The mystery of the moon's "exodus": it is moving away from the Earth at a speed of 3.8 cm per year

From the total solar eclipse

On May 28, 585 BC, soldiers from the Lydian and Median cities were fighting fiercely for land. They had been fighting for six years, but the outcome was still undecided. In the early summer dusk, amidst the flying dust, the two teams of soldiers suddenly found to their horror that the sun in the sky was swallowed by a shadow! After six minutes of darkness, the panicked soldiers laid down their weapons one after another, because they regarded the solar eclipse as a sign of the end of the war, so they stopped fighting. A war came to an abrupt end because of a spectacular "miracle". We now know that a total solar eclipse is not a miracle, but is caused by the moon passing in front of the sun and completely blocking the sun. The premise for such an astronomical wonder to appear is that the distance between the moon and the earth is just right for us to see the moon as big as the sun, which is perfect like a cleverly designed conspiracy. Such a coincidence makes some people wonder if the moon is a surveillance spacecraft placed near the earth by a more advanced civilization? Or is it intentional by the Creator?

Distance from the Moon to the Earth

When the Apollo program landed on the moon, astronauts installed mirrors on the moon to reflect the laser from the earth, and measured the distance between the earth and the moon by the speed of light and time difference, which was about 384,000 kilometers. But according to computer deduction, when the moon was born 4.5 billion years ago, it was only 25,000 kilometers away from the earth. At that time, the moon looked very huge, and its gravity could set off huge waves several kilometers high on the earth. As time went by, the moon gradually moved away from the earth. By the time the dinosaurs appeared in the Triassic period, the moon was already more than 370,000 kilometers away from the earth, and it looked 5% larger than it is now.

The Mystery of the Runaway

Why is the moon getting farther and farther away from the earth?

Simply put, a force called "tidal effect" causes the moon to gradually move away from the earth. The gravitational pull of the moon causes the sea surface to bulge at both ends of the earth facing and away from the moon, which is the tide. Since the earth only needs one day to rotate once, and the moon takes one month to orbit the earth, which is much slower than its rotation, the tide appears to be moving in the opposite direction of the earth's rotation. The huge friction between the sea water and the seabed will gradually slow down the earth's rotation, just like the moon is dragging the earth with an invisible rope. At the same time, because the force is mutual, the moon also seems to be pulled by the earth to speed up. This is like a skater who is spinning quickly in place and pulls a companion who is walking slowly beside him. The skater's rotation will slow down, and his companion will be pulled by him faster than before.

The faster the moon revolves around the earth, the easier it is for it to break free from the pull of the earth's gravity, so the moon will move farther and farther away from the earth.

Will the Moon leave the Earth?

After a lot of observations and calculations, scientists have concluded that the tidal effect will slow down the Earth's rotation speed by about 2 milliseconds every hundred years. At the same time, the moon will move away from the Earth by about 3.8 centimeters every year. Compared with the vast universe, 3.8 centimeters seems insignificant, but the universe is so long that the moon will get farther and farther away from the Earth over billions of years. Will the moon eventually leave the Earth? The simple answer is: no. On the one hand, as the distance between the moon and the Earth gets farther and farther, the tidal effect of the moon's gravity on the Earth becomes weaker and weaker; on the other hand, as the Earth's rotation slows down due to the tidal effect, the dragging effect will become less and less obvious. If we only consider the combination of the Earth and the Moon, as long as the time is long enough, such as tens of billions of years later, the Earth and the Moon will become like Pluto and its satellite Charon, always with only one side facing each other, that is, "tidal lock" to each other. If the time is close to infinity, considering reasons such as gravitational wave radiation, the moon will eventually fall to the Earth.

The future of Earth and the Moon

However, before the moon falls, the earth may no longer exist. The life expectancy of the sun is limited. The sun's hydrogen nuclear fusion can continue for about another 5 billion years, after which the sun will become a red giant and expand violently, engulfing the earth and the moon. Even without waiting for 5 billion years, the earth will no longer be suitable for human civilization in about 1 billion years, because the sun's radiation power will increase by 10% at that time, and the heat generated will evaporate all the oceans on the earth. Without the effect of ocean tides, the moon's pace away from the earth will suddenly slow down, and then it will quietly wait for the fate of being swallowed by the sun together with the earth. Therefore, at least in the years of human civilization, the moon will always be the satellite of the earth.

Excerpted from: Inside and Outside the Classroom Junior High School Edition·Science Boys July 2023 Weekly No. 3 (Total No. 1149) Text/Space Monk Editor/Shan Yu