What did Galileo observe about Venus that supported the heliocentric model?

Galileo observed the phases of Venus, a sequence of changing illuminated shapes that supported the heliocentric model. Through his telescope in 1610, he saw Venus appear as a crescent, half-lit disk, gibbous globe, and nearly full planet.

These phases made sense if Venus orbited the Sun inside Earth's orbit. As Venus moved around the Sun, observers on Earth saw different portions of its sunlit hemisphere, just as the Moon shows changing phases. The traditional Ptolemaic model could not naturally produce the complete set of Venusian phases because its arrangement kept Venus from moving to the far side of the Sun as viewed from Earth.

The observation was powerful evidence against the strict Ptolemaic system, but it did not uniquely prove Copernicus's exact arrangement. Tycho Brahe's geo-heliocentric model also allowed Venus to circle the Sun while the Sun circled Earth. Galileo's observation therefore strongly favored Sun-centered planetary motion, while later evidence helped establish full heliocentrism.

Galileo reported the discovery publicly in 1613 in Letters on Sunspots. Venus takes about 224.7 Earth days to orbit the Sun.