Transits
The Venusian orbit is slightly inclined relative to Earth's orbit; thus, when the planet passes between Earth and the Sun, it usually does not cross the face of the Sun. Transits of Venus occur when the planet's inferior conjunction coincides with its presence in the plane of Earth's orbit. Transits of Venus occur in cycles of 243 years with the current pattern of transits being pairs of transits separated by eight years, at intervals of about 105.5 years or 121.5 years—a pattern first discovered in 1639 by the English astronomer Jeremiah Horrocks.
The latest pair was June 8, 2004 and June 5–6, 2012. The transit could be watched live from many online outlets or observed locally with the right equipment and conditions.
The preceding pair of transits occurred in December 1874 and December 1882; the following pair will occur in December 2117 and December 2125. The 1874 transit is the subject of the oldest film known, the 1874 Passage de Venus. Historically, transits of Venus were important, because they allowed astronomers to determine the size of the astronomical unit, and hence the size of the Solar System as shown by Horrocks in 1639. Captain Cook's exploration of the east coast of Australia came after he had sailed to Tahiti in 1768 to observe a transit of Venus.
Pentagram of Venus
The pentagram of Venus is the path that Venus makes as observed from Earth. Successive inferior conjunctions of Venus repeat very near a 13:8 ratio (Earth orbits eight times for every 13 orbits of Venus), shifting 144° upon sequential inferior conjunctions. The 13:8 ratio is approximate. 8/13 is approximately 0.61538 while Venus orbits the Sun in 0.61519 years. The pentagram of Venus is sometimes also referred to as the petals of Venus due to the path's visual similarity to a flower.
Daylight apparitions
Naked-eye observations of Venus during daylight hours exist in several anecdotes and records. Astronomer Edmund Halley calculated its maximum naked eye brightness in 1716, when many Londoners were alarmed by its appearance in the daytime. French emperor Napoleon Bonaparte once witnessed a daytime apparition of the planet while at a reception in Luxembourg. Another historical daytime observation of the planet took place during the inauguration of the American president Abraham Lincoln in Washington, D.C., on 4 March 1865. Although naked eye visibility of Venus's phases is disputed, records exist of observations of its crescent.
Ashen light
A long-standing mystery of Venus observations is the so-called ashen light—an apparent weak illumination of its dark side, seen when the planet is in the crescent phase. The first claimed observation of ashen light was made in 1643, but the existence of the illumination has never been reliably confirmed. Observers have speculated it may result from electrical activity in the Venusian atmosphere, but it could be illusory, resulting from the physiological effect of observing a bright, crescent-shaped object. The ashen light has often been sighted when Venus is in the evening sky, when the evening terminator of the planet is towards to Earth.
Observation and exploration
Early observation
Because the movements of Venus appear to be discontinuous (it disappears due to its proximity to the sun, for many days at a time, and then reappears on the other horizon), some cultures did not recognize Venus as a single entity; instead, they assumed it to be two separate stars on each horizon: the morning and evening star. Nonetheless, a cylinder seal from the Jemdet Nasr period and the Venus tablet of Ammisaduqa from the First Babylonian dynasty indicate that the ancient Sumerians already knew that the morning and evening stars were the same celestial object. In the Old Babylonian period, the planet Venus was known as Ninsi'anna, and later as Dilbat. The name "Ninsi'anna" translates to "divine lady, illumination of heaven", which refers to Venus as the brightest visible "star". Earlier spellings of the name were written with the cuneiform sign si4 (= SU, meaning "to be red"), and the original meaning may have been "divine lady of the redness of heaven", in reference to the colour of the morning and evening sky.
The Chinese historically referred to the morning Venus as "the Great White" (Tàibái 太白) or "the Opener (Starter) of Brightness" (Qǐmíng 啟明), and the evening Venus as "the Excellent West One" (Chánggēng 長庚).
