A planet is an astronomical body orbiting a star or stellar remnant, massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, has cleared its neighbouring region of planetesimals. The term planet is ancient, with ties to history, science and religion. Five planets in the Solar System are visible to the naked eye; these were regarded by many early cultures as emissaries of deities. As scientific knowledge advanced, human perception of the planets changed, incorporating a number of disparate objects. In 2006, the International Astronomical Union adopted a resolution defining planets within the Solar System; this definition is controversial because it excludes many objects of planetary mass based on where or what they orbit. Although eight of the planetary bodies discovered before 1950 remain "planets" under the modern definition, some celestial bodies, such as Ceres, Pallas and Vesta, Pluto, that were once considered planets by the scientific community, are no longer viewed as such.
The planets were thought by Ptolemy to orbit Earth in epicycle motions. Although the idea that the planets orbited the Sun had been suggested many times, it was not until the 17th century that this view was supported by evidence from the first telescopic astronomical observations, performed by Galileo Galilei. About the same time, by careful analysis of pre-telescopic observational data collected by Tycho Brahe, Johannes Kepler found the planets' orbits were elliptical rather than circular; as observational tools improved, astronomers saw that, like Earth, each of the planets rotated around an axis tilted with respect to its orbital pole, some shared such features as ice caps and seasons. Since the dawn of the Space Age, close observation by space probes has found that Earth and the other planets share characteristics such as volcanism, hurricanes and hydrology. Planets are divided into two main types: large low-density giant planets, smaller rocky terrestrials. There are eight planets in the Solar System.
In order of increasing distance from the Sun, they are the four terrestrials, Venus and Mars the four giant planets, Saturn and Neptune. Six of the planets are orbited by one or more natural satellites. Several thousands of planets around other stars have been discovered in the Milky Way; as of 1 April 2019, 4,023 known extrasolar planets in 3,005 planetary systems, ranging in size from just above the size of the Moon to gas giants about twice as large as Jupiter have been discovered, out of which more than 100 planets are the same size as Earth, nine of which are at the same relative distance from their star as Earth from the Sun, i.e. in the circumstellar habitable zone. On December 20, 2011, the Kepler Space Telescope team reported the discovery of the first Earth-sized extrasolar planets, Kepler-20e and Kepler-20f, orbiting a Sun-like star, Kepler-20. A 2012 study, analyzing gravitational microlensing data, estimates an average of at least 1.6 bound planets for every star in the Milky Way.
Around one in five Sun-like stars is thought to have an Earth-sized planet in its habitable zone. The idea of planets has evolved over its history, from the divine lights of antiquity to the earthly objects of the scientific age; the concept has expanded to include worlds not only in the Solar System, but in hundreds of other extrasolar systems. The ambiguities inherent in defining planets have led to much scientific controversy; the five classical planets, being visible to the naked eye, have been known since ancient times and have had a significant impact on mythology, religious cosmology, ancient astronomy. In ancient times, astronomers noted how certain lights moved across the sky, as opposed to the "fixed stars", which maintained a constant relative position in the sky. Ancient Greeks called these lights πλάνητες ἀστέρες or πλανῆται, from which today's word "planet" was derived. In ancient Greece, China and indeed all pre-modern civilizations, it was universally believed that Earth was the center of the Universe and that all the "planets" circled Earth.
The reasons for this perception were that stars and planets appeared to revolve around Earth each day and the common-sense perceptions that Earth was solid and stable and that it was not moving but at rest. The first civilization known to have a functional theory of the planets were the Babylonians, who lived in Mesopotamia in the first and second millennia BC; the oldest surviving planetary astronomical text is the Babylonian Venus tablet of Ammisaduqa, a 7th-century BC copy of a list of observations of the motions of the planet Venus, that dates as early as the second millennium BC. The MUL. APIN is a pair of cuneiform tablets dating from the 7th century BC that lays out the motions of the Sun and planets over the course of the year; the Babylonian astrologers laid the foundations of what would become Western astrology. The Enuma anu enlil, written during the Neo-Assyrian period in the 7th century BC, comprises a list of omens and their relationships with various celestial phenomena including the motions of the planets.
