The beginning
Astronomy is one of the oldest sciences, dating back to the most ancient civilizations that lived thousands of years ago. Possibly the oldest star chart we have found is dated some 32000 years old. At the beginning, astronomical observations were mostly used for practical purposes such as timekeeping or navigation. One thing that was fastly understood by ancient astronomers, was that throughout the night the stars moved the same way as the Sun did during the day: they rose in the east and set to the west.
The Ancient Greeks
As civilizations were getting more advanced, and developed tools such as mathematics to study the patterns of nature, soon the question of how objects moved in the sky was a central problem to solve. The first ones to develop a scientific model that could explain the motion of celestial bodies were the ancient Greeks. In 140 AD the ancient Greek astronomer Ptolemy published the now famously known Ptolemaic model. It was a geocentric model, where the Earth is at the center and everything else moves around it in perfect circles. Despite its complexities and inaccuracies, Ptolemy’s model was widely accepted for nearly two thousand years. It’s worth mentioning that around 260 B.C. Aristarchus suggested a heliocentric model, where the Sun is at the center of the Solar system. But this did not catch on until the Copernican Revolution.
Renaissance – What made everyone change their mind
In the late 1400s, Nicolaus Copernicus, started to notice that the tables of planetary motions based on the Ptolemaic model had been growing very inaccurate. He went back to Aristarchus’ idea, and proposed a system with the Sun at the center and planets revolving around it. However, Copernicus made the mistake of keeping the planets in perfectly circular orbits. For it to work, he had to add several complexities to his model, making it just as inaccurate and convoluted as the Ptolemaic one. A big contribution to astronomical observations came from Tycho Brahe, in the mid 1500s. Tycho dedicated his life work to recording precise naked-eye observations of celestial bodies which were the basis for his apprentice, Johannes Kepler, to turn the world upside down. Kepler understood that the Sun is at the center of our solar system, and planets orbit around it in ellipses. This heliocentric model has been proven correct time and time again, and it is what we use today.
Of course it wasn’t that easy for people to get onboard with these ideas. They were very reluctant to change a picture they’d upheld for almost 2000 years, and humanity has a bit of an issue admitting they’re not at the center of the Universe.
The scientist that brought the so-called Copernican revolution to reality was Galileo Galilei. Galileo was among the first to use (not invent!) a telescope to look at the night sky. His observations destroyed some of the most fundamental beliefs of his time. Galileo is especially famous for the discovery in 1610 of the four largest moons of Jupiter, which are now collectively known as the Galilean moons. This was the first ever observation of satellites orbiting another planet! It proved that it was possible for other objects in the Solar System to orbit something other than the Earth! So would the Earth be at the Center of the Universe? Why couldn’t it orbit the Sun instead? Thanks to Galileo the world of science and astronomy had revolutionized, and the heliocentric model was finally on the table.
(On) Planet Nine 