The ancient Greeks also initially believed Venus to be two separate stars: Phosphorus, the morning star, and Hesperus, the evening star. Pliny the Elder credited the realization that they were a single object to Pythagoras in the sixth century BC, while Diogenes Laërtius argued that Parmenides was probably responsible for this discovery. Though they recognized Venus as a single object, the ancient Romans continued to designate the morning aspect of Venus as Lucifer, literally "Light-Bringer", and the evening aspect as Vesper, both of which are literal translations of their traditional Greek names.
In the second century, in his astronomical treatise Almagest, Ptolemy theorized that both Mercury and Venus are located between the Sun and the Earth. The 11th-century Persian astronomer Avicenna claimed to have observed the transit of Venus, which later astronomers took as confirmation of Ptolemy's theory. In the 12th century, the Andalusian astronomer Ibn Bajjah observed "two planets as black spots on the face of the Sun"; these were thought to be the transits of Venus and Mercury by 13th-century Maragha astronomer Qotb al-Din Shirazi, though this cannot be true as there were no Venus transits in Ibn Bajjah's lifetime.
When the Italian physicist Galileo Galilei first observed the planet in the early 17th century, he found it showed phases like the Moon, varying from crescent to gibbous to full and vice versa. When Venus is furthest from the Sun in the sky, it shows a half-lit phase, and when it is closest to the Sun in the sky, it shows as a crescent or full phase. This could be possible only if Venus orbited the Sun, and this was among the first observations to clearly contradict the Ptolemaic geocentric model that the Solar System was concentric and centered on Earth.
The 1639 transit of Venus was accurately predicted by Jeremiah Horrocks and observed by him and his friend, William Crabtree, at each of their respective homes, on 4 December 1639 (24 November under the Julian calendar in use at that time).
The atmosphere of Venus was discovered in 1761 by Russian polymath Mikhail Lomonosov. Venus's atmosphere was observed in 1790 by German astronomer Johann Schröter. Schröter found when the planet was a thin crescent, the cusps extended through more than 180°. He correctly surmised this was due to scattering of sunlight in a dense atmosphere. Later, American astronomer Chester Smith Lyman observed a complete ring around the dark side of the planet when it was at inferior conjunction, providing further evidence for an atmosphere. The atmosphere complicated efforts to determine a rotation period for the planet, and observers such as Italian-born astronomer Giovanni Cassini and Schröter incorrectly estimated periods of about 24 h from the motions of markings on the planet's apparent surface.
Ground-based research
Little more was discovered about Venus until the 20th century. Its almost featureless disc gave no hint what its surface might be like, and it was only with the development of spectroscopic, radar and ultraviolet observations that more of its secrets were revealed. The first ultraviolet observations were carried out in the 1920s, when Frank E. Ross found that ultraviolet photographs revealed considerable detail that was absent in visible and infrared radiation. He suggested this was due to a dense, yellow lower atmosphere with high cirrus clouds above it.
Spectroscopic observations in the 1900s gave the first clues about the Venusian rotation. Vesto Slipher tried to measure the Doppler shift of light from Venus, but found he could not detect any rotation. He surmised the planet must have a much longer rotation period than had previously been thought. Later work in the 1950s showed the rotation was retrograde. Radar observations of Venus were first carried out in the 1960s, and provided the first measurements of the rotation period, which were close to the modern value.
Radar observations in the 1970s revealed details of the Venusian surface for the first time. Pulses of radio waves were beamed at the planet using the 300 m (1,000 ft) radio telescope at Arecibo Observatory, and the echoes revealed two highly reflective regions, designated the Alpha and Beta regions. The observations also revealed a bright region attributed to mountains, which was called Maxwell Montes. These three features are now the only ones on Venus that do not have female names.
Exploration
The first robotic space probe mission to Venus and any planet was Venera 1 of the Soviet Venera program launched in 1961, though it lost contact en route.
