1. Early Views of the Universe

2. Greco-Roman Times • Patterns in the night sky • Are the objects in the sky similar each night? • What changes over time? • Use Stellarium to look at the movement of stars, including the sun, the moon and the planets

3. Greco-Roman Times • Aristotle’s Geocentric Model (or Earth-centered model) • 384BC – 322 BC • Dominated thinking for 2000 years • Earth at the centre of a giant sphere • Stars were attached to a fixed outer sphere • Sun, moon and known planets were attached to inner spheres

4. Greco-Roman Times • Aristotle’s Geocentric Model • Was able to predict the movement celestial bodies that moved in the same direction around the Earth • Did not explain the fact that some planets were observed to sometimes reverse their direction

5. Greco-Roman Times • Ptolemy’s geocentric model • Modified Aristotle’s model by giving planets another level of circular motion called “epicycles” • Successful in describing the sometimes retrograde (or backwards) movement of planets • Video

6. Copernicus • In the early 16th century, Copernicus began to question the notion of the Earth being at the centre of the universe • He proposed a heliocentric model of the universe • The sun was at the centre • The Earth and other planets revolved around the sun • Planets still had “epicycles”

7. Galileo • Suspected that Copernicus was correct but felt that it was important to test theories and to gather observations to support ideas. • In 1609, Galileo constructed a telescope that allowed him to observe the heavens. The telescope allowed him to make the observations that supported a heliocentric model of the universe • Galileo's telescope video

8. Galileo’s Observations • Venus - size changes as an object gets closer or further away from us. This would not happen if Venus was revolving around the Earth in a circular orbit as predicted by Aristotle and Ptolemy Venus video • Phases of the moon depend on what fraction of the sunlit hemisphere can be seen from Earth.

9. Galileo’s Observations • Jupiter • If there are objects revolving around Jupiter just like our moon revolves around the Earth, then this means that other planets also have moons and suggests that Earth is not at the centre of the universe and acts in a similar fashion to other planets. • Jupiter video