Venus and the outer planets Mars and Saturn were all identified by Babylonian astronomers. These would remain the only known planets until the invention of the telescope in early modern times; the ancient Greeks did not attach as much significance to the planets as the Babylonians. The Pythagoreans, in the 6th and 5t
Bellerophon or Bellerophontes is a hero of Greek mythology. He was "the greatest hero and slayer of monsters, alongside Cadmus and Perseus, before the days of Heracles", his greatest feat was killing the Chimera, a monster that Homer depicted with a lion's head, a goat's body, a serpent's tail: "her breath came out in terrible blasts of burning flame." One possible etymology, suggested is: Βελλεροφόντης from βέλεμνον, βελόνη, βέλος and -φόντης from φονεύω. However, Geoffrey Kirk says that "Βελλεροφόντης means'slayer of Belleros'". Belleros could have been a Lycian, a local daimon or a Corinthian nobleman—Bellerophon's name "clearly invited all sorts of speculation". Bellerophon was born at Corinth and was the son of the mortal Eurynome by either her husband Glaucus, or Poseidon, he was the brother of Deliades Bellerophon was the father of Isander Hippolochus and Laodamia by Philonoe, daughter of King Iobates of Lycia. Philonoe was known under several other names: Alkimedousa, Pasandra or Cassandra.
In some accounts, Bellerophon fathered Hydissos by Asteria, daughter of Hydeus. The Iliad vi.155–203 contains an embedded narrative told by Bellerophon's grandson Glaucus, named for his great-grandfather, which recounts Bellerophon's myth. Bellerophon's father was Glaucus, the king of Corinth and the son of Sisyphus. Bellerophon's grandsons Sarpedon and the younger Glaucus fought in the Trojan War. In the Epitome of pseudo-Apollodorus, a genealogy is given for Chrysaor that would make him a double of Bellerophon. Chrysaor has no myth save that of his birth: from the severed neck of Medusa, with child by Poseidon, he and Pegasus both sprang at the moment of her death. "From this moment we hear no more of Chrysaor, the rest of the tale concerning the stallion only... also for his brother's sake, by whom in the end he let himself be caught, the immortal horse by his mortal brother." Bellerophon's brave journey began in the familiar way, with an exile: he had murdered either his brother, whose name is given as Deliades, Peiren or Alcimenes, or killed a shadowy "enemy", a "Belleros" or "Belleron", a ruler of the Corinthians, in expiation of his crime arrived as a suppliant to Proetus, king in Tiryns, one of the Mycenaean strongholds of the Argolid.
Proetus, by virtue of his kingship, cleansed Bellerophon of his crime. The wife of the king, whether named Anteia or Stheneboea, took a fancy to him, but when he rejected her, she accused Bellerophon of attempting to ravish her. Proetus dared not satisfy his anger by killing a guest, so he sent Bellerophon to King Iobates his father-in-law, in the plain of the River Xanthus in Lycia, bearing a sealed message in a folded tablet: "Pray remove the bearer from this world: he attempted to violate my wife, your daughter." Before opening the tablets, Iobates feasted with Bellerophon for nine days. On reading the tablet's message Iobates too feared the wrath of the Erinyes; the Chimera was a fire-breathing monster consisting of the body of a goat, the head of a lion and the tail of a serpent. This monster had terrorized the nearby countryside. On his way he encountered the famous Corinthian seer Polyeidos, who gave him advice about his oncoming battle. Polyeidos told Bellerophon. To obtain the services of the untamed winged horse, Polyeidos told Bellerophon to sleep in the temple of Athena.
While Bellerophon slept, he dreamed that Athena set a golden bridle beside him, saying "Sleepest thou, prince of the house of Aiolos? Come, take this charm for the steed and show it to the Tamer thy father as thou makest sacrifice to him of a white bull." It was there. Bellerophon had to approach Pegasus. Other accounts say that Athena brought Pegasus tamed and bridled, or that Poseidon the horse-tamer, secretly the father of Bellerophon, brought Pegasus, as Pausanias understood. Bellerophon flew off to where the Chimera was said to dwell; when he arrived in Lycia, the Chimera was ferocious, he could not harm the monster while riding on Pegasus. He felt the heat of the breath the Chimera expelled, was struck with an idea, he mounted it on his spear. He flew head-on towards the Chimera, holding out the spear as far as he could. Before he broke off his attack, he managed to lodge the block of lead inside the Chimera's throat; the beast's fire-breath melted the lead, blocked its air passage. The Chimera suffocated, Bellerophon returned victorious to King Iobates.
