Venus

Venus, the second planet from the Sun, is named after the Roman goddess Venus. A terrestrial planet, it is sometimes called Earth's "sister planet", as the two are very similar in size and bulk composition. Although all planets' orbits are elliptical, Venus's orbit is the closest to circular, with an eccentricity of less than 1%. Venus is closer to the Sun than the Earth, it always appears in roughly the same direction from Earth as the Sun (the greatest elongation is 47.8°), so on Earth it can usually only be seen a few hours before sunrise or a few hours after sunset. However, when at its brightest, Venus may be seen during the daytime, making it one of only two heavenly bodies that can be seen both day and night (the other being the Moon). It is sometimes referred to as the "Morning Star" or the "Evening Star", and when it is visible in dark skies it is by far the brightest star-like object in the sky.

The cycle between one maximum elongation and the next lasts 584 days. After these 584 days Venus is visible in a position 72 degrees away from the previous one. Since 5 * 584 = 2920, which is equivalent to 8 * 365 Venus returns to the same point in the sky every 8 years (minus two leap days). This was known as the Sothis cycle in ancient Egypt. Another association is with the Moon, because 2920 days equal almost exactly 99 lunations (29.5 * 99 = 2920.5).

Venus was known to ancient Babylonians around 1600 BC, and to the Mayan civilization (the Mayans developed a religious calendar based on Venus's motion) and must have been known long before in prehistoric times, given that it is the third brightest object in the sky after the Sun and Moon. The Maasai people in Africa named the planet Kileken, and have a myth about it called "The Orphan Boy." Venus was called Lucifer by St. Jerome, who is the fallen angel "cast out of heaven" in the Christian scripture.

Its symbol is the sign also used in biology for the female sex, a stylized representation of the goddess Venus's hand mirror: a circle with a small cross underneath (Unicode: ♀). The Venus symbol also represents feminity, and in ancient alchemy stood for copper. Alchemists constructed the symbol from a circle (representing spirit) above a cross (representing matter).

The association with sex and femininity is supposed to relate to the period of 266 days between the conjunction and maximum elongation of Venus, which corresponds more or less to the length of human pregnancy.

The adjective Venusian is commonly used for Venus, but it is etymologically incorrect. The true adjective coming from Latin, Venereal, is avoided because of its modern association with sexually transmitted diseases. Some astronomers use Cytherean, which comes from Cythera. Other less common adjectives include Venerean, Venerian, and Veneran. The Chinese, Korean, Japanese and Vietnamese cultures refer to the planet as the Metal Star, based on the Five Elements.

1 Physical characteristics
2 Observations and explorations of Venus
3 Appearance
4 Cultural references
5 See also
6 References

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Physical characteristics

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Atmosphere

Venus has an atmosphere consisting mainly of carbon dioxide and a small amount of nitrogen, with a pressure at the surface about 90 times that of Earth (a pressure equivalent to a depth of 1 kilometre under Earth's oceans). This enormously CO₂-rich atmosphere results in a strong greenhouse effect that raises the surface temperature more than 400 °C (750 °F) above what it would be otherwise, causing temperatures at the surface to reach extremes as great as 500 °C (930 °F) in low elevation regions near the planet's equator. This makes Venus's surface hotter than Mercury's, even though Venus is nearly twice as distant from the Sun and only receives 25% of the solar irradiance (2613.9 W/m² in the upper atmosphere, and just 1071.1 W/m² at the surface). Owing to the thermal inertia and convection of its dense atmosphere, the temperature does not vary significantly between the night and day sides of Venus despite its extremely slow rotation of less than one rotation per Venusian year, meaning that, at the equator, Venus' surface rotates at a mere 6.5 km/h (4 mph). Upper atmosphere winds circling the planet approximately every 4 days help distribute the heat to other areas on the surface.

The solar irradiance is so much lower at the surface of Venus because the planet's thick cloud cover reflects the majority of the sunlight back into space. This prevents most of the sunlight from ever heating the surface. Venus's bolometric albedo is approximately 60%, and its visual light albedo is even greater. Thus, despite being closer to the Sun than Earth, the surface of Venus is not as well heated and even less well lit by the Sun. In the absence of any greenhouse effect, the temperature at the surface of Venus would be quite similar to Earth. A common conceptual misunderstanding regarding Venus is the mistaken belief that its thick cloud cover traps heat, as the opposite is actually true. The cloud cover keeps the planet much cooler than it would be otherwise. The immense quantity of CO₂ in the atmosphere is what traps the heat by the greenhouse mechanism.

There are strong 300 km/h (200 mph) winds at the cloud tops, but winds at the surface are very slow, no more than a few miles per hour. However, owing to the high density of the atmosphere at Venus's surface, even such slow winds exert a significant amount of force against obstructions. The clouds are mainly composed of sulfur dioxide and sulfuric acid droplets and cover the planet completely, obscuring any surface details from the human eye. The temperature at the tops of these clouds is approximately −45 °C (−50 °F). The mean surface temperature of Venus, as given by NASA, is 464 °C (864 °F). The minimal value of the temperature, listed in the table, refers to cloud tops —the surface temperature is never below 400 °C (750 °F). (This makes the surface temperature hot enough to melt lead.)

