MOTION AND PHASES OF VENUS

Galileo was the first to observe the phases of Venus. He argued that the fact it exhibits a gibeous phase proves that it orbits the Sun and not the Earth. When Venus is closest to the Earth it is in its new phase and cannot be seen. When Venus is in its full phase it is at its most distant from the Earth but still cannot be seen as it is behind the Sun. Venus is brightest when it is 39 degrees from the Sun. This occurs 36 days before and after its full phase. When Venus is close to the Earth it has an angular diameter five times that of when it is furthest away.

The image to the right shows the motion of Venus against the background stars. Move the mouse pointer over the numbers on the bar at the bottom of the image to view a stage in the orbits of the planets. The yellow line represents the line of sight from the Earth to Venus. The panel at the bottom of the image shows the view of Venus as seen from the Earth, although the planet should appear noticably larger when it is closer to the Earth. As you view progressive stages of the two planets moving around their orbits Venus seems to show phases as it orbits the Sun. Move the mouse pointer over 'Autorun' to watch the animation in full.
The Earth also goes around the Sun, but for the sake of simplicity the Earth is shown as being stationary. There would be no difference in the phases or relative motions exhibited, but the background stars would change as the Earth orbited the Sun.