I don’t know why I never thought about this before as it seems obvious to me now, but the distance from the earth to the moon is not constant. I’m not saying that it’s min and max distance varies or wobbles, I mean that the moon is getting further and further away every year. I don’t know why I thought that it would have settled down by now to be fixed.
The obvious implication is that one day, the moon will escape earth’s orbit! That’s kind of important. I’ve seen programs on what happens when the sun expands and goes Red Giant on us in about 5 billion years, but no programs on what happens when the moon shoots off into outer space. I guess that way before it actually “leaves”, it’s gravitational pull will have diminished to the point that we no longer have tides, which would be kind of bad. Tides have been credited with being fundamental to the evolution of life.
But another new thing I learnt yesterday was that the moons relative size and orbit, i.e. it’s gravitational pull is critical to maintaining our “polar tilt”. Apparently without the moons gravity, our polar axis would vary wildly from 0degs to 90degs (currently 23degs if memory serves). This would of course cause chaos as the north and south poles turned into deserts, and Africa (say) grew the next polar ice cap!
How far away do you think the moon needs to get to cause a change? Let’s say that the gravitation effect needs to reduce by a half for us to significantly notice it. Newton’s Universal Gravity Equation says: F = GMm/R² The important term for us here is R, the distance between the 2 objects. For 1 over R squared to result in a number half it’s original value, R must become approximately 1.4R. I.e. the distance increases by 40%.
Currently the distance from the earth to the moon is about 385,000km so the question is, when will this happen? Assuming constant drift (which of course it isn’t but makes the maths a little easier for our simple needs) the moon gets:
- 3.8cm further away each year
- 3.8m each century
- 38m each millennia
- 38km every million years
So in a million years it will have got 0.01% further away. But we need it to get 40% further away, which will take approximately 4,000 million years or 4 billion years. Ok panic over, that’s about the same timescale as the sun going Red Dwarf on us, which I think will be a far more significant event than the moon getting 40% further away and it’s gravitational pull on us halving.
Aren’t Friday mornings fun! 🙂