History of Gravitation

Solar system proposed by Nicolaus Copernicus

When we studied universal gravitation, we saw that the Sun is the central star of our planetary system. We also know that all the planets move around it, following the following increasing order of distance: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The figure above shows us this order.

We call the movement that each planet describes around the Sun the orbital motion of translation ; while the movement that the planet makes on its own axis is called rotational movement.

The laws of physics behind the motions of the planets are the result of thousands of years of observing the universe. These observations go back to the time of the Egyptians, Chaldeans, Phoenicians, and other peoples of antiquity. These observations were intended to try to understand the movements – not only the movement of the planets, but of every body in space. Interest in this movement was always linked to human activities, such as agriculture and navigation. The first explanations about the celestial bodies involved interventions by gods, that is, they presented as fundamentals religious, mystical and mythical concepts.

Historically it is known that the first scientific studies of the stars were carried out by Greek philosophers. It was they who, without relying on religion, tried to explain the movements of the planets, or rather, of the entire planetary system.

The astronomical model proposed in antiquity was the geocentric model . This model was supported by Claudius Ptolemy. This model considered the Earth to be the center of the Universe, that is, all the stars in the universe revolved around the Earth. In this model, also called the Ptolemaic model, Claudius Ptolemy said that the Sun and Moon described circular orbits around the Earth. The other planets, according to this model, each described a circular orbit around a center, which, in turn, described another circular orbit around the Earth.

The Ptolemaic model, that is, the model suggested by Claudius Ptolemy, was accepted for many years without any refutation. However, in the sixteenth century new hypotheses about the motion of the universe began to emerge. A new model was then proposed by Nicolaus Copernicus. In his model, Nicolaus Copernicus proposed that the Sun was the center of the universe and the other planets, until then discovered, revolved in circular orbits around the Sun. His model became known as the heliocentric model .

Another scientist who vigorously defended the heliocentric model was Galileo Galilei. Through the use of optical instruments in astronomical observations, Galileo obtained strong evidence that proved the Copernican model to be correct. One of the most plausible proofs of the time was the discovery of Jupiter’s moons. If there were bodies that revolved around a planet, the Earth could not be the center of the Universe.

But it was up to the young astronomer Johannes Kepler to definitively determine how the planets move around the Sun. Kepler was a disciple and assistant to astronomer Tycho Brahe. Kepler inherited the records of the precise observations left by his master. From these records, and after a long and laborious study, Kepler was able to enunciate the three laws that describe the motion of the planetary system. These laws are called Kepler’s Laws .

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