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Surface features

Radar image of the surface of Venus, centered at 180 degrees east longitude

For more details on this topic, see Geology of Venus.

Venus has slow retrograde rotation, meaning it rotates from east to west, instead of west to east as most of the other major planets do. (Pluto and Uranus also have retrograde rotation, though Uranus's axis, tilted at 97.86 degrees, almost lies in its orbital plane.) It is not known why Venus is different in this manner, although it may be the result of a collision with a very large asteroid at some time in the distant past. If the Sun could be seen from Venus' surface, it would appear to rise and set in a 116.75 day cycle (Venus' synodic rotation period), and a Venusian year would thus last 1.92 Venusian "days".

In addition to this unusual retrograde rotation, the periods of Venus' rotation and of its orbit are synchronized in such a way that it always presents the same face toward Earth when the two planets are at their closest approach (5.001 Venusian days between each inferior conjunction). This may simply be a coincidence, but there is some speculation that this may be the result of tidal locking, with tidal forces affecting Venus' rotation whenever the planets get close enough together —although the tides raised by Earth on Venus are vanishingly small.

Venus has two major continent-like highlands on its surface, rising over vast plains. The northern highland is named Ishtar Terra and has Venus's highest mountains, named the Maxwell Montes (roughly 2 km taller than Mount Everest) after James Clerk Maxwell, which surround the plateau Lakshmi Planum. Ishtar Terra is about the size of Australia. In the southern hemisphere is the larger Aphrodite Terra, about the size of South America. Between these highlands are a number of broad depressions, including Atalanta Planitia, Guinevere Planitia, and Lavinia Planitia. With only the exception of Maxwell Montes, all surface features on Venus are named after real or mythological females. Venus' thick atmosphere causes meteors to decelerate as they fall toward the surface, and even large meteors will strike the surface at too low a speed to form an impact crater if they have less than a certain threshold kinetic energy. Because of this, no impact crater smaller than about 3 km (2 mi) in diameter can form.

Nearly 90% of Venus's surface appears to consist of recently (in the geological sense) solidified basaltic lava, with very few meteorite craters. The oldest features present on Venus seem to be only around 800 million years old, with most of the terrain being considerably younger (though still not less than several hundred million years for the most part). This suggests that Venus underwent a major resurfacing event in the not too distant geological past. The interior of Venus is probably similar to that of Earth: an iron core about 3000 km in radius, with a molten rocky mantle making up the majority of the planet. Recent results from the Magellan gravity data indicate that Venus's crust is stronger and thicker than had previously been assumed. It is theorized that Venus does not have mobile plate tectonics as Earth does, but instead undergoes massive volcanic upwellings at regular intervals that inundate its surface with fresh lava. Other recent findings suggest that Venus is still volcanically active in isolated geological hotspots.

Venus's intrinsic magnetic field has been found very weak compared to other planets in the solar system. This may be due to its slow rotation being insufficient to drive an internal dynamo of liquid iron. As a result, solar wind strikes Venus's upper atmosphere without mediation. It is thought that Venus originally had as much water as Earth, but that under the Sun's assault water vapor in the upper atmosphere was split into hydrogen and oxygen, with the hydrogen escaping into space owing to its low molecular mass; the ratio of hydrogen to deuterium (a heavier isotope of hydrogen which doesn't escape as quickly) in Venus's atmosphere seems to support this theory. Molecular oxygen is thought to have combined with atoms in the crust (large amounts of oxygen, however, remain in the atmosphere in the form of carbon dioxide). Because of their dryness, Venus's rocks are much harder than Earth's, which leads to steeper mountains, cliffs and other features.

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Venus' moon

Venus was once thought to possess a moon, named Neith after the chief goddess of Sais, Egypt (whose veil no mortal raised), first observed by Giovanni Domenico Cassini in 1672. German astronomers called the moon Kleinchen (literally "tiny"), and sporadic sightings by astronomers continued until 1892. These sightings have since been discredited, and are thought to have been mostly faint stars that happened to be in the right place at the right time, or maybe even asteroids passing by the planet. Venus is now known to be moonless.

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Observations and explorations of Venus

Main article: Observations and explorations of Venus

Venus has been observed and explored several times within the past 4000 years by a number of people, including the Greeks.

