* Galileo Galilei is credited with first publishing the observations of the phases of Venus in 1610. He used his newly improved telescope to observe the planet and saw it changing shape from a thin crescent to a full disc. This supported his theory of heliocentricity, where the Earth revolves around the Sun.
* However, there is evidence that Thomas Harriot (an English mathematician and astronomer) may have observed the phases of Venus earlier than Galileo, in 1610 or even late 1609. But he did not publish his findings.
* There are also claims that the ancient Greeks had already observed the phases of Venus. The philosopher Pythagoras is said to have written about the phases of Venus, though the exact details are lost.
So, while Galileo is generally recognized for the discovery of the phases of Venus, the timeline and the exact first observer are still debated.