Iobates, on Bellerophon's return, was unwilling to credit his story. A series of daunting further quests ensued: he was sent against the warlike Solymi and against the Amazons who fought like men, whom Bellerophon vanquished by dropping boulders from his winged horse. In defense the palace women sent him and the flood in retreat by rushing from the gates with their robes lifted high, offering themselves, to which the mo
International Astronomical Union
The International Astronomical Union is an international association of professional astronomers, at the PhD level and beyond, active in professional research and education in astronomy. Among other activities, it acts as the internationally recognized authority for assigning designations and names to celestial bodies and any surface features on them; the IAU is a member of the International Council for Science. Its main objective is to promote and safeguard the science of astronomy in all its aspects through international cooperation; the IAU maintains friendly relations with organizations that include amateur astronomers in their membership. The IAU has its head office on the second floor of the Institut d'Astrophysique de Paris in the 14th arrondissement of Paris. Working groups include the Working Group for Planetary System Nomenclature, which maintains the astronomical naming conventions and planetary nomenclature for planetary bodies, the Working Group on Star Names, which catalogs and standardizes proper names for stars.
The IAU is responsible for the system of astronomical telegrams which are produced and distributed on its behalf by the Central Bureau for Astronomical Telegrams. The Minor Planet Center operates under the IAU, is a "clearinghouse" for all non-planetary or non-moon bodies in the Solar System; the Working Group for Meteor Shower Nomenclature and the Meteor Data Center coordinate the nomenclature of meteor showers. The IAU was founded on 28 July 1919, at the Constitutive Assembly of the International Research Council held in Brussels, Belgium. Two subsidiaries of the IAU were created at this assembly: the International Time Commission seated at the International Time Bureau in Paris and the International Central Bureau of Astronomical Telegrams seated in Copenhagen, Denmark; the 7 initial member states were Belgium, France, Great Britain, Greece and the United States, soon to be followed by Italy and Mexico. The first executive committee consisted of Benjamin Baillaud, Alfred Fowler, four vice presidents: William Campbell, Frank Dyson, Georges Lecointe, Annibale Riccò.
Thirty-two Commissions were appointed at the Brussels meeting and focused on topics ranging from relativity to minor planets. The reports of these 32 Commissions formed the main substance of the first General Assembly, which took place in Rome, Italy, 2–10 May 1922. By the end of the first General Assembly, ten additional nations had joined the Union, bringing the total membership to 19 countries. Although the Union was formed eight months after the end of World War I, international collaboration in astronomy had been strong in the pre-war era; the first 50 years of the Union's history are well documented. Subsequent history is recorded in the form of reminiscences of past IAU Presidents and General Secretaries. Twelve of the fourteen past General Secretaries in the period 1964-2006 contributed their recollections of the Union's history in IAU Information Bulletin No. 100. Six past IAU Presidents in the period 1976–2003 contributed their recollections in IAU Information Bulletin No. 104. The IAU includes a total of 12,664 individual members who are professional astronomers from 96 countries worldwide.
83% of all individual members are male, while 17% are female, among them the union's former president, Mexican astronomer Silvia Torres-Peimbert. Membership includes 79 national members, professional astronomical communities representing their country's affiliation with the IAU. National members include the Australian Academy of Science, the Chinese Astronomical Society, the French Academy of Sciences, the Indian National Science Academy, the National Academies, the National Research Foundation of South Africa, the National Scientific and Technical Research Council, KACST, the Council of German Observatories, the Royal Astronomical Society, the Royal Astronomical Society of New Zealand, the Royal Swedish Academy of Sciences, the Russian Academy of Sciences, the Science Council of Japan, among many others; the sovereign body of the IAU is its General Assembly. The Assembly determines IAU policy, approves the Statutes and By-Laws of the Union and elects various committees; the right to vote on matters brought before the Assembly varies according to the type of business under discussion.
The Statutes consider such business to be divided into two categories: issues of a "primarily scientific nature", upon which voting is restricted to individual members, all other matters, upon which voting is restricted to the representatives of national members. On budget matters, votes are weighted according to the relative subscription levels of the national members. A second category vote requires a turnout of at least two-thirds of national members in order to be valid. An absolute majority is sufficient for approval in any vote, except for Statute revision which requires a two-thirds majority. An equality of votes is resolved by the vote of the President of the Union. Since 1922, the IAU General Assembly meets every three years, with the ex
The term apsis refers to an extreme point in the orbit of an object. It denotes either the respective distance of the bodies; the word comes via Latin from Greek, there denoting a whole orbit, is cognate with apse. Except for the theoretical possibility of one common circular orbit for two bodies of equal mass at diametral positions, there are two apsides for any elliptic orbit, named with the prefixes peri- and ap-/apo-, added in reference to the body being orbited. All periodic orbits are, according to Newton's Laws of motion, ellipses: either the two individual ellipses of both bodies, with the center of mass of this two-body system at the one common focus of the ellipses, or the orbital ellipses, with one body taken as fixed at one focus, the other body orbiting this focus. All these ellipses share a straight line, the line of apsides, that contains their major axes, the foci, the vertices, thus the periapsis and the apoapsis; the major axis of the orbital ellipse is the distance of the apsides, when taken as points on the orbit, or their sum, when taken as distances.
