As it moves around its orbit, Venus displays phases like those of the Moon: it is new when it passes between the Earth and the Sun, small and full when it is on the opposite side of the Sun, and a half-phase when it is at its maximum elongations from the Sun. Venus is brightest when it is a large but thin crescent and much closer to the Earth. In 1610 Galileo saw the phases of Venus through his telescope.

The heliocentric model of the solar system developed by Copernicus predicted that all phases would be visible since the orbit of Venus around the Sun would cause its illuminated hemisphere to face the Earth when it was on the opposite side of the Sun and to face away from the Earth when it was on the Earth-side of the Sun. In contrast, the geocentric model of Ptolemy predicted that only crescent and new phases would be seen, since Venus was thought to remain between the Sun and Earth during its orbit around the Earth. Galileo's observations of the phases of Venus proved that it orbited the Sun and lent support to (but did not prove) the heliocentric model.

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