What changes will occur on Earth as the moon moves further away from us?
Ron Dippold
San Diego, California, US
Usually, my answers are: 鈥淥h, this is so much worse than you even imagined!鈥� But in this case, the answer is 鈥渘ot much鈥�.
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The moon is about 384,400 kilometres from Earth right now and our days are about 24 hours long. The moon pulls on Earth鈥檚 oceans as the planet rotates below it, causing tidal bulges at the points closest to and furthest away from the moon. Those bulges drag on Earth through the seafloor, slowing down its rotation and causing its days to be about 1.8 milliseconds longer per century. That energy is transferred to the moon, which is kicked into a slightly higher orbit, and is thus retreating from Earth at about 3.78 centimetres per year. We have fossil proof from corals that roughly 1.8 billion years ago, our year was about 485 days long because Earth was rotating much faster 鈥� leading to approximately 18-hour days. In a billion years or so, at current rates, an Earth day will be around 27 hours long and the moon will be some 10 per cent further from Earth, about 400,000 km away.
Usually my answers are: 鈥楾his is so much worse than you imagined!鈥� But in this case, the answer is 鈥榥ot much鈥�
This is the time in the future when everything changes, because the sun will be about 10 per cent brighter. This will trigger a runaway greenhouse effect on Earth, if we haven鈥檛 already done that to ourselves, and the oceans will completely boil away. The moon will still have Earth鈥檚 rocks to drag on, but those aren鈥檛 nearly as flexible as water, and without the massive friction of tides on Earth鈥檚 seafloor, the moon will almost stop moving further away and the length of Earth鈥檚 day will almost be fixed. And then, about 4 billion years after that, the growing sun may or may not envelop Earth and the moon, but will at least turn their surfaces to magma. At that point, the moon may start moving further away again, but we won鈥檛 care.
Practically, the moon continuing to move further away in the next billion years only causes two minor effects. First, because tidal effects fall off by the cube of the distance between the moon and Earth, if the moon moves 10 per cent further away, this would reduce tides by about 25 per cent. Four-metre tides become 3-metre ones. Second, it is entirely a coincidence that the moon is just the right size to cover the sun as seen from Earth and cause a total solar eclipse. In about half a billion years, as the moon continues to move away, its apparent size will shrink and it will no longer cover the sun. There will only be annular eclipses 鈥� a dark moon surrounded by the ring of fire of the sun peeking around it. But for the sentient rodent people of Earth in 600 million years, the moon moving further away will be the least of their concerns.
Mike Follows
Sutton Coldfield, West Midlands, UK
Tidal interactions cause the moon to move away from Earth, currently at about 3.8 cm per year. This process also lengthens both the moon鈥檚 orbital period and Earth鈥檚 spin period. Assuming these processes continue for tens of billions more years, Earth鈥檚 spin period will eventually equal the moon鈥檚 orbital period, and the two bodies will both become tidally locked.
The same face of the moon is always turned towards Earth. Eventually, Earth will reciprocate: anyone on the moon鈥檚 near side would always see the same face of Earth fixed at the same point in the sky. Earth and the moon will each then permanently show the same face to the other.
Lunar tides will become weaker because the moon鈥檚 tide-raising effect decreases roughly with the inverse cube of its distance from Earth. Over millions of years, this would alter coastal environments and ocean mixing, with ecosystems evolving in response.
The moon will also appear smaller in the sky. In about 600 million years, it will no longer be able to cover the sun completely, so total solar eclipses will cease.
The moon helps to stabilise Earth鈥檚 axial tilt, which is presently about 23.4 degrees and naturally varies only between about 22.1 and 24.5 degrees. As the moon moves further away, this stabilising influence will weaken, allowing larger variations in Earth鈥檚 tilt and potentially more extreme long-term changes in climate and the seasons.
The moon may have influenced Earth鈥檚 history. Some researchers have suggested that the repeated wetting and drying of intertidal environments, driven by the moon鈥檚 tides, may have favoured the chemistry leading to life, echoing Charles Darwin鈥檚 famous speculation about 鈥渟ome warm little pond鈥�. Together with its stabilising influence on axial tilt, the moon may therefore have played a significant role in making Earth habitable. If so, were it not for the moon, there might have been nobody to observe its slow recession.
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