• Question: Gravity is pulling the moon closer into the earth (very slowly) when will it hit us? Why don't they send a rocket out to space to push it a little bit further away every decade or something.

    Asked by Kate and Peckasso :) to Chris, Josh, Rebecca, Rob, Susan on 22 Jun 2015.
    • Photo: Chris Armstrong

      Chris Armstrong answered on 22 Jun 2015:

      I had to look this up as I thought the moon was fairly stable. Buuut it seems to be that the moon is actually moving away from us very very slowly in fact the speed its moving away is the same rate at which your finger nails grow so there’s no need to give the moon a farewell card yet.

      To put some numbers to it, the moon is currently around 400,000km away and your fingernails grow at 4cm a year, that means we would be to sending a rocket up to the moon every 10 years to have the moon be pushed in by 40cm… That being said the moon is pretty important.

    • Photo: Susan Cartwright

      Susan Cartwright answered on 22 Jun 2015:

      Wrong, wrong, wrong. The Moon is moving AWAY from us.

      It is true that gravity causes the Moon to accelerate towards us. This just deflects its velocity vector, converting the straight line path it would follow in the absence of gravity into an elliptical orbit. If the Moon and the Earth were point masses, this would be completely stable.

      However, the Earth is not a point mass, and in particular it has oceans. Because the Moon’s gravity on the side of the Earth closest to the Moon is a little bit stronger (since gravity is an inverse square law, i.e. if depends on 1/r squared), and the Moon’s gravity on the side furthest from the Moon is a bit less, two bulges of water develop, one on the side facing the Moon (the water is pulled towards the Moon more than the Earth is) and one on the opposite side (the water is pulled less than the Earth is, and is slightly left behind). These are the tides.

      The tidal bulges want to stay lined up with the Moon, but the Earth rotates much faster than the Moon orbits, and friction between the seabed and the ocean drags the tidal bulges further east than directly under the Moon. This causes a slight gravitational pull that acts to speed the Moon up (the bulge is ahead of the Moon and is pulling it forward). Because the Moon speeds up, it is going a little bit too fast for its current orbit, so it spirals slowly outwards.

      This motion is measured. Both one of the Apollo missions and one of the Russian unmanned landers left “corner reflectors” (like those in road cat’s-eyes) on the Moon, and astronomers regularly reflect a laser off these reflectors to measure the Moon’s distance very precisely (using the travel time of the laser pulse and the speed of light). This verifies the theoretical prediction that the Moon is moving away from us.

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