Laws of planetary motion – Kepler's law

To fully appreciate the brilliance of the discovery of the Law of universal gravitation, let us return to its prehistory. There is a legend that while walking in the Apple orchard at his parents ' estate, Newton saw the moon in the daytime sky, and immediately before his eyes, an Apple broke off from a branch and fell to the ground. Since Newton was working on the laws of motion at the same time, he already knew that the Apple had fallen due to the gravitational field of the Earth. He also knew that the Moon was not just hanging in the sky, but was orbiting the Earth, and that therefore it was being affected by some force that was keeping it from falling out of orbit and flying straight away into outer space. Then it occurred to him that perhaps it was the same force that caused the Apple to fall to earth and the moon to remain in earth orbit – the force of gravity that exists between all bodies.

So, when Newton's great predecessors studied the equidistant motion of bodies falling to the surface of the Earth, they were sure that they were observing a phenomenon of purely terrestrial nature — existing only close to the surface of our planet. When other scientists, studying the movement of celestial bodies, believed that in the celestial spheres there are very different laws of motion than the laws governing movement here on Earth.

The very idea of a universal gravitational force has been expressed many times before: Epicurus, Gassendi, Kepler, Borelli, Descartes, Roberval, Huygens, and others have speculated about it. Descartes believed it to be the result of vortices in the ether. The history of science shows that almost all arguments concerning the motion of celestial bodies before Newton were basically that the celestial bodies, being perfect, move in circular orbits because of their perfection, since the circle is an ideal geometric figure.

Thus, in modern terms, it was believed that there were two types of gravity, and this idea was firmly fixed in the minds of people of that time. Everyone believed that there is an earthly gravity acting on an imperfect Earth, and there is a heavenly gravity acting on a perfect heaven. The study of the motion of planets and the structure of the Solar system led, in the end, to the creation of the theory of gravity – the discovery of the law of universal gravitation.

The first attempt to create a model of the Universe was made by Ptolemy (~140). in the center of the universe, Ptolemy placed the Earth, around which the planets and stars moved in large and small circles, as in a dance. The Ptolemaic geocentric system lasted for more than 14 centuries and was replaced only in the middle of the XVI century by the heliocentric Copernican system.

In the early XVII century on the basis of the system of Copernicus, the German astronomer J. Kepler formulated three empirical laws of motion of planets of the Solar system using observations of the motions of the planets, the Danish astronomer Brahe, Etc.

Kepler's first law (1609): "all planets move in elliptical orbits, in one of the foci of which the Sun is located".

The elongation of the ellipse depends on the speed of movement of the planet; on the distance at which the planet is located from the center of the ellipse. Changing the speed of a celestial body leads to the transformation of an elliptical orbit into a hyperbolic one, moving along which you can leave the Solar system.