Phases of Venus

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The phases of Venus vary from a thin crescent to full phase in 584 days.

They can be seen without a telescope by those with exceptionally acute eye-sight, at the limit of human perception. The angular resolution of the naked eye is about 1 minute of arc. The apparent disk of Venus measures between 60.2 and 68 seconds of arc, according to the distance from Earth. Nevertheless it is possible for observers with an extreme acute eyesight to see a crescent Venus under ideal atmospheric circumstances.

There have been numerous reports stating such observations. The phases of Venus are alleged to have been seen in Mesopotamian times by priest-astronomers. Ishtar (Venus) is described in cuneiform text as 'having horns'. However, other Mesopotamian deities were depicted with horns, so the phrase could have been simply a symbol of divinity.

The first known observation of the full planetary phases of Venus were first observed by Galileo at the end of 1610 (though not published until 1613). Using a telescope, Galileo was able to observe Venus going through a full set of phases, something prohibited by the Ptolemaic system (which would never allow Venus to be fully lit from the perspective of the Earth). This observation essentially ruled out the Ptolemaic system, and was compatible only with the Copernican system and the Tychonic system.

[edit] References

Campbell, William Wallace (1916). "Is the Crescent Form of Venus Visible to the Naked Eye?". Publications of the Astronomical Society of the Pacific 28: 85-86.
Reinhardt, Carl (1929). "Notes and Queries: Phase of Venus seen with the Naked Eye". The Journal of the Royal Astronomical Society of Canada 23: 48-49.
Goines, David Lance (1992). "Observer’s Notebook: Naked Eye Crescent of Venus". Sky & Telescope 83.