According to tradition, it was the Greek mathematician and philosopher Pythagoras of Samos (570-500 BC), who was the first to realize that the evening and morning star is one and the same celestial body.
According to unclear historical sources, Heraclides Pontikos (390 to 322 BC), a pupil of Plato, concluded that Venus orbits the sun.
As an inner planet, Venus shows phases quite similar to those of the moon when observed with the telescope: it does not always appear full, but often only partially illuminated.
If it moves towards the earth as the evening star, its sickle shape becomes ever narrower.
As Morgenstern, however, after the overtaking process, the Venussichel slowly increases again. When Galileo Galileo made his first astronomical observations with the just invented telescope, he also recognized the phase shape of Venus.
The researcher saw this as confirmation that the planet revolves around the sun and not around the earth.
However, the sickle shape alone is not proof of the heliocentric world view.
For this purpose, the observation of the temporal course of the Venus phases is necessary.