The major axes of the individual ellipses around the barycenter the contributions to the major axis of the orbital ellipses are inverse proportional to the masses of the bodies, i.e. a bigger mass implies a smaller axis/contribution. Only when one mass is sufficiently larger than the other, the individual ellipse of the smaller body around the barycenter comprises the individual ellipse of the larger body as shown in the second figure. For remarkable asymmetry, the barycenter of the two bodies may lie well within the bigger body, e.g. the Earth–Moon barycenter is about 75% of the way from Earth's center to its surface. If the smaller mass is negligible compared to the larger the orbital parameters are independent of the smaller mass. For general orbits, the terms periapsis and apoapsis are used. Pericenter and apocenter are equivalent alternatives, referring explicitly to the respective points on the orbits, whereas periapsis and apoapsis may refer to the smallest and largest distances of the orbiter and its host.
For a body orbiting the Sun, the point of least distance is the perihelion, the point of greatest distance is the aphelion. The terms become apastron when discussing orbits around other stars. For any satellite of Earth, including the Moon, the point of least distance is the perigee and greatest distance the apogee, from Ancient Greek Γῆ, "land" or "earth". For objects in lunar orbit, the point of least distance is sometimes called the pericynthion and the greatest distance the apocynthion. Perilune and apolune are used. In orbital mechanics, the apsides technically refer to the distance measured between the barycenters of the central body and orbiting body. However, in the case of a spacecraft, the terms are used to refer to the orbital altitude of the spacecraft above the surface of the central body; these formulae characterize the pericenter and apocenter of an orbit: Pericenter Maximum speed, v per = μ a, at minimum distance, r per = a. Apocenter Minimum speed, v ap = μ a, at maximum distance, r ap = a.
While, in accordance with Kepler's laws of planetary motion and the conservation of energy, these two quantities are constant for a given orbit: Specific relative angular momentum h = μ a Specific orbital energy ε = − μ 2 a where: a is the semi-major axis: a = r per + r ap 2 μ is the standard gravitational parameter e is the eccentricity, defined as e = r ap − r per r ap + r per = 1 − 2 r ap r per + 1 Note t
Pegasus is a famous pterippus, a mythical winged divine stallion, one of the most recognized creatures in Greek mythology. Although misused in popular culture, the term "Pegasus" is a proper noun, referring to a particular character, whereas the term "pterippus" is the generic name for the species of winged horses. Pegasus is depicted as pure white in color. Pegasus is a child of the Olympian god Poseidon, he was foaled by the Gorgon Medusa upon her death. Pegasus is the uncle of Geryon. Greco-Roman poets wrote about the ascent of Pegasus to heaven after his birth, his subsequent obeisance to Zeus, king of the gods, who instructed him to bring lightning and thunder from Olympus. Friend of the Muses, Pegasus created the fountain on Mt. Helicon. Pegasus was caught by the Greek hero Bellerophon, near the fountain Peirene, with the help of Athena and Poseidon. Pegasus allowed Bellerophon to ride him in order to defeat the monstrous Chimera, which led to many other exploits. Bellerophon fell from the winged horse's back while trying to reach Mount Olympus.
Afterwards, Zeus transformed Pegasus into the eponymous constellation. The symbolism of Pegasus varies with time. Symbolic of wisdom and fame from the Middle Ages until the Renaissance, Pegasus became associated with poetry around the 19th century, as the fountainhead of sources from which the poets gained their inspiration. Pegasus is the subject of a rich iconography throughout ancient Greek pottery and paintings and sculptures of the Renaissance. Hypotheses have been proposed regarding the relationship between Pegasus and the Muses, the gods Athena, Zeus and the hero Perseus; the poet Hesiod presents a folk etymology of the name Pegasus as derived from πηγή pēgē "spring, well": "the pegai of Okeanos, where he was born."A proposed etymology of the name is Luwian pihassas, meaning "lightning", Pihassassi, a local Luwian-Hittite name in southern Cilicia of a weather god represented with thunder and lightning. The proponents of this etymology adduce Pegasus' role, reported as early as Hesiod, as the bringer of thunderbolts to Zeus.
