Each year, the Moon increases its orbit and moves away from Earth by about 3.8 centimeters. A distance which seems very ridiculous on the scale of our gigantic Universe, but this drift has major repercussions on our beautiful planet. Indeed, this movement away modifies not only the amplitude of ocean tides, but also the Earth’s rotation, thus gradually extending the length of our days.
A slow gravitational divorce
To understand this phenomenon, we must first return to the functioning of the tides. Under the attraction of the Moon, the oceans deform and create two large bumps of water oriented towards and away from our satellite. However, the rotation of the Earth is faster than the orbital movement of our satellite, which is 27.3 times slower.
This shift means that these water masses are never directly aligned under the Moon, because the Earth’s rotation always drags them slightly ahead of the lunar position. This tidal ridge then exerts a pulling force on the Moon, pulling slightly forward in its orbit.
This constant force (or tidal torque) has the effect of accelerating our satellite. In celestial mechanics, when a body gains orbital momentum, it is propelled into a higher and wider orbit. This transfer of energy therefore naturally moves away our satellite of 3.8 cm annually.
One of the consequences is that the length of the day increases very slowly. On the scale of a human lifetime, change is imperceptible, which is not the case over millions of years. Cretaceous mollusk fossils show us, for example, that at the time of the dinosaurs, the Earth made a complete revolution in approximately 23.5 hours and a year lasted 372 days. If it was rotating faster, it was simply because the Moon was closer and the Earth-Moon couple had not yet transferred as much energy.
This mechanism is never destined to stop, and when the Moon has moved far enough away, Earth days will reach 25 hours, or even more. So this means, potentially, that we could be more productive and get more done in a day? Unfortunately no, since the process will take hundreds of millions of yearsand it is very likely that the human species became extinct long before. The 25-hour day is ultimately a bit like retiring at 60: a magnificent concept, but for a civilization that will no longer exist to benefit from it.
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