The hidden face of the Moon

2008/04/01 Azkargorta Aretxabala, Jon - Fisikan doktorea. Bilboko Ingeniaritza Goi Eskolako irakasle eta ikertzaileaEHU Iturria: Elhuyar aldizkaria

• Astronomia
• Sariak

The Moon existed before man emerged, and has always fascinated us, for thousands of years, but we have never seen its back.

In 1959 the Soviets sent the Luna-3 mission and managed to photograph the back… 50 years have not yet been fulfilled [1]. As we have said, from Earth you can never see that back of the Moon, if not in a photo.

In principle there is no physical relationship between the rotation of a body and the duration of an orbit. So why does the duration of a 'day' of the Moon coincide with the duration of a 'month'? Is it chance? The whim of the Moon?

There is a reason and it has to do with the tides. We know that on Earth the sea goes up and down twice a day. The reason is explained in depth in the article [2], but here I will make a brief summary:

Just as the Earth attracts the Moon, the Moon also attracts the Earth. But due to the size of the Earth, the area near the Moon feels greater attraction than the central one and the sea approaches the Moon. On the other hand, the remote area of the Moon feels less attracted than the central and the sea away from the Moon. As a result, the Earth suffers a "deformation", with two sides of the fold (G) and two of the low tide (B) simultaneously on Earth (Figure 1).

To this we must add that the Earth rotates once a day, so we have two pleamares every day and two bajamares every day.

Just as the Moon creates tides on Earth, the Earth also creates them on the Moon. Okay! On the Moon there is no sea but solid bodies deform as much as liquids, but deform as a rubber ball. The deformation of the solid earth produced by the Moon is a few centimeters and we do not realize it, but seismographs, for example, detect very well the tides of earth. For on the Moon there are also solid tides and the ground rises and falls every time the Moon gets upset, that is, twice a month.

How the Earth affects the rotation of the Moon

If the Moon were fully elastic, it would respond 'immediately' to Earth's attraction and its 'deformation' would be aligned with Earth, as shown in Figure 2A. However, throughout the universe there is no fully elastic material, and the internal friction and inertia of solid material cause the Moon's response to occur later, i.e. with a delay, as in Figure 2B.

The conclusion is spectacular (Figure 3): When the Earth attracts the Moon, the force of side A tends to turn the Moon towards the clock hands, while the force of side B turns the Moon against the clock hands. The force of side A is somewhat greater, because it is closer to the Earth, which causes the Earth to turn the Moon towards the clockwise, or what is the same, slows the rotation of the Moon.

How long does this cushioning last? For the rotation of the Moon has been slowly and for thousands of years, until today it reaches a position of equilibrium, that is, a symmetrical position, where point A and point B are located in the same direction of the Earth. Therefore, from Earth we only see zone A of the Moon, because the Earth has synchronized or "stopped" the rotation of the Moon for a long time.

Most of the natural satellites or 'moon' of the solar system, like our Moon, are synchronized; the satellites of Mars, Jupiter, Saturn, etc. have the same period of rotation and orbit around the planet [3].

The Sun also generates tidal forces that have made the rotations of Mercury and Venus not synchronized at all, but if they are rather dampened, they are closest. The rest of the planets are far away and the effect is weaker, but it exists.

How the Moon affects Earth's rotation

If the Earth has paralyzed the rotation of the Moon, it is logical to think that the Moon will have a similar effect on the Earth. It is true. The Earth does not always show the same difference to the Moon, that is, the Earth is not 'synchronized', but is slowing down.

Since the mass of the Earth is much larger than the mass of the Moon, more time will have to be spent until the Earth 'stops', but in that it is: The duration of a day on Earth was shorter in the past, that is, the Earth revolved faster.

There are paleontological studies that have gone far in time and that demonstrate this result [5]: for example, coral fossils have rings or growth layers called ephitecas. As in tree trunks, each ring shows coral growth over a one-year cycle. More detailed studies have shown that within each layer there are other thinner layers: they are monthly layers, within which there are other even finer layers. The finest layers have been considered as indicators of daily growth. The number of thin layers has been counted within an annual layer of coral fossils from the Paleozoic Devonian era (400 million years ago): 400 rings. On the contrary, within the one-year ring of Garai Carbonífero's coral-fossil (300 million years) there are 380 layers. This means that the Earth revolved 400 times a year in the Devonian era and made 380 tours a year in the Charcoal. Everyone can make a small calculation and see that in the Devonian era the Earth day lasted about 22 hours and in the Carbonífera era about 23 hours. We know it currently lasts about 24 hours.

The same result has been proven with other methods, that is, the Earth is increasingly slowed down. In fact, with the accuracy of atomic clocks it has been shown that the duration of the day is increasing: 2.3 milliseconds per century [4].

Moving away

As shown in the previous section, the Moon exerts the same effect as the Earth on the Moon: cushion rotation. Figure 4 shows, on the left, the Earth and the forces of the Moon on Earth (A and B). These two forces are the ones that are stopping the rotation of the Earth, as mentioned in the previous section.

On the other hand, the right part of the image shows the Moon and the forces that both parts of the Earth exert on the Moon (A' and B'), which are reactions of the previous, exactly the same but opposite.

Forces A' and B' that support the Moon have a component (and the principal) in the direction of the Earth, but have another component perpendicular to this direction. The force 'A' rises a little and the force 'B' drops, but the force 'A' is something greater because the Earth is closer than the B. Therefore, another consequence of the tidal forces is that the Moon has a tangential acceleration ( to t) or that it is increasing its speed. If speed is increased, the centrifugal force of your orbit also increases, causing an increase in the radius of the orbit. Thus, over the years, the Moon is moving away from Earth and its 'month' is getting longer.

Physicists say that energy is not produced or erased, that is, it is preserved, but it is changed from one type to another: If the rotation of Earth and Moon is slowing down, kinetic energy will lose, although this energy is becoming potential energy as the Moon and Earth move away.

In fact, the orbit of the Moon is not a circle or an ellipse, but gradually grows like a spiral. Exact measurements have shown that the Moon is moving almost four centimeters a year [6].

How long will this slowdown of Earth's rotation last? And how long will the Moon be away from Earth? It is estimated that the Moon will keep moving away until the current distance increases by sixty percent. By then the rotation of the Earth will be slowed down and the duration of the day will be the same as the month of the Moon, and both will be about fifty-five days [7] (the day of this phrase is the duration of the current day). At that time, the deformation of the Earth and that of the Moon will be symmetrical about the direction between the two, that is, the Earth, seen from the Moon, will always show the same face and keep the other in secret, but until this happens it will take billions of years.