The time of the Greek Civilisation (800 BC - 140 BC) was the period in history where Astronomy really did grow during its infant stages. Many people often misconfuse history to think of the Greeks as the pioneers of cosmic and scientific thinking but thanks to the previous Egyptian and Mesopotamian Civilisations, we already had observing tools, mathematics, models, and writing systems (the Egyptian Hieroglyphics and the Mesopotamian Cuneiform) by the time we got on to the Greeks. Below are a few of the standout scientists from this period along with their most significant ideas/inventions:

Anaxagoras (510 BC - 428 BC): He understood eclipses and proposed the idea that moonlight wasn't light being produced by the moon itself but was sunlight being reflected.

Aristotle (384 BC - 322 BC): Believed the Earth was static and the centre of the universe

Euclid (324 BC - 270 BC): Proposed and developed Euclidian Geometry

Aristarchus (310 BC - 230 BC): Believed in the revolution of the Earth around the Sun

Eratosthenes (276 BC - 194 BC): Measured the Earth's circumference and tilt of the spin axis of the Earth

Hipparchus (190 BC - 120 BC):

Measured the positions and brightness of the stars in the sky, more than 850 and created a catalogue

Invented magnitude system, to catalogue the brightness of stars

Figured out the precession of the equinoxes and measured the duration of year and distance to the moon more accurately than anyone in the prehistoric period

Pythagoras (570 BC - 490 BC):

The Pythagorean Theorem, which states that in a right-angle triangle, the sum of the squares of the sides containing the right angle is equal to the square of the longest side, i.e. the hypotenuse.

He believed in the Geocentric model of the Universe

He introduced the musical scale, which consisted of eight musical notes, to science. He believed that each of the eight astronomical bodies (the Sun, Mercury, Venus, Mars, Jupiter, Saturn, and the Stars) were attached to spherical shells with varying radii all attached to the Earth at their centres. There was a harmony of the spheres and their motions similar to the harmony of the eight musical notes.