Millions of years ago, the Moon was 18 times nearer from the Earth than it is in our days, and its apparent size in the sky was much bigger than the one that we observe actually. We now know that the Moon moves away from us at a distance of 3,8 cm per year. It can seem very little, but after many years these insignificant measures convert into gigantic numbers. The distancing of the Moon will also affect us in many aspects.
The Apollo missions left on the lunar surface retro-reflector units covered with little mirrors. Thanks to this, we can calculate the distance our satellite is at with millimetre accuracy, by sending lasers that reflect against these mirrors and that return to the Earth.
Duration of the days
The fact that the Moon gets away from us will bring consequences to the Earth, One of them is the duration of days. Little after the Moon’s formation, this one was situated at a distance of 22 500 kilometres from the Earth (in comparison, the geostationary satellites that we send to orbit are situated at a distance of 35 786 kilometres). In that epoch, the days on the Earth only lasted for 5 hours. Little after, however, the Moon started to get away, for the same reason that causes it nowadays, but with greater intensity. For consequent, as the Moon moved away, the days got longer until they arrived the 24 hours of nowadays.
Why does it move away?
This distancing of the Moon is due to that the friction between the Earth’s surface and the enormous mass of water that it is on it, which causes, over time, the Earth to rotate a little slower on its axis.
According to Newton’s third law, each action brings with it a reaction of the same magnitude and of opposite direction. The Moon and the Earth are united by a gravitational attraction. Then, as the rotation movement of the Earth gets slower, that of the Moon accelerates.
Consequently, when an object that is in orbit accelerates, this acceleration pushes it towards the outside. This causes the Moon to get further and further away from the Earth.
The effects that it provokes
The distancing of the Moon affects our planet in different ways. To start with, as the Earth will turn slower, the days will be longer. In fact, they are now getting longer at a rhythm of 2 milliseconds each century. The days will continue getting longer until the point to last for months and even to get eternal, as one face of the Earth will always look to the Sun and the other one will be plunged in the darkness. However, much before this will happen the Sun will have already converted into a red giant and will have burnt the interior planets, from which the Earth takes part.
Furthermore, if the gravity force of the Moon on the Earth gets weaker, the tides on the Earth won’t be so noticeable. Even so, even if the Moon didn’t provoke tides because it is too far away from the Earth, there would exist tides, even if very weak, by the effect of the Sun.
Even if the distancing of the Moon will affect us, we haven’t of what worrying. The changes are too subtle for us to notice in our lifetime. Additionally, a lot before all this will take place, the Sun will have already expanded until it will have converted into a red giant and in the process, it will burn the Earth and its satellite in approximately 4 500 million years.
Will the Moon escape from its Earth’s orbit?
The answer is no, the Moon will never escape from the Earth’s gravity. Eventually, the Moon would be sufficiently far away from the Earth for the deformation of the space-time to affect any of the two bodies, thus being in synchronic orbit (that to say, that the Earth would last to rotate onto its axis the time that the Moon would take to complete its orbit around the Earth, whatever its duration is). Arrived at this point, the Moon would stop accelerating in its orbit, and, therefore, of moving away.
In any case, if it is the case that the Earth will survive at the moment of expansion of the Sun, some astronomers calculate that the synchronic orbit would take place in 50 000 million years.