thumb|right|200px|The phases of Venus and evolution of its [[apparent diameter]]

The phases of Venus are the variations of lighting seen on the planet's surface, similar to lunar phases. The first recorded observations of them are thought to have been telescopic observations by Galileo Galilei in 1610. Although the extreme crescent phase of Venus has since been observed with the naked eye, there are no indisputable historical pre-telescopic records of it being described or known.

Contrary to other planets its apparent magnitude around inferior conjunction does not decrease consistently but rather spikes before dimming further. This is caused by sulfuric acid droplets in Venus' atmosphere reflecting more light at a certain angle and thus phase, an effect similar to a glory on Earth.

History

The first observations of the full planetary phases of Venus were by Galileo at the end of 1610 (though not published until 1613 in the Letters on Sunspots). Using a telescope, Galileo was able to observe Venus going through a full set of phases, something prohibited by the Ptolemaic system that assumed Venus to be a perfect celestial body. In the Ptolemaic system, the Sun and Venus circle the earth, with Venus orbiting around a point on the Earth-Sun axis, so that Venus is never on the far side of the sun. One could never expect an alignment Sun-Earth-Venus or Venus-Sun-Earth to occur, so that a full Venus could never be observed. Galileo's observations of the phases of Venus essentially ruled out the Ptolemaic system, and was compatible only with the Copernican system and the Tychonic system and other models such as the Capellan and Riccioli's extended Capellan model.

There is some controversy about Galileo's claim to first observing the phases of Venus: In December of 1610, Galileo received a letter from fellow scientist Benedetto Castelli, asking if the phases of Venus were observable through Galileo's new telescope. Days later, Galileo wrote in a letter to Johannes Kepler saying that he had observed Venus going through phases, but took complete credit for himself. It is unclear, lacking copies of any earlier correspondence, whether Castelli was telling Galileo of it for the first time, or responding to Galileo having previously informed him of it.

Galileo's letter to Kepler was encrypted so that Kepler could not pre-empt Galileo before he had made more exhaustive observations. Galileo wrote a sentence stating that Venus went through phases:

:Cynthiae figuras aemulatur mater amorum (The mother of love imitates the shape of Cynthia)

And scrambled the letters into a strange anagram:

:Haec immatura a me iam frustra leguntur o.y. (These are now too young to be read by me)

Cynthia was a popular epithet for the Moon, the mother of love of course being Venus. He sent the anagram to Kepler, then a few months later sent the decoded version. This way he had proof of having made the observation, without Kepler being able to publish it earlier.

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References

  • Observations and Theories of Planetary Motion
  • The crescent Venus seen with the naked eye
  • Owen Gingerich - Empirical proof and/or persuasion &mdash; lecture on Galileo's observation of the phases of Venus from a renowned historian of science
  • YouTube animation of the phases of Venus predicted by the pure geocentric Ptolemaic model [https://www.youtube.com/watch?v=OlnlbZHP-jk]
  • YouTube animation of the phases of Venus predicted by the heliocentric model (and implicitly also by the geo-heliocentric models such as the Tychonic) [https://www.youtube.com/watch?v=0Z_v7Tag4fg]