The ancient Greeks were among the first to propose that the Earth is a sphere. The idea of a spherical Earth can be traced back to the 6th century BCE with the philosopher Pythagoras and his followers. However, it was the philosopher and mathematician Pythagoras's student, Parmenides, who provided one of the earliest arguments for a spherical Earth based on observations and reasoning.
The Greek philosopher and mathematician Eudoxus of Cnidus, who lived in the 4th century BCE, developed a cosmological model known as the "celestial spheres." According to this model, the celestial bodies, including the Sun, Moon, planets, and stars, were each attached to a separate transparent sphere that revolved around the Earth. These spheres were often imagined as being made of a crystalline substance.
The understanding that stars were not fixed on a set of crystalline spheres and instead had their own independent motion developed gradually over several centuries. The ancient Greek astronomer Hipparchus, who lived around the 2nd century BCE, made observations that challenged the idea of fixed celestial spheres.
Nicolaus Copernicus, a Polish astronomer and mathematician, proposed the heliocentric model of the solar system, where the Sun is at the center and the Earth and other planets orbit around it. "De Revolutionibus Orbium Coelestium" (On the Revolutions of the Celestial Spheres), was published in 1543, the year of his death.
In 1923, Hubble observed a Cepheid variable in the Andromeda Nebula and determined that it was located much farther away than previously believed. This discovery, along with subsequent observations, led Hubble to conclude that the Andromeda Nebula was not part of our Milky Way galaxy but a separate galaxy.
2623, 2423, 2223, 480, and 100 years ago we had huge revelations about the nature of our universe. It feels like we talk about science, the universe and everything in a way which assumes we have everything figured out now. We forget, science is just out best guess at what's going on, and so often (if you consider hundred year intervals often) gets overturned in huge ways. Can you imagine living in any one of these times and hearing the universe you grew up with is not what it seemed? Often people could not believe it and it took many years after these discoveries for them to be widely accepted. Fortunately in recent times we seem much more likely to accept changes.