1. The Early History of Astronomy
The ancient people saw seven celestial bodies moving through the stars; Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn.

2. Five of these objects appeared to “wander” through the constellations
Five of these objects appeared to “wander” through the constellations. These objects were considered to be special stars, which they called planets.

3. Retrograde Motion
The planets move eastward and then loop back briefly, then continue moving eastward.

4. Ptolemy ( AD)
Determined the positions of about 1,000 bright stars.
Summarized the geocentric model of the universe, which stated that the Earth is motionless and everything revolves around it.

5. In the Earth-centred model, planets orbit on epicycles – wheels whose centers – are attached to the planets’ celestial sphere. (placing epicycles upon epicycles.)

6. Aristarchus BC
He proposed that all of the planets, including Earth revolved around the Sun, and that the Earth rotates on its axis once a day. This is the heliocentric model.

7. Aristotle 384 BC
Developed the concept of celestial sphere, stating that the universe is a perfect circle.

8. Galileo Galilei
Was the first to use the telescope to observe the sky. Galileo asserted, but never proved, that the Earth moves.

9. Johannes Kepler
Suggested the elliptical orbits of planets, and developed his Laws of Planetary Motion.

10. Isaac Newton
Developed the Law of Universal Gravitation.

11. Edwin Hubble 1920s
Realized that the Milky Way is just one of the millions of Galaxies and discovered the expansion of the Universe.

12. Video Link for Retrograde Motion