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Tuesday, August 6, 2024

# The Earth has more than one satellite

In fact, the question “How many satellites does the Earth have?” – with a catch. And we mean, of course, natural satellites, not artificial ones. Many will say that there is only one – the Moon. But if you think about it, there will be as many as seven or even more.

If you have one object, say a planet, orbiting another object – a star – then everything is relatively simple. The orbit can be circular or more elliptical (elongated) or have many other mathematical shapes. The time it takes for one revolution of the star remains relatively constant.

Now imagine that there is a second object in the orbit of the star. Let’s say it’s an asteroid. It can also move in a circular or more elliptical path. From the outside it will seem that both of these objects follow their paths at different speeds and with different periods of time.

And here comes the strangeness. Let’s assume that the asteroid’s orbit is very similar in size to the planet’s orbit, but slightly more elliptical. Both the planet and the asteroid revolve around the star, completing each revolution one orbit in the same amount of time. But sometimes the asteroid will spin a little faster than the planet, and sometimes slower.

If you look at the asteroid from the point of view of the planet, it will appear that it is moving around it. And this is very similar to the movement of the Moon. However, this is an illusion because the asteroid does not actually orbit the planet. Instead, it moves around the Sun.

The analogy for this movement is the scenario in which you are driving in the center lane of a three-lane road. Imagine that the car will first move ahead of you on the left, overtake you and change lanes to the right. Then she will slow down, get behind you, speed up and pass you on the left again.

Objects with similar orbits are known as quasi-satellites, or quasi-moons. But these are not actually satellites, because they orbit a star, not a planet. Also, these objects are usually too far away in terms of gravity from the planet to be gravitationally bound to it.

The Earth has several such quasi-moons. For example, 469219 Kamoaleva is an asteroid about 50 meters wide, orbiting with an orbital period of 1.002 Earth years—only about 17 hours longer than Earth’s period.

This situation once again proves that our ideas about things are much more malleable than they seem. For this reason, in science it is better to avoid any rigid definitions and allow yourself to be flexible in your thinking.

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