The answer is not as simple as it might seem. It largely depends on whether we are talking about the current period or about the history of the Earth in general? And what size celestial object has the right to be called the moon, and what not? In what orbit should it move? Let's try to figure it out …
Did you know that in the history of our planet there was a period when the Moon generally occupied almost half of the earth's sky? No, it was not larger than it is now, but simply located much closer.
This follows from the current astronomical observations, according to which our Moon is annually removed from the Earth by several centimeters. This means that once it was much closer than it is now. And someday the time will come when she will say goodbye to us.
And were there once a time when the moon did not exist at all, and where did it come from? Scientists do not have an exact answer to this question. But there are as many as three hypotheses explaining the appearance of the moon.
According to one of them, the Moon was formed together with the Earth about 5 billion years ago, when the entire solar system was forming.
According to another, the Moon was once a wandering celestial body and, flying past the Earth, fell into its gravitational trap. The gravity of our planet has caused the moon to revolve around the earth.
Perhaps the most interesting is the third hypothesis, according to which the Moon was formed as a result of the collision of the Earth with another planet, the size of Mars, which some astronomers call Theia.
Promotional video:
Theia, like the moon, flew past our planet. But she did not calculate her route and crashed into her from all over. The collision released so much energy that Theija, according to geologist Daniel Herwartz of the German University in Cologne, completely melted and turned into vapor, like most of the earth's surface.
Part of the evaporated rock material then returned to the Earth and formed its shell, while the other took on a solid state, turning into Selenium. So the Moon appeared on the Earth.
However, many planets in the solar system have more than one satellite. According to NASA's Jet Propulsion Laboratory, Mars has 2 moons, Jupiter has 66, Saturn has 62, Uranus has 27, and Neptune has 13, and these numbers are constantly changing as new moons are discovered.
And only our planet has only one satellite - Selena. But is it? It turns out, besides the Moon, which everyone knows, our planet has two more subtle satellites. They were discovered by the Polish astronomer A. Kordylevsky in 1956. However, to this day, very few astronomers can boast that they have observed these accumulations of cosmic dust - they are too rarefied, since they consist of … cosmic dust.
And he moves along the same path as the real moon, and at the same speed. But one cloud of dust is 60 degrees ahead of Selena, and another is just as much behind. And both are located at the so-called libration points calculated by the French scientist L. Lagrange in the 18th century.
He proved mathematically that if three bodies at the beginning of their movement are at the vertices of an equilateral triangle, their movement will be stable and mutual attraction will not last long. The same cannot be said about objects that can be called a second or false moon.
Such pseudo-satellites or quasi-satellites are celestial objects whose orbits are similar to the orbit of the Earth. In this case, an orbital resonance occurs between our planet and this object. But there is a pseudo-satellite in the orbit of the Sun, like the Earth. And only occasionally do they intersect.
The most famous false satellite of the Earth, often referred to as the second moon, is the object discovered in 1986, named 3753 Cruithne. It is an asteroid orbiting the Sun, one of several thousand whose orbits cross the Earth.
The object is not a satellite of the Earth, as it revolves around the Sun. However, the gravity of the Earth acts on Cruithney in such a way that the Earth and this asteroid "meet" every year in almost the same place. But the collision does not occur, because the asteroid moves away from the plane of the Earth's rotation around the Sun.
According to computer models, it has been established that Cruithney will remain in its orbit for approximately another 5 thousand years. Then the asteroid can be in another orbit and temporarily become a satellite of the Earth. But it will not last long. Astronomers have established that after rotating around the Earth for 3 thousand years, Cruithney will return to orbit around the Sun.
Cruithney is not the only false satellite whose orbit intersects Earth's orbit. In March 2012, astronomers at Cornell University published the result of a computer study of asteroids that revolve around the Sun, but may temporarily become Earth's natural satellites.
These small moons are no more than a few kilometers across and may orbit Earth for less than a year before returning to the Sun.