The first successful mission to Venus (as well as the world's first successful interplanetary mission) was the Mariner 2 mission by the United States, passing on 14 December 1962 at 34,833 km (21,644 mi) above the surface of Venus and gathering data on the planet's atmosphere.
On 18 October 1967, the Soviet Venera 4 successfully entered as the first to probe the atmosphere and deployed science experiments. Venera 4 showed the surface temperature was hotter than Mariner 2 had calculated, at almost 500 °C (932 °F), determined that the atmosphere was 95% carbon dioxide (CO
2), and discovered that Venus's atmosphere was considerably denser than Venera 4's designers had anticipated. The joint Venera 4–Mariner 5 data were analyzed by a combined Soviet–American science team in a series of colloquia over the following year, in an early example of space cooperation.
On 15 December 1970, Venera 7 became the first spacecraft to soft land on another planet and the first to transmit data from there back to Earth.
In 1974, Mariner 10 swung by Venus to bend its path toward Mercury and took ultraviolet photographs of the clouds, revealing the extraordinarily high wind speeds in the Venusian atmosphere. This was the first interplanetary gravity assist ever used, a technique which would be used by later probes, most notably Voyager 1 and 2.
In 1975, the Soviet Venera 9 and 10 landers transmitted the first images from the surface of Venus, which were in black and white. In 1982 the first colour images of the surface were obtained with the Soviet Venera 13 and 14 landers.
NASA obtained additional data in 1978 with the Pioneer Venus project that consisted of two separate missions: Pioneer Venus Orbiter and Pioneer Venus Multi-probe. The successful Soviet Venera program came to a close in October 1983, when Venera 15 and 16 were placed in orbit to conduct detailed mapping of 25% of Venus's terrain (from the north pole to 30°N latitude)
Several other missions explored Venus in the 1980s and 1990s, including Vega 1 (1985), Vega 2 (1985), Galileo (1990), Magellan (1994), Cassini–Huygens (1998), and MESSENGER (2006). All except Magellan were gravity assists. Then, Venus Express by the European Space Agency (ESA) entered orbit around Venus in April 2006. Equipped with seven scientific instruments, Venus Express provided unprecedented long-term observation of Venus's atmosphere. ESA concluded the Venus Express mission in December 2014.
As of 2020, Japan's Akatsuki is in a highly eccentric orbit around Venus since 7 December 2015, and there are several probing proposals under study by Roscosmos, NASA, ISRO, ESA, and the private sector (e.g. by Rocketlab).
In culture
Venus is a primary feature of the night sky, and so has been of remarkable importance in mythology, astrology and fiction throughout history and in different cultures.
In Sumerian religion, Inanna was associated with the planet Venus. Several hymns praise Inanna in her role as the goddess of the planet Venus. Theology professor Jeffrey Cooley has argued that, in many myths, Inanna's movements may correspond with the movements of the planet Venus in the sky. The discontinuous movements of Venus relate to both mythology as well as Inanna's dual nature. In Inanna's Descent to the Underworld, unlike any other deity, Inanna is able to descend into the netherworld and return to the heavens. The planet Venus appears to make a similar descent, setting in the West and then rising again in the East. An introductory hymn describes Inanna leaving the heavens and heading for Kur, what could be presumed to be, the mountains, replicating the rising and setting of Inanna to the West. In Inanna and Shukaletuda and Inanna's Descent into the Underworld appear to parallel the motion of the planet Venus. In Inanna and Shukaletuda, Shukaletuda is described as scanning the heavens in search of Inanna, possibly searching the eastern and western horizons. In the same myth, while searching for her attacker, Inanna herself makes several movements that correspond with the movements of Venus in the sky.
Classical poets such as Homer, Sappho, Ovid and Virgil spoke of the star and its light. Poets such as William Blake, Robert Frost, Letitia Elizabeth Landon, Alfred Lord Tennyson and William Wordsworth wrote odes to it.
