Final answer:
Galileo Galilei used a telescope to observe all the phases of Venus, which supported the heliocentric Copernican theory and challenged the geocentric Ptolemaic model.
Step-by-step explanation:
The individual who used a telescope to observe all the phases of Venus, which supported the Copernican theory and disproved Ptolemy's model, was Galileo Galilei. Galileo's observations of Venus's phases were similar to those of the Moon, indicating that Venus must orbit the Sun, not the Earth. This was a pivotal moment in the history of astronomy as it helped to shift the consensus from the geocentric model proposed by Ptolemy to the heliocentric model supported by Copernicus. Although Johannes Kepler and Tycho Brahe significantly advanced astronomical understanding, it was Galileo's telescopic observations that provided clear evidence against the prevailing Ptolemaic system.