History+of+Astronomy

People have been observing the night sky from the beginning of time, using it for navigation, and signs for planting and harvesting times.

Not only did they study the '**fixed stars**', that is the stars that appear to stay in the same position all of the time, but the stars that wandered among the fixed stars; The Planets (Greek (astēr planētēs), meaning "wandering star"). It was fairly obvious that all of these orbit the Earth. The Sun rises in the East and sets in the west, the stars rotate during the night, the planets move however they move around the Earth.

Early astronomers observed the periodic motion of the planets, sun and moon, the movement against the fixed stars and their rising and setting times and eclipses. They wanted to easily predict these events.They developed models to help with these predictions.

 The idea that everything moved around the Earth is referred to as a **Geocentric** model (geo-earth, centric-centered on). Because all of this was taking place in the 'heavens' all objects needed to be perfect, so all planets were spheres, and their movement was in circles.  or more simplified

However, this model does not explain all phenomena and movements, such as the **Retrograde motion** of the planets, that is the planets moving backwards against the fixed stars, or that Venus shows phases, similar to the moon. By adding **epicycles** to the model, retrograde motion could be explained. Epicycles are orbits within orbits.

This became too cumbersome. If the earth was place in an orbit, like the other planets, that would take care of retrograde motion and phases of Venus. Retrograde motion is similar to a faster car overtaking a slower car, from the perspective of the faster car, the apparent motion of the slower car changes. Venus shows phases as it orbits the Sun, with the illuminated portion showing differing amounts to us. This can be explained by the heliocentric system (helio meaning sun) and difficult to explain by the geocentric model.

Using this model proved to be somewhat problematic due to it not being as useful as the geocentric model in predicting planet motions over time. Having Kepler describe the actual orbits as not being circular helped

But this brought up another problem. The lack of parallax shown by the fixed stars.