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Appearance

Greatest Eastern Elongation	Maximum brightness	Stationary, begin retrograde	Inferior Conjunction	Stationary, resume direct	Maximum brightness	Greatest Western Elongation	Superior Conjunction
March 29, 2004 46°	May 3, 2004	May 18, 2004	June 8, 2004	June 29, 2004	July 13, 2004	August 17, 2004 45.8°	March 31, 2005
November 3, 2005 47.1°	December 12, 2005	December 23, 2005	January 13, 2006	February 3, 2006	February 14, 2006	March 25, 2006 46.5°	October 27, 2006
June 9, 2007 45.4°	July 14, 2007	July 25, 2007	August 18, 2007	September 7, 2007	September 23, 2007	October 28, 2007 46.5°	June 9, 2008
January 14, 2009 47.1°	February 20, 2009	March 5, 2009	March 27, 2009	April 15, 2009	April 29, 2009	June 5, 2009 45.9°	January 11, 2010
August 20, 2010 46°	September 27, 2010	October 7, 2010	October 29, 2009	November 16, 2010	December 2, 2010	January 8, 2011 47°	August 16, 2011
March 27, 2012 46°	April 30, 2012	May 15, 2012	June 6, 2012	June 27, 2012	July 10, 2012	August 15, 2012 45.8°	March 28, 2013
November 1, 2013 47.1°	December 10, 2013	December 20, 2013	January 11, 2014	January 31, 2014	February 11, 2014	March 22, 2014 46.6°	October 25, 2014
June 6, 2015 45.4°	July 12, 2015	July 23, 2015	August 15, 2015	September 5, 2015	September 20, 2015	October 26, 2015 46.4°	June 6, 2016
January 12, 2017 47.1°	February 18, 2017	March 2, 2017	March 25, 2017	April 12, 2017	April 26, 2017	June 3, 2017 45.9°	January 9, 2018
August 17, 2018 45.9°	September 25, 2018	October 5, 2018	October 26, 2018	November 24, 2018	November 30, 2018	January 6, 2019 47°	August 14, 2019
March 24, 2020 46.1°	April 28, 2020	May 13, 2020	June 3, 2020	June 24, 2020	July 8, 2020	August 13, 2020 45.8°	March 26, 2021

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Cultural references

See also Venus in fiction

Until it was penetrated by probes, Venus's opaque cloud layer gave science fiction writers free rein in imagining the planet's surface, and they frequently imagined it to be Earthlike. There are some religious sects who believe that Hell may be located on Venus. Its extremely high surface temperature and impenetrable cloud cover cause people to believe that the fires of Hell burn on the surface, obscured from our earthly view. Conversely, other sects consider Venus to be some form of paradise or an advanced secret base for angels/aliens to operate from.

In Olaf Stapledon's epic Last and First Men (1930), Venus is an oceanic idyll where humans evolve the power of flight.
In the mythology of Middle-earth (1937), by J. R. R. Tolkien, Venus is the Star of Eärendil. The star was created when Eärendil the Mariner was set in the sky on his ship, with a Silmaril bound to his brow. In fact, Tolkien chose the name directly from the ancient Old English word for the planet Venus.
In H. P. Lovecraft's Cthulhu Mythos (1928–), there are mentions of the 'Lords of Venus', and conflicting indications that the Serpent People originated there.
The H. P. Lovecraft and Kenneth Sterling short story 'In the Walls of Eryx' (1939), takes place on Venus, but is not considered part of the Cthulhu Mythos.
The second book of the Space Trilogy (1938–1945) by C.S. Lewis, Perelandra 1943) takes place on Venus (called by the natives Perelandra), the site of a second garden of Eden.
In the military SF classic Clash by Night (1943) by Henry Kuttner (writing as Lawrence O'Donnell) and C. L. Moore, underwater city-states hire mercenary companies and their battleships to fight their wars on the surface.
Venus was the home planet of the Mekon, arch-enemy of the 1950s comic book hero Dan Dare.
Many science-fiction movies and serials of the '50s and '60s, such as Abbott and Costello Go to Mars and Space Patrol, have used Venus' namesake goddess and her domain to contrive planetary populations of nubile women welcoming (or attacking) all-male astronaut crews.
Venus is the location of several Starfleet Academy training facilities and terraforming stations in the fictional Star Trek universe (1966–).
In Jacqueline Susann's Yargo (1979), Venus is inhabited by bees that are as big as horses.
Venus is briefly mentioned in Arthur C. Clarke's 3001: The Final Odyssey (1997).
A presumably terraformed Venus was the setting of one episode of the anime Cowboy Bebop (1998). In the show, Venus was revealed to be an arid but habitable world. Much of the population lived in floating cities in the sky. In the cartoon Exosquad, terraformed Venus was portrayed as one of the three habitable planets in the solar system (the others being Earth and Mars).
A more scientifically accurate depiction of the planet is offered in Ben Bova's novel Venus (2000, ISBN 031287216X)-

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References

Arnett, Bill (2005). Venus. Retrieved March 27, 2005.
European Space Agency (2005). Venus Express overview. Retrieved March 27, 2005.
Grayzeck, Ed (2004). Venus Fact Sheet. NASA. Retrieved March 27, 2005.
Grieger, Bjoern (2004). Picture “Real Venus”. Retrieved March 27, 2005.
The Maya Astronomy Page (2002). Venus. Retrieved March 27, 2005.
Mitchell, Don P. (2004). The Soviet Exploration of Venus. Retrieved March 27, 2005.
Rosenthal, David. (2003). THE SOUTHERNMOST RISE OF VENUS AT UXMAL, 1997. Retrieved March 27, 2005.
Vienna University of Technology (2004). Venus Three-Dimensional Views. Retrieved March 27, 2005.
Mallama, A. (1996). Schroeter's Effect and the twilight model for Venus, Journal of the British Astronomical Association, 106(1): 16–18 [1]
3D VRML Venus globe