Who first recorded telescopic observations of the phases of Venus in 1610?

Galileo Galilei first recorded telescopic observations of the phases of Venus in 1610.

Using an improved refracting telescope, Galileo observed Venus changing through crescent, quarter, gibbous, and nearly full phases. The observations were made late in 1610, after his earlier telescopic studies of the Moon and Jupiter’s satellites. He later published the discovery in 1613 in Letters on Sunspots.

The phases were historically important because they challenged the Ptolemaic Earth-centered model. In that system, Venus could not pass behind the Sun as seen from Earth, so it could not display a full set of phases. The observations supported the Copernican arrangement, although they were also compatible with Tycho Brahe’s geo-heliocentric model.

A minor historical wrinkle is that Galileo’s claim to priority has been discussed because Benedetto Castelli wrote to him about Venus in December 1610. Still, Galileo is the canonical answer for the first recorded telescopic observations.