a smaller moon may have been captured.

   I will further explore this theory in a later section; but for the present I will  look at the widely accepted theory of how the moon came into existence today through a giant impact. This was first proposed in the mid-1970's to explain how the moon formed stated that after the Earth had (somewhat) cooled down and got a solid crust, a Mars-sized proto-planetary object hits Earth, coursing a huge re-melting of the crust.

  Although it wasn't as some would imagine as a 'head-on collision', it was more of a slow glancing blow. This object which is thought to have collided with the Earth is believed to have originally accreted in what is, in astronomical distances at least - close to (being either just outside or just inside) the Earth's present orbit; because of the low speed of the impact. Current estimates based on computer simulations of such an event suggest that approx. 2% of this original bodies mass ended up as a ring of debris orbiting the Earth, while 50% of that material eventually coalesced into our present day Moon - the rest falling back to Earth in the progress of time.

  The mostly iron cores of both these bodies later melted down into Earth's core explaining (part therein) why the planets core is abundant (by percentage to Mars and Venus) in iron.

  The angular momentum of the Earth-Moon system, contained in both the Earth's spin and the Moon's orbit after the impact is quite large and which implies that the terrestrial day was only about five hours long when the Moon first formed close to the Earth.

  The Moon finished forming approx. 4 billion years ago. Although the surface continued to be bombarded by remnant solar material - the moon has remained practically dormant in diameter and mass. During this later period the moon warmed, and experienced some volcanism, there being abundant evidence today of volcanic plains on the surface.

  By this Mars-sized body hitting the Earth at exactly the angle it did, it avoid completely destroying our planet while producing a moon of suitable size and orbit to stabilize the recession of the Earth's axis.

A Fateful Rendezvous