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

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.

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

Ptolemy (100-200 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.

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

Aristarchus 310-230 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.

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

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

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

Isaac Newton Developed the Law of Universal Gravitation.

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

Video Link for Retrograde Motion http://honolulu.hawaii.edu/distance/sci122/SciLab/L2/MarsRetroMany.html