Galileo and the Phases of Venus
Created September 30, 1997
- Copernicus proposed that the
Sun was in the center of the Solar System.
- This hypothesis was accepted because of its simplicity, but
no supporting observations existed at the time.
- If Venus and the Earth both orbited the Sun
then the Copernican model made a definite prediction about what
phases it should show at various places in its orbit.
- The phases predicted by the Earth-centered Ptolemaic model
were quite different from the Copernican predictions.
- Galileo used the newly available telescope
to observe the phases of Venus.
- The phases matched the predictions of the Copernican
model. The Copernican model was validated by Galileo's
- Aside: The definitive test of the Earth's motion about
the Sun was the observation of stellar
The parallax of the nearby stars was finally observed in 1838.