Formation of the Earth and the Moon, Tides and Gravity


By Dan Green, BSc


No one will ever know how the Moon and Earth system came into being.  Many believe that the Earth formed first and the Moon was either captured or blasted off from the Earth by the impact of a passing planet the size of Mars.  If the latter had happened, either the Earth as we know it would never have come into being, since it would have ended up somewhere else in the solar system, or, the incoming and impacting planet mass could have replaced the original planet mass and itself became Earth, with the original planet mass speeding off to be absorbed by, say, Jupiter.  What is highly unlikely is that such an impact could have led to the formation of the Moon from debris since the debris would have been gravitationally attracted to the impacting, and solid, incoming planet. There is a more ridiculous supposition here, though.  There is a claim that a planet the size of Mars (again!) smashed into Earth, when Earth was one third of its present size and molten, and blasted of it parts of Earth’s crust!  First, at the time Earth was one third of its present size, a planet the size of Mars would have been considerably larger than Earth and would have completely annihilated it.  Second, planet Earth would have been planet Gloop, without any layer which one could describe as a crust, so that any body slamming into it would have increased the size of planet Gloop and splashed gloop into space.  Thus, no mini-gloop Moon.


The theory of capture is too far fetched.


The theory I support is this:  contrary to every image one sees of the early Earth, the forming planet was not round.  It was of an irregular shape.  After all, anything forming from accretion is more likely to have an irregular than a regular shape.  This irregular shape, not necessarily solid, would have been spinning and tumbling very rapidly. The rapidly spinning and tumbling body became sufficiently elongated at one point that a piece of the accretion broke away to form a satellite body that eventually became the Moon.  Alternatively, a small piece of the mass that was accreting to form the early Earth remained un-accreted and this formed the Moon.  Many theories are put forward from time to time, based on one set of gathered information – I will not say facts – or another but, when studied it is soon realised that we will go on having such theories for ever.  All will be equally worthless.


The Moon, the tides and gravity


It is often said that the Moon causes the tides.  It does, but not in the way which we are led to believe, which is that it is the Moon’s gravitational pull which is responsible for the tides.  The relationship between the Moon and the tides is lost in antiquity.  It was observed in early human history that whenever tidal levels rose, the Moon was visible and so the relationship between the two became part of folklore.  The Moon is where it is when the tide is high because it is one end of the barycentre mentioned above.  Because the Moon and the Earth form a dual body system, both the Moon and the Earth revolve around the barycentre each month – the Moon in its orbit 384,400 km away, while the Earth revolves around a point 1763 metres below the surface of the Earth.  This monthly revolution provides the outward force which raises the sea level at two diametrically opposite points on the Earth, one being almost at the point on the Earth directly below the Moon. 


In ‘modern’ times, following the introduction of Newton’s Laws, it has been erroneously perpetuated that it is the gravitational pull of the Moon which ‘raises’ the sea levels.  It is perpetuated that the Moon raises the sea level ‘below’ it directly by gravity, and that the sea level on the other side of the Earth is so raised because of the differential gravitational pull of the Moon across the diameter of the Earth.  Those who repeat this fallacy overlook the fact that at the surface of the Earth (which is where the sea is!)  the gravitational pull of the Earth itself is 3000 times the gravitational pull of the Moon.