page_title: Phases of Venus
section_title: History
hierarchical_section_title: Phases of Venus / History
context_page_description: 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.
context_section_description: The official 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. (The Ptolemaic system would never allow Venus to be fully lit from the perspective of the Earth, as this would require it to be on the far side of the Sun, which is impossible if Venus's orbit in its entirety is between the Earth and the Sun, as the Ptolemaic system requires). This observation essentially ruled out the Ptolemaic system, and was compatible only with the Copernican system and the Tychonic system and other heliocentric 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.
  Curiously, Galileo's letter to Kepler was encrypted so that Kepler could not scoop Galileo before he had made more exhaustive observations: Galileo took 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. This technique of hiding encoded announcements in letters was not uncommon at the time.
caption: Diagram of the orbit of Venus in relationship to the Earth, observed by Galileo Galilei in 1610