Tuesday, 15 December 2009

The moon and the age of the earth

In my defence of a young earth I wanted to address the philosophical issues which I think are foundational to the argument. Discussions that fail to identify these issues end up with proponents of an old earth indirectly defending their presuppositions as if they are conclusions.

Consider 2 dating systems that give contradictory results. Which do we take as preferable? They both cannot be true. One or neither is true. Frequently the position is taken that dating system A gives the correct result and dating system B in in error because of incorrect assumptions X, Y, and Z. But it may be just as reasonable to take B as the correct result and explain why A is in error. Unfortunately proponents of A fail to see the philosophical validity of this. And even if they do, their subsequent arguments still frequently assume A.

I am not saying that all systems are equally convincing in their arguments. Rather that if B can be questioned then so can A.

I anticipated giving further specific arguments in favour of a young earth, or at least against a 4 billion year old earth. One argument is the maximum age of the moon.

The moon is known to be receding from earth. The rate is currently about 4 cm per year, though it is decreasing; the moon receded more quickly in the past. The recession is due to a transfer of angular momentum from the earth to the moon. The loss of angular momentum on earth is due to ocean tidal friction.

If we calculate how long it would take the moon to get to its current position if the moon was initially at the surface of the earth we get a figure of ~1 billion years. This is the maximum possible age for the earth-moon system. It can be much younger than this.

This maximum age is slightly, but negligibly, shorter if we consider the Roche limit. The earth's gravity exerts a force on the moon dependant on the distance of the moon from the earth. At a certain distance the force exerted from the earth on the near-side of the moon compared to the lesser force on the far-side of the moon is greater than the gravitational force holding the moon together. This is called the Roche limit. This ignores added force from any internal tensile strength that holds the moon together.

The Roche limit for the moon is ~18,000 km from the centre of the earth. The moon is currently ~384,000 km from the (centre of the) earth. The earth's radius is ~6,300 km.

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