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Galileo discovered that the planet Venus goes through phases similar to the phases of our moon, but Ptolemy’s geocentric model implied that we should never see Venus fully illuminated. Venus always appears near the horizon closest to the sun: it is visible near the eastern horizon just before sunrise, or near the western horizon just after sunset. Therefore, in order for sunlight to shine on the side of Venus that is facing us, Venus must be farther away than the sun is. According to Ptolemy’s model, however, Venus is always closer to Earth than the sun is. Thus, Galileo’s observations of the phases of Venus contradicted the Ptolemaic model and agreed with the heliocentric model. The left panel of this video shows the orbits of Venus and Earth around the sun. (Their sizes are greatly exaggerated to show detail.) The right panel shows the phases of Venus as seen from Earth’s perspective. We see the fully illuminated phase whenever Earth and Venus are located across from each other, on opposite sides of the su
