Thephases ofVenus are the variations of lighting seen on the planet's surface, similar tolunar phases. The first recorded observations of them are thought to have been telescopic observations byGalileo Galilei in 1610. Although the extreme crescent phase of Venus has since been observed with thenaked eye, there are no indisputable historical pre-telescopic records of it being described or known.[1]
Theorbit of Venus is 224.7 Earth days (7.4 avg. Earthmonths [30.4 days]). The phases of Venus result from the planet's orbit around theSun inside theEarth's orbit giving thetelescopic observer a sequence of progressive lighting similar in appearance to theMoon's phases. It presents afull image when it is on the opposite side of the Sun. It is agibbous phase when it approaches or leaves the opposite side of the Sun. It shows aquarter phase when it is at its maximumelongation from the Sun. Venus presents athin crescent in telescopic views as it comes around to the near side between the Earth and the Sun and presents itsnew phase when it is between the Earth and the Sun. Since the planet has an atmosphere it can be seen atnew in a telescope by the halo of light refracted around the planet. The full cycle fromnew tofull tonew again takes 584 days (the time it takes Venus to overtake the Earth in its orbit). Venus (like the Moon) has 4 primary phases of 146 days each.
The planet also changes in apparent size from 9.9arc seconds atfull (superior conjunction) up to a maximum of 68 arc seconds atnew (inferior conjunction).[1] Venus reaches its greatestmagnitude of about −4.5 when it is an intermediate crescent shape at the point in its orbit, when it is 68 million km away from the Earth, at which point the illuminated part of its disk reaches its greatest angular area as seen from the Earth (a combination of its closeness and the fact that it is 28% illuminated).[2]
Contrary to other planets its apparent magnitude aroundinferior conjunction does not decrease consistently but rather spikes before dimming further. This is caused bysulfuric acid droplets in Venus' atmosphere reflecting more light at a certain angle and thus phase, an effect similar to aglory on Earth.[3]
The first observations of the full planetary phases of Venus were byGalileo at the end of 1610 (though not published until 1613 in theLetters on Sunspots). Using atelescope, Galileo was able to observe Venus going through a full set of phases, something prohibited by thePtolemaic 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[4] essentially ruled out the Ptolemaic system, and was compatible only with theCopernican system and theTychonic system and other models such as the Capellan andRiccioli'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 scientistBenedetto Castelli, asking if the phases of Venus were observable through Galileo's new telescope.[5] Days later, Galileo wrote in a letter toJohannes 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.[6]
Galileo's letter to Kepler wasencrypted 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:
And scrambled the letters into a strange anagram:
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.[7][6] This technique of hiding encoded announcements in letters was not uncommon at the time.
The extreme crescent phase of Venus can be seen without atelescope by those with exceptionally acute eyesight, at the limit of human perception. Theangular resolution of the naked eye is about 1 minute of arc (60 seconds). The apparent disk of Venus' extreme crescent measures between 60.2 and 66seconds of arc,[8] depending on the distance from Earth.
Mesopotamian priest-astronomers describedIshtar (Venus) incuneiform text ashaving horns which has been interpreted as indicating observation of a crescent. However, other Mesopotamian deities were depicted with horns, so the phrase could have been simply a symbol of divinity.[1]
