Cerebral Friday Fun – When will the Moon Leave Our Orbit?

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!  🙂

{ 10 comments… add one }
  • krustan 6 November 2009, 2:01 am

    Woah i neva thought that the moon would leave us
    at least i’ll b dead by then lol:)

  • Mary 10 February 2010, 3:38 am

    Goodbye, Moon!

  • Josh 24 March 2011, 3:54 pm

    What about the compounding effect. In 10,000 years won’t the fact that the moon is further away mean that its distance travelled per year will have increased (due to the corrosponding reduction of gravitational pull)? I still don’t think its a concern for our species, but wouldn’t that cut your estimate about in half? Meaning that the moon’s departure will be a problem long before the explosion of the sun?

  • OB 4 August 2011, 2:42 am


    The period of the moons orbit is not a relevant variable in these calculations.

  • J.H. 29 December 2011, 1:59 pm

    What? Josh was not talking about the moons circular orbit. He meant that as the moon travels outward, it is held by less and less of Earth’s gravity. Therefore, as time goes on, the moon wouldn’t just be traveling outward at the same speed, but it would be accelerating away. I’d say that’s very relevant.

  • skwigget-wigget 12 February 2012, 11:05 am

    Not too many people have the mental capacity 2 even attempt 2 phathom this topic. It’s a shame that nobody perceives the gravity of the matter. We’ll all be long gone before this cataclysmic event, but it’s still something worth a bit more attention. -skwigget

  • Yusuf 20 February 2012, 11:16 pm

    I heard one possibility would be humans would not be able to reproduce, because the menstrual cycles in women are directed effected by the moon, so if that the moon were gone it would effect her cycles,, what do you think?

  • Ark 25 May 2012, 3:35 am

    Why does it seem all of destruction has a roughly parallel time scale? The moon, the sun going red, and that Andromeda will collide with the milky way in 3 – 5 billion years. hey ho. Cant wait.

  • Steph 7 June 2012, 10:07 pm

    Seems to me the moon is smarter than we thought.It’s trying to leave the earth orbit before the sun blows and we get hit by the Andromeda galaxy. 😉

  • Alex w 10 September 2012, 12:27 am

    A+++!!!!! Two thumbs up, u made my day. The question I’ve always wanted answered finally answered in a funny way, u rock! XD thank you!!

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