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== Ancient knowledge and naming ==

== Ancient knowledge and naming ==

Venus has been known to man for all of recorded history.<ref name=Arnett>Arnett, Bill. "[http://www.nineplanets.org/venus.html Entry for Venus]." ''The <s>Nine</s> 8 Planets'', July 17, 2006. Accessed May 20, 2008.</ref> The ancients originally misidentified the "Evening Star" and the "Morning Star" as two separate objects.<ref name=Arnett/><ref name=Hamilton/> Tradition credits [[Pythagoras]] with realizing that evening and morning star were the same object.

Venus has been known to man for all of recorded history.<ref name=Arnett>Arnett, Bill. "[http://www.nineplanets.org/venus.html Entry for Venus]." ''The <s>Nine</s> 8 Planets'', July 17, 2006. Accessed May 20, 2008.</ref> The ancients originally misidentified the "Evening Star" and the "Morning Star" as two separate objects.<ref name=Hamilton/><ref name=Arnett/> Tradition credits [[Pythagoras]] with realizing that evening and morning star were the same object.

Venus is the Roman name of the goddess of love and beauty. Venus is identified with [[Astarte]],<ref name=Hislop>Hislop, Alexander. ''[http://www.biblebelievers.com/babylon/00index.htm The Two Babylons, or The Papal Worship Proved to be the Worship of Nimrod and His Wife].'' 1853. Accessed May 20, 2008.</ref> or [[Ishtar]], chief goddess of [[Babylon]]. Venus was also known to the ancient civilizations of the Americas; the Aztecs called it Tlahuizcalpantecuhtli, or Quetzalcoatl<ref name=Tlahuizcalpantecuhtli>"[http://www.windows.ucar.edu/tour/link=/mythology/tlahuizcal_venus.html&edu=high Tlahuizcalpantecuhtli]." ''Windows to the Universe'', University Corporation for Atmospheric Research, University of Michigan, March 27, 1997. Accessed May 20, 2008.</ref>, and the Mayans Kukulcan.<ref name=Kukulcan>"[http://www.windows.ucar.edu/tour/link=/mythology/Kukulcan_venus.html&edu=high Kukulcan]." ''Windows to the Universe'', University Corporation for Atmospheric Research, University of Michigan, March 25, 1997. Accessed May 20, 2008.</ref> In addition, all the geographical features of Venus are named for goddesses from one body of [[mythology]] or another.

Venus is the Roman name of the goddess of love and beauty. Venus is identified with [[Astarte]],<ref name=Hislop>Hislop, Alexander. ''[http://www.biblebelievers.com/babylon/00index.htm The Two Babylons, or The Papal Worship Proved to be the Worship of Nimrod and His Wife].'' 1853. Accessed May 20, 2008.</ref> or [[Ishtar]], chief goddess of [[Babylon]]. Venus was also known to the ancient civilizations of the Americas; the Aztecs called it Tlahuizcalpantecuhtli, or Quetzalcoatl,<ref name=Tlahuizcalpantecuhtli>"[http://www.windows.ucar.edu/tour/link=/mythology/tlahuizcal_venus.html&edu=high Tlahuizcalpantecuhtli]." ''Windows to the Universe'', University Corporation for Atmospheric Research, University of Michigan, March 27, 1997. Accessed May 20, 2008.</ref> and the Mayans Kukulcan.<ref name=Kukulcan>"[http://www.windows.ucar.edu/tour/link=/mythology/Kukulcan_venus.html&edu=high Kukulcan]." ''Windows to the Universe'', University Corporation for Atmospheric Research, University of Michigan, March 25, 1997. Accessed May 20, 2008.</ref> In addition, all the geographical features of Venus are named for goddesses from one body of [[mythology]] or another.

Venus is important historically as a calendar reference, and is still of high standing in astrology. More to the point, Claudius [[Ptolemy]] considered that Venus orbited the earth, in an orbital sphere one level closer to Earth than the sun, but subject to a wide epicycle.

Venus is important historically as a calendar reference, and is still of high standing in astrology. More to the point, Claudius [[Ptolemy]] considered that Venus orbited the earth, in an orbital sphere one level closer to Earth than the sun, but subject to a wide epicycle.

== Orbital characteristics ==

== Orbital characteristics ==

Venus' orbit is more nearly circular than that of any other planet in the [[solar system]].<ref name=Arnett/> It maintains an average distance of 108,210,000 km from the sun. Its closeness to both the earth and the sun, and its very high geometric albedo (65%), make it the brightest object in the night sky except for the [[Moon]].

Venus' orbit is more nearly circular than that of any other planet in the [[solar system]].<ref name=Arnett/> It maintains an average distance of 108,210,000&nbsp;km from the sun. Its closeness to both the earth and the sun, and its very high geometric albedo (65%), make it the brightest object in the night sky except for the [[Moon]].

The sidereal period of Venus is 224.7 days, and the synodic period is 583.92 days. Remarkably, the Mayans calculated Venus' synodic period at 584 days, a figure accurate to within an incredible three significant digits.<ref name=Kukulcan/> Moreover, the ratio of the synodic year of Venus to the year of Earth is very nearly 13:8, close enough to suggest that Earth and Venus participate in an orbital resonance.

The sidereal period of Venus is 224.7 days, and the synodic period is 583.92 days. Remarkably, the Mayans calculated Venus' synodic period at 584 days, a figure accurate to within an incredible three significant digits.<ref name=Kukulcan/> Moreover, the ratio of the synodic year of Venus to the year of Earth is very nearly 13:8, close enough to suggest that Earth and Venus participate in an orbital resonance.

== Physical characteristics ==

== Physical characteristics ==

Venus's mass is 4.8685 * 10²⁴ kg, or about 81.5% that of earth.<ref name=venusfact/> Its radius is 6051.8 km. Perhaps on account of Venus' very long sidereal day, Venus is an almost perfect sphere.

Venus's mass is 4.8685 * 10²⁴ kg, or about 81.5% that of earth.<ref name=venusfact/> Its radius is 6051.8&nbsp;km. Perhaps on account of Venus' very long sidereal day, Venus is an almost perfect sphere.

=== Atmosphere ===

=== Atmosphere ===

=== Magnetosphere ===

=== Magnetosphere ===

Venus's magnetosphere is far too weak to protect it from the radiation of the solar wind. Its magnetic dipole moment cannot be more than 8 * 10¹⁷ N-m/T, or about 10^-5 times that of the earth. Given its mass, it should have had a magnetic dipole moment at creation of 1.15 * 10²⁴ N-m/T. Thus its decay time is very short (433 Julian years) and its half-life even shorter (300 Julian years). The conventional explanation is that the very slow rotation of Venus precludes any dynamo effect. [[Russell Humphreys]], however, suggests that Venus simply has a small and relatively non-conductive core and was thus less able to maintain its magnetic field since creation as well as Earth has.

Venus's magnetosphere is far too weak to protect it from the radiation of the solar wind. Its magnetic dipole moment cannot be more than 8 * 10¹⁷ N-m/T, or about 10⁻⁵ times that of the earth. Given its mass, it should have had a magnetic dipole moment at creation of 1.15 * 10²⁴ N-m/T. Thus its decay time is very short (433 Julian years) and its half-life even shorter (300 Julian years). The conventional explanation is that the very slow rotation of Venus precludes any dynamo effect. [[Russell Humphreys]], however, suggests that Venus simply has a small and relatively non-conductive core and was thus less able to maintain its magnetic field since creation as well as Earth has.

== Satellites ==

== Satellites ==