Moon Moving Away

The earth’s relationship with the moon; are they drifting apart?

On the 21st July 1969 Neil Armstrong and Buzz Aldrin set up mirrors on the Sea of tranquillity on the moon as part of the first lunar landing by Apollo 11. From then up until around 7 years ago a laser beam of light was sent to the mirror and the time it took to reflect back had been accurately measured, thus being able to calculate the exact distance between the earth and the moon. These measurements proved what scientists already knew that the moon was indeed moving away from the earth at around 3.8cm per year. But why is this happening and what could the consequences be for both?

The relationship between the earth and the moon began in an incredibly violent way when a proto-planet about the size of Mars collided with the earth about 4.5 billion years ago. The debris that was flung out from this collision eventually coalesced into what is now our moon. It was much closer then and would have appeared huge in the night sky. It is thanks to the moon that our planet became stable enough to survive and not begin to spin too slowly that it would destroy itself. The gravitational force that each exerts on the other has also had many other consequences. Firstly, the length of the earth’s day has steadily got longer, and is still doing so. When the moon first formed our day would have been only 5 hours slowly increasing at a rate of just over 4 hours per billion years to what it is today! The moon literally puts the brakes on the earth. Secondly, the influence of the tides.  As the moon orbits earth, it creates a bulge of water that travels round the planet behind it. This bulge, which we experience as tides, exerts a gravitational pull on the moon, slowing it down as it orbits the earth at a distance of 240,000 miles. As a consequence of being held back by this pull, the orbit of the moon becomes altered and it is this which is causing it to move slowly away from Earth. So the tidal bulge will eventually get less affecting how we experience tides on earth. Another consequence will be that when the sun, moon and earth align there will no longer be a total eclipse as the moon will be closer to the sun and it will not be big enough in relation to it to completely block out the light.

Clearly all of these effects will not be seen in our lifetime. In reality there may not even be a human race as we know it by the time these phenomena are realised but who knows how our future ancestors may see their night sky?