In Chinese the planet is called Jīn-xīng (金星), the golden planet of the metal element. In India Shukra Graha ("the planet Shukra") is named after the powerful saint Shukra. Shukra which is used in Indian Vedic astrology means "clear, pure" or "brightness, clearness" in Sanskrit. One of the nine Navagraha, it is held to affect wealth, pleasure and reproduction; it was the son of Bhrgu, preceptor of the Daityas, and guru of the Asuras. The word Shukra is also associated with semen, or generation. Venus is known as Kejora in Indonesian and Malaysian Malay. Modern Chinese, Japanese and Korean cultures refer to the planet literally as the "metal star" (金星), based on the Five elements.
The Maya considered Venus to be the most important celestial body after the Sun and Moon. They called it Chac ek, or Noh Ek', "the Great Star". The cycles of Venus were important to their calendar and were described in some of their books such as Maya Codex of Mexico and Dresden Codex.
The Ancient Egyptians and Greeks believed Venus to be two separate bodies, a morning star and an evening star. The Egyptians knew the morning star as Tioumoutiri and the evening star as Ouaiti. The Greeks used the names Phōsphoros (Φωσϕόρος), meaning "light-bringer" (whence the element phosphorus; alternately Ēōsphoros (Ἠωσϕόρος), meaning "dawn-bringer"), for the morning star, and Hesperos (Ἕσπερος), meaning "Western one", for the evening star. Though by the Roman era they were recognized as one celestial object, known as "the star of Venus", the traditional two Greek names continued to be used, though usually translated to Latin as Lūcifer and Vesper.
Modern fiction
With the invention of the telescope, the idea that Venus was a physical world and possible destination began to take form.
The impenetrable Venusian cloud cover gave science fiction writers free rein to speculate on conditions at its surface; all the more so when early observations showed that not only was it similar in size to Earth, it possessed a substantial atmosphere. Closer to the Sun than Earth, the planet was frequently depicted as warmer, but still habitable by humans. The genre reached its peak between the 1930s and 1950s, at a time when science had revealed some aspects of Venus, but not yet the harsh reality of its surface conditions. Findings from the first missions to Venus showed the reality to be quite different and brought this particular genre to an end. As scientific knowledge of Venus advanced, science fiction authors tried to keep pace, particularly by conjecturing human attempts to terraform Venus.\
Symbol
The astronomical symbol for Venus is the same as that used in biology for the female sex: a circle with a small cross beneath. The Venus symbol also represents femininity, and in Western alchemy stood for the metal copper. Polished copper has been used for mirrors from antiquity, and the symbol for Venus has sometimes been understood to stand for the mirror of the goddess although that is unlikely to be its true origin. In the Greek Oxyrhynchus Papyri, the symbols for Venus and Mercury didn't have the cross-bar on the bottom stroke.
Habitability
Speculation on the possibility of life on Venus's surface decreased significantly after the early 1960s when it became clear that the conditions are extreme compared to those on Earth. Venus's extreme temperature and atmospheric pressure make water-based life as currently known unlikely.
Some scientists have speculated that thermoacidophilic extremophile microorganisms might exist in the cooler, acidic upper layers of the Venusian atmosphere). Such speculations go back to 1967, when Carl Sagan and Harold J. Morowitz suggested in a Nature article that tiny objects detected in Venus's clouds might be organisms similar to Earth's bacteria (which are of approximately the same size):
While the surface conditions of Venus make the hypothesis of life there implausible, the clouds of Venus are a different story altogether. As was pointed out some years ago, water, carbon dioxide and sunlight—the prerequisites for photosynthesis—are plentiful in the vicinity of the clouds.
In August 2019, astronomers led by Yeon Joo Lee reported that long-term pattern of absorbance and albedo changes in the atmosphere of the planet Venus caused by "unknown absorbers", which may be chemicals or even large colonies of microorganisms high up in the atmosphere of the planet, affect the climate. Their light absorbance is almost identical to that of micro-organisms in Earth's clouds. Similar conclusions have been reached by other studies.
