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The phases of Venus

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.

gallieo Galileo (portait by Giusto Sustermans) 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 Ptolomy’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.

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 Ptolomy, 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]."

Observations of the evolution of the phases of Venus in the evenings February to May 2004. Galileo observed the same phenomenon between September 1610 and February1611, suggesting that it did not turn around the Earth, but around the Sun

Observations of the evolution of the phases of Venus in the evenings February to May 2004. Galileo observed the same phenomenon between September 1610 and February1611, suggesting that it did not turn around the Earth, but around the Sun
(credit: TBGS Observatory, photo by Chris Proctor)

Explanation of the phases of Venus

Explanation of the phases of Venus
(credit: ESO)

It is impossible to explain the phases of Venus with Ptolomy’s geocentric system

It is impossible to explain the phases of Venus with Ptolomy’s geocentric system
(credit: www.astrosurf.com)


Click to enlarge the pictures...

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.

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