Copernicus, Galileo, Kepler and Newton: the revolution that created astrophysics.
For most of human history, the night sky was an enthralling mystery. The stars appeared as unchanging points of light, scattered across a great sphere rotating around the Earth. For millennia this was the dominant view: our planet fixed at the centre of the universe. This geocentric conception found its most famous formulation in the model proposed by Claudius Ptolemy in the second century AD. To explain the planets’ seemingly complex motions, Ptolemy introduced a system of circular orbits and smaller circles called epicycles. The model worked surprisingly well for predicting planetary positions, but it was extremely complicated. In particular, it had to be made increasingly elaborate, potentially without limit, to account for the long‑term motions of celestial objects. The revolution had yet to come.
From an old idea to a historic revolution
Few people realise that if Ptolemy had simply considered the possibility of elliptical orbits and epicycles, his model could have reproduced the observed motions remarkably well, even though it would not have reflected physical reality. This offers an important lesson: a mathematical model can describe the world adequately, at least in broad terms, even if it does not correspond to the true nature of things. In fact, this is precisely the difference between mathematics and physics: only physics selects the mathematical model that actually describes reality, because there may be more than one mathematical model that “works”.
In the sixteenth century, a conceptual revolution arrived. Nicolaus Copernicus proposed that the Sun, not the Earth, lay at the centre of the Solar System, and that the Earth was simply one of the planets orbiting it. The idea was not entirely new, Aristarchus of Samos had suggested something very similar shortly after 300 BC, but Copernicus developed it in a mathematically coherent way. The shift became irreversible thanks to Galileo Galilei’s telescopic observations in the early seventeenth century. With an instrument extremely simple by modern standards, Galileo discovered that Jupiter has four satellites and observed the phases of Venus. These observations demonstrated unequivocally that not everything revolved around the Earth.
Towards modern astrophysics
Johannes Kepler then took another decisive step. By studying the observational data collected by the astronomer Tycho Brahe, he discovered that planets do not move in circular orbits but in elliptical ones. This insight greatly simplified the description of planetary motion and paved the way for the final synthesis achieved by Isaac Newton in the seventeenth century. Newton showed that the same force that causes objects to fall on Earth, gravity, also governs the motion of the planets and the Moon. With the law of universal gravitation, the heavens and the Earth were finally unified under the same physical laws.
From that moment on, astronomy became a truly quantitative science. It was no longer just about observing the sky, but about understanding its underlying mechanisms. And from this revolution, modern astrophysics was born: the science that today studies stars, galaxies, and the very origin of the universe.
