Venus phases explained
As for all other planets, including our Moon, Venus shines by reflecting the light of the Sun. In fact, since its orbit is inside that of the Earth, Venus displays phases much like those of the Moon.
Galileo was the first astronomer to notice this phenomenon, using a telescope, which allowed him to confirm that the planets are bodies like the Earth. The phases of Venus could not be explained by Ptolemy’s system centred on the Earth. The major difference with the Moon is that, due to the significant changes in distance between Venus and the Earth, its size seems to change considerably, unlike the Moon.
Galileo endangers Ptolemy’s system centred on the Earth
In 1610, Galileo published his Sidereus Nuncius, a compilation of his observations of the Moon and Jupiter carried out between 1609 and 1610. During the autumn of 1610, Galileo turned his telescope towards Venus, which at that time was just visible having been hidden by the Sun.
The results of his observations began to endanger the cosmological system established by Ptolemy (geocentric system), and confirmed the system proposed by Copernicus. Galileo’s announcement of his discovery of the phases of Venus was hidden in the form of an anagram in one of his letters to the Tuscan ambassador to Prague:
Haec immatura a me iam frustra leguntur o y which should be read as: Cynthiae figuras aemulatur mater amorum, or : "The mother of love [Venus] imitates the figures of Cynthia [the Moon]."
When is Venus brightest?
Another difference between the phases of Venus and those of the moon is that, seen from the Earth, it is not normally possible to see Venus as ‘full’ because it is than on the other side of the Sun (at superior conjunction and very close to the Sun in the sky).
Venus is brightest when we see it as a crescent. This is the case when Venus is in the evening sky and is at its maximum distance (called greatest elongation) from the Sun. When we see Venus in the morning sky, it is brightest just before the greatest elongation.