500 BC. The Greek philosopher Heraclitus of Ephesus maintains that permanence is an illusion, and that all things are flowing, in a state of becoming. He also thought that most people sleepwalk through life, not understanding what is going on about them.

450 BC. The Greek philosopher Anaxagoras born in Clazomenae, Lydia (now Turkey), proposes that the Moon shines by reflected sunlight, and is thus able to explain total eclipses of the Sun and Moon. He was imprisoned for claiming that the Sun was a “red-hot stone”, and not a god, and that the Moon reflects the Sun’s light.

Another Greek philosopher Empedocles of Acragas (Agrigentum) in Sicily is one of the earliest to propose that terrestrial objects are made up of four elements or basic principles – fire, air, water, and earth. He viewed these as united or divided by attraction and repulsion, or more poetically, love and strife.

435 BC. Greek philosopher Leucippus of Miletus is the first to propose the atomic theory, in which is composed of very small, invisible particles now called atoms. His pupil Democritus subsequently develops the theory.

420 BC. Greek philosopher Democritus, born in Abdera, Thrace, states that all matter consists of an infinite number of eternal, invisible and indivisible particles, which he called atomon, or atoms, and that the space between the atoms is a vacuum, or void, which gives the atoms a place to move into.

366 BC. The Greek mathematician and astronomer Eudoxus of Cnidus builds an observatory and constructs a model of nested, geocentric, rotating spheres to explain the motions of the Sun, Moon, and planets as viewed from Earth, which was at the center of the system. By using a total of twenty-seven concentric spheres he was able to calculate the Sun’s annual motion through the zodiac, the Moon’s motion including its wobble, and the planet’s motions, including the retrograde movements of some of them.

Source is: A Companion to Astronomy and Astrophysics, Chronology and Glossary with Data Tables, Kenneth R Lang, Springer 2006, Singapore.