In September 2020, a team of astronomers led by Jane Greaves from Cardiff University announced the likely detection of phosphine, a gas not known to be produced by any known chemical processes on the Venusian surface or atmosphere, in the upper levels of the planet's clouds. One proposed source for this phosphine is living organisms. The phosphine was detected at heights of at least 30 miles above the surface, and primarily at mid-latitudes with none detected at the poles. The discovery prompted NASA administrator Jim Bridenstine to publicly call for a new focus on the study of Venus, describing the phosphine find as "the most significant development yet in building the case for life off Earth".
Subsequent analysis of the data-processing used to identify phosphine in the atmosphere of Venus has raised concerns that the detection-line may be an artifact. The use of a 12th-order polynomial fit may have amplified noise and generated a false reading. Observations of the atmosphere of Venus at other parts of the electromagnetic spectrum in which a phosphine absorption line would be expected did not detect phosphine. By late October 2020, re-analysis of data with a proper subtraction of background did not show a statistically significant detection of phosphine.
Planetary protection
The Committee on Space Research is a scientific organization established by the International Council for Science. Among their responsibilities is the development of recommendations for avoiding interplanetary contamination. For this purpose, space missions are categorized into five groups. Due to the harsh surface environment of Venus, Venus has been under the planetary protection category two. This indicates that there is only a remote chance that spacecraft-borne contamination could compromise investigations.
Human presence
Venus is the place of the very first interplanetary human presence, mediated through robotic missions, with the first successful landings on another planet and extraterrestrial body other than the Moon. Venus was at the beginning of the space age frequently visited by space probes until the 1990s. Currently in orbit is Akatsuki, and the Parker Solar Probe routinely uses Venus for gravity assist maneuvers.
The only nation that has sent lander probes to the surface of Venus has been the Soviet Union, which has been used by Russian officials to call Venus a "Russian planet".
Habitation
While the surface conditions of Venus are very inhospitable, the atmospheric pressure and temperature fifty kilometers above the surface are similar to those at Earth's surface. With this in mind the Soviet engineer Sergey Zhitomirskiy (Сергей Житомирский, 1929–2004) in 1971 and more con temporarily NASA aerospace engineer Geoffrey A. Landis in 2003 suggested the use of aerostats for crewed exploration and possibly for permanent "floating cities" in the Venusian atmosphere, an alternative to the popular idea of living on planetary surfaces such as Mars. Among the many engineering challenges for any human presence in the atmosphere of Venus are the corrosive amounts of sulfuric acid in the atmosphere.
The High Altitude Venus Operational Concept (HAVOC) by NASA is a mission concept that proposed a crewed aerostat design.
Notes
Misstated as "Ganiki Chasma" in the press release and scientific publication.
It is important to be clear about the meaning of "closeness". In the astronomical literature, the term "closest planets" often refers to the two planets that approach each other the most closely. In other words, the orbits of the two planets approach each other most closely. However, this does not mean that the two planets are closest over time. Essentially because Mercury is closer to the Sun than Venus, Mercury spends more time in proximity to Earth; it could, therefore, be said that Mercury is the planet that is "closest to Earth when averaged over time". However, using this time-average definition of "closeness", it turns out that Mercury is the closest planet to all other planets in the solar system. For that reason, arguably, the proximity-definition is not particularly helpful. An episode of the BBC Radio 4 programme "More or Less" explains the different notions of proximity well.
Several claims of transit observations made by medieval Islamic astronomers have been shown to be sunspots. Avicenna did not record the date of his observation. There was a transit of Venus within his lifetime, on 24 May 1032, although it is questionable whether it would have been visible from his location.
The American Pioneer Venus Multi-probe has brought the only non-Soviet probes to enter the atmosphere, as atmospheric entry probes only briefly signals were received from the surface.
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