In the distant past, people gave a simple answer to this question: the three whales. True, it remained unclear what was holding the whales up. However, this did not disturb our naïve forefathers.

Correct ideas about the nature of the Earth’s motion, the Earth’s form and many regularities in the motion of the planets around the Sun had arisen long before an answer was given to the question of the causes for the motion of the planets.

And really, what “holds up” the Earth and the planets? Why do they move around the Sun along definite paths instead of flying away from it?

There was no answer to these questions for a long time, and the Church, struggling against the Copernican system of the Universe, used this to negate the fact of the Earth’s motion.

We are obliged to the great English scientist Isaac Newton for his discovery of the true answers.

A well-known historical anecdote asserts that while sitting in an orchard under an apple-tree, thoughtfully observing how one apple after another fell to the ground because of gusts of wind, Newton arrived at the idea of the existence of gravitational forces between all bodies in the Universe.

As a result of Newton’s discovery, it became clear that many apparently miscellaneous phenomena—the free fall of bodies to the Earth, the apparent motions of the Moon and the Sun, the ocean tides, etc.—are manifestations of one and the same law of nature—the law of universal gravitation.

Between all bodies in the Universe, asserts this law, be they grains of sand, peas, stones or planets, forces of mutual attraction are exerted.

At first sight, this law seems false: we somehow haven’t noticed that the objects surrounding us were attracted to each other. The Earth attracts all bodies to itself; no one will have any doubt about this. But perhaps this is a special property of the Earth? No, that isn’t so. The attraction of two arbitrary objects is slight, and this is the only reason why it doesn’t arrest our attention. Nevertheless, it can be detected by means of special experiments. But more about that later.

The presence of universal gravitation, and nothing else, explains the stability of the solar system and the motion of the planets and other celestial bodies.

The Moon is kept in orbit by terrestrial gravitational forces, and the Earth on its trajectory by solar gravitational forces.

The circular motion of celestial bodies occurs in the same way as the circular motion of a stone twirled on a string. The forces of universal gravitation are invisible “ropes” compelling celestial bodies to move along definite paths.

The assertion of the existence of universal gravitational forces didn’t really mean much. Newton discovered the law of gravitation and showed what these forces depend on.