The phases of

Venus Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never fa ...

are the variations of lighting seen on the planet's surface, similar to

lunar phase Concerning the lunar month of ~29.53 days as viewed from Earth, the lunar phase or Moon phase is the shape of the Moon's directly sunlit portion, which can be expressed quantitatively using areas or angles, or described qualitatively using the t ...

s. The first recorded observations of them are thought to have been telescopic observations by

Galileo Galilei Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He was ...

in 1610. Although the extreme crescent phase of Venus has since been observed with the

naked eye Naked eye, also called bare eye or unaided eye, is the practice of engaging in visual perception unaided by a magnifying, light-collecting optical instrument, such as a telescope or microscope, or eye protection. Vision corrected to normal ...

, there are no indisputable historical pre-telescopic records of it being described or known.

Observation

The

orbit In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a p ...

of Venus is 224.7 Earth days (7.4 avg. Earth months 0.4 days. The phases of Venus result from the planet's orbit around the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

inside the

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

's orbit giving the telescopic observer a sequence of progressive lighting similar in appearance to the Moon's phases. It presents a ''full'' image when it is on the opposite side of the Sun. It is a ''gibbous'' phase when it approaches or leaves the opposite side of the Sun. It shows a ''quarter phase'' when it is at its maximum elongation from the Sun. Venus presents a ''thin crescent'' in telescopic views as it comes around to the near side between the Earth and the Sun and presents its ''new phase'' when it is between the Earth and the Sun. Since the planet has an atmosphere it can be seen at ''new'' in a telescope by the halo of light refracted around the planet. The full cycle from ''new'' to ''full'' to ''new'' 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 9.9 arc seconds at ''full'' (

superior conjunction In astronomy, a conjunction occurs when two astronomical objects or spacecraft have either the same right ascension or the same ecliptic longitude, usually as observed from Earth. When two objects always appear close to the ecliptic—such as t ...

) up to a maximum of 68 arc seconds at ''new'' (

inferior conjunction In astronomy, a conjunction occurs when two astronomical objects or spacecraft have either the same right ascension or the same ecliptic longitude, usually as observed from Earth. When two objects always appear close to the ecliptic—such as t ...

). Venus reaches its greatest

magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematics), a term for the size or length of a vector *Order of ...

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). Contrary to other planets its apparent magnitude around

does not decrease consistently but rather spikes before dimming further. This is caused by

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular formu ...

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 Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He was ...

at the end of 1610 (though not published until 1613 in the ''

Letters on Sunspots '' Letters on Sunspots '' (''Istoria e Dimostrazioni intorno alle Macchie Solari'') was a pamphlet written by Galileo Galilei in 1612 and published in Rome by the Accademia dei Lincei in 1613. In it, Galileo outlined his recent observation of dar ...

''). Using a

telescope A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observe ...

, Galileo was able to observe Venus going through a full set of phases, something prohibited by the

Ptolemaic system In astronomy, the geocentric model (also known as geocentrism, often exemplified specifically by the Ptolemaic system) is a superseded description of the Universe with Earth at the center. Under most geocentric models, the Sun, Moon, stars, an ...

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 The Tychonic system (or Tychonian system) is a model of the Universe published by Tycho Brahe in the late 16th century, which combines what he saw as the mathematical benefits of the Copernican system with the philosophical and "physical" bene ...

and other models such as the Capellan and

Riccioli Giovanni Battista Riccioli, SJ (17 April 1598 – 25 June 1671) was an Italian astronomer and a Catholic priest in the Jesuit order. He is known, among other things, for his experiments with pendulums and with falling bodies, for his discussion ...

'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 Benedetto Castelli (1578 – 9 April 1643), born Antonio Castelli, was an Italian mathematician. Benedetto was his name in religion on entering the Benedictine Order in 1595. Life Born in Brescia, Castelli studied at the University of Padua and ...

, asking if the phases of Venus were observable through Galileo's new telescope. Days later, Galileo wrote in a letter to

Johannes Kepler Johannes Kepler (; ; 27 December 1571 – 15 November 1630) was a German astronomer, mathematician, astrologer, natural philosopher and writer on music. He is a key figure in the 17th-century Scientific Revolution, best known for his laws ...

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 In cryptography, encryption is the process of encoding information. This process converts the original representation of the information, known as plaintext, into an alternative form known as ciphertext. Ideally, only authorized parties can deci ...

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.Galileo Galilei's Anagram
/ref> This technique of hiding encoded announcements in letters was not uncommon at the time.

Naked eye observations

The extreme crescent phase of Venus can be seen without a

by those with exceptionally acute eyesight, at the limit of human perception. The angular resolution of the naked eye is about 1 minute of arc. The apparent disk of Venus' extreme crescent measures between 60.2 and 66 seconds of arc, depending on the distance from Earth.

Mesopotamia Mesopotamia ''Mesopotamíā''; ar, بِلَاد ٱلرَّافِدَيْن or ; syc, ܐܪܡ ܢܗܪ̈ܝܢ, or , ) is a historical region of Western Asia situated within the Tigris–Euphrates river system, in the northern part of the F ...

n priest-astronomers described

Ishtar Inanna, also sux, 𒀭𒊩𒌆𒀭𒈾, nin-an-na, label=none is an ancient Mesopotamian goddess of love, war, and fertility. She is also associated with beauty, sex, divine justice, and political power. She was originally worshiped in S ...

(Venus) in

cuneiform Cuneiform is a logo-syllabic script that was used to write several languages of the Ancient Middle East. The script was in active use from the early Bronze Age until the beginning of the Common Era. It is named for the characteristic wedge-sha ...

text as ''having 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.

Notes

References

* * *

External links

Observations and Theories of Planetary Motion
*

Owen Gingerich Owen Jay Gingerich (; born 1930) is professor emeritus of astronomy and of the history of science at Harvard University and a senior astronomer emeritus at the Smithsonian Astrophysical Observatory. In addition to his research and teaching, he has ...