# The hidden force holds everything together

Gravity is also called gravitation and is one of the four fundamental forces of nature that hold the universe together (the other three are the weak and strong nuclear force and the electromagnetic force).

Gravity has an infinite range, but the amount of force that objects attract to each other is determined by the objects’ mass – “weight” – and their mutual distance.

The Sun is more than 300,000 times heavier than Earth, and since its gravity is infinite, you might think our star would drag you and everything else in the solar system into its glowing inferno. But the center of the sun is 150,000,000 kilometers away from us, so the attraction is relatively weak.

The distance from you to the center of the Earth is only about 6,000 km, so even though the Earth is much lighter than the Sun, the short distance means you maintain contact with the Earth.

If you travel a little farther from the Earth’s surface, the force of gravity decreases.

The International Space Station (ISS) is located at an altitude of about 400 km, and 6,400 km from the center of the Earth. Here, the gravitational force is about ten percent less than that at Earth’s surface.

Astronauts on the International Space Station do not escape Earth’s gravity even when they go into orbit. In fact, they are in a continuous free fall. But it never hit the Earth because the International Space Station orbits the Earth at 28,000 km/h.

### Everything revolves around each other

The Earth experiences the same equilibrium. It is pulled by the mass of the Sun, but since the Earth is moving around the Sun at 100,000 km/h, it is pushed outward with the same force at the same time.

It was precisely the movements in the solar system that put scientists on the path to gravity.

A popular – but incorrect – legend says that naturalist Isaac Newton got the idea for gravity when he placed an apple on his head.

The real story is that the German astronomer Johannes Kepler proved that the planets do not move around the sun in perfect circles, but rather in elliptical orbits. This began the search for an explanation for the type of invisible force that affects the planets, and Newton provided this explanation with his law of gravitation in 1687.

All bodies exert an attractive force on each other that depends on the bodies’ masses and their mutual distance.

The result is that the earth does not only revolve around the sun. The sun also revolves around the earth.

Our planet attracts the sun, so the two bodies rotate around a common center of gravity. But since the weight of the Sun is much greater than the weight of the Earth, this point is closer to the center of the Sun than the center of the Earth.

The orbits of the planets are more complex because they also naturally pull on each other with different forces.

In theory you are also contributing to the calculation because you are drawing on the Earth, the Sun and the rest of the universe.