The Fate Of The Moon.
The clockwork motion of our solar system enables a certain degree of predictability – eclipses and comets are classic examples of this. One of the lesser known predictions is the fate of our Moon.
Astronomers have calculated that the Moon will eventually be be torn apart by the Earth’s gravitational field, forming a ring-like system akin to those seen around Saturn. Thanks to the reflective mirrors left on the Moon by Apollo 11, 14 and 15 astronauts between 1969 and 1971, scientists have been able to accurately measure the distance to the Moon. Facilities such as Electro Optic Systems’ Satellite Laser Ranging Observatory at Mount Stromlo Observatory in Canberra are capable of firing a laser beam at these mirrors.
By recording the time taken for the laser light to bounce back, and knowing the speed at which light travels (299,792 kilometres per second), they can determine the distance travelled by the pulse of light. Such measurements over the past three and a half decades have revealed that our Moon is currently receding from the Earth at a rate of 3.8 centimetres per year.
The reason for this retreat is well understood, albeit somewhat complicated to explain. Because the force of gravity declines with distance, the Moon’s gravitational pull on the Earth is slightly stronger on the side of the Earth facing the Moon. In fact, this is what produces our tides. They are gravitationally-induced distortions in our planet, with the greatest affect evidenced in our oceans.
There are of course two high tides on our planet at any one time; while one of these faces the Moon, a weaker tidal-bulge also exists on the opposing side of the Earth – the part of our planet experiencing the weakest gravitational tug from the Moon because it is the furthest from the Moon.
Due to the Earth’s rotation, these bulges are rotated slightly forward with respect to the Earth-Moon axis. That is, as the Earth spins on its axis, completing one rotation every 24 hours, it can’t quite re-shape itself quickly enough to keep the tidal bulges perfectly aligned with the Moon.
As the Moon’s gravity tries to pull these bulges back into alignment – and given the law of conservation of momentum – the Earth slows down by transferring some of its rotational momentum to the Moon’s angular momentum. The Moon is consequently forced into a higher orbit, i.e., further from the Earth, where it has a decreased period of revolution around the Earth.
Face to Face
The action of tides also causes friction, especially as waves crash into shore lines. The net affect of tides is to slow a rotating object so that eventually the same side of the rotating object always faces the external object that is causing the tides. While such a “tidal lock” has not yet happened to the Earth, it has happened to our Moon; this is why it always presents the same face to us.
Due to the Moon’s tidal-braking on the Earth’s spin, the time taken for the Earth to complete one rotation, i.e. one day, is currently increasing by a couple of milliseconds per century. In a billion years from now, the length of a day will be 3 to 4 hours longer than it is at present, and the Moon will be 10% further away.
Given enough time, the Moon would have moved from its current average distance of 384,400 kilometres (a distance spanned by placing 30 Earths side-by-side) to a distance of roughly 550,000 kilometres, and the length of a day will equal 47 current Earth days.
By this stage, the length of a month would have also increased to 47 current Earth days and lunar tides will no longer exist on Earth because the Earth will always show the same face to the Moon. Just as there is a so-called “dark side of the Moon” – the side we cannot see from Earth – in billions of years time there will also be a “dark side of the Earth” as viewed from the Moon.
But the story does not end here. The Sun also generates tides on Earth; we experience these most notably in the form of spring tides, when the Sun and Moon are aligned with the Earth and their gravitational tugs combine to produce extra large tides in our oceans. At present, the tide-raising force of the Moon is more than double that of the Sun.
With the decline of lunar-induced tides on Earth, the solar-induced tides will start to dominate and result in further tidal-breaking that slows the Earth down until the length of a day (the time taken for the Earth to complete one rotation around its axis of spin) equals the length of a year (the time to complete one orbit around the Sun).
At this point the Earth will always present the same face to the Sun, and there truly will be a “dark side of the Earth”. If you were on the Earth at this time, relative to the Sun it would appear as though the Earth has stopped spinning, although if you looked out into space, at the distant stars, you would see that the world is still turning.
During this period of additional rotational slow down, the Earth will actually start to rotate more slowly than the Moon revolves around the Earth. This will result in the generation of lunar tides on Earth again. However, this time the situation will be reversed.
The Moon will now tug on the Earth’s tidal bulges in an attempt to speed up the Earth, but in so doing the Moon’s orbit will gradually but surely decay.
At a distance of only 18,500 kilometres from Earth, the Moon would have reached the Earth-Moon Roche limit, inside of which the Earth’s gravitational pull on the closest and furthest side of the Moon is so different that the Moon will quite literally be torn asunder.
The Earth-induced tidal bulge on the Moon will be so great that the Moon will split into two. This shredding process will continue until the Moon has fragmented enough to form a set of ring-like distributions around the Earth.
One should perhaps note that the time frame for the above process to unfold is on the order of hundreds of billions of years, and understandably a lot is expected to happen before then. For example, in a mere 3 billion years our galaxy, the Milky Way, will collide with our neighbouring galaxy Andromeda.
In 5 billion years our Sun will dramatically expand to become a red giant – increasing the temperature on Earth to above the boiling point of water – before fading to become a white dwarf. Still, if one is around, what a majestical sight it would be, akin to the rings that circle Saturn.
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EOS operates research centres in Australia, the US and Germany, and has significant production facilities in Australia and the US. The EOS Group of companies was founded in 1983 and became a publicly-traded company on the Australian Stock Exchange in 2002.Source: EOS
- Supermoon? Super Phooey! (davidreneke.com)
- What Would Happen If The Sun Disappeared? (davidreneke.com)