It was first suggested in 1952 and remains accepted, but Robin Lane Fox has criticized it as implausible. According to legend, everywhere the winged horse struck his hoof to the earth, an inspiring water spring burst forth. One of these springs was upon the Muses' Mount Helicon, the Hippocrene, Antoninus Liberalis suggested, at the behest of Poseidon to prevent the mountain swelling with rapture at the song of the Muses. Hesiod relates how Pegasus was peacefully drinking from a spring when the hero Bellerophon captured him. Hesiod says Pegasus carried thunderbolts for Zeus. There are several versions of the birth of the winged stallion and his brother Chrysaor in the far distant place at the edge of Earth, Hesiod's "springs of Oceanus, which encircles the inhabited earth, where Perseus found Medusa: One is that they sprang from the blood issuing from Medusa's neck as Perseus was beheading her, similar to the manner in which Athena was born from the head of Zeus. In another version, when Perseus beheaded Medusa, they were born of the Earth, fed by the Gorgon's blood.
A variation of this story holds that they were formed from the mingling of Medusa's blood and sea foam, implying that Poseidon had involvement in their making. The last version bears resemblance to Hesiod's account of the birth of Aphrodite from the foam created when Uranus's severed genitals were cast into the sea by Cronus. Pegasus aided the hero Bellerophon in his fight against the Chimera. There are varying tales about; the next morning, still clutching the bridle, Bellerophon found Pegasus drinking at the Pierian spring, caught him and tamed him. Michaud's Biographie universelle relates that when Pegasus was born, he flew to where thunder and lightning are released. According to certain versions of the myth, Athena tamed him and gave him to Perseus, who flew to Ethiopia to help Andromeda. In fact, Pegasus is a late addition to the story of Perseus, who flew on his own with the sandals lent to him by Hermes. Pegasus and Athena left Bellerophon and continued to Olympus where he was stabled with Zeus' other steeds, was given the task of carrying Zeus' thunderbolts, along with other members of his entourage, his attendants/handmaidens/shield bearers/shieldmaidens and Bronte.
Because of his years of faithful service to Zeus, Pegasus was honoured with transformation into a constellation. On the day of his catasterism, when Zeus transformed him into a constellation, a single feather fell to the earth near the city of Tarsus. During World War II, the silhouetted image of Bellerophon the warrior, mounted on the winged Pegasus, was adopted by the United Kingdom's newly raised parachute troops in 1941 as their upper sleeve insignia; the image symbolized a warrior arriving at a battle by air, the same tactics used by paratroopers. The square upper-sleeve insignia comprised Bellerophon/Pegasus in light blue on a maroon background. One source suggests that the insignia was designed by famous English novelist Daphne du Maurier, wife of the commander of the 1st Airborne Division, General Frederick "Boy" Browning. According to the British Army Website, the insignia was designed by the celebrated East Anglian painter Major Edward Seago in May 1942; the maroon background on
An exoplanet or extrasolar planet is a planet outside the Solar System. The first evidence of an exoplanet was not recognized as such; the first scientific detection of an exoplanet was in 1988. The first confirmed detection occurred in 1992; as of 1 April 2019, there are 4,023 confirmed planets in 3,005 systems, with 656 systems having more than one planet. There are many methods of detecting exoplanets. Transit photometry and Doppler spectroscopy have found the most, but these methods suffer from a clear observational bias favoring the detection of planets near the star. In several cases, multiple planets have been observed around a star. About 1 in 5 Sun-like stars have an "Earth-sized" planet in the habitable zone. Assuming there are 200 billion stars in the Milky Way, it can be hypothesized that there are 11 billion habitable Earth-sized planets in the Milky Way, rising to 40 billion if planets orbiting the numerous red dwarfs are included; the least massive planet known is Draugr, about twice the mass of the Moon.
The most massive planet listed on the NASA Exoplanet Archive is HR 2562 b, about 30 times the mass of Jupiter, although according to some definitions of a planet, it is too massive to be a planet and may be a brown dwarf instead. There are planets that are so near to their star that they take only a few hours to orbit and there are others so far away that they take thousands of years to orbit; some are so far out. All of the planets detected so far are within the Milky Way. Nonetheless, evidence suggests that extragalactic planets, exoplanets farther away in galaxies beyond the local Milky Way galaxy, may exist; the nearest exoplanet is Proxima Centauri b, located 4.2 light-years from Earth and orbiting Proxima Centauri, the closest star to the Sun. The discovery of exoplanets has intensified interest in the search for extraterrestrial life. There is special interest in planets that orbit in a star's habitable zone, where it is possible for liquid water, a prerequisite for life on Earth, to exist on the surface.
The study of planetary habitability considers a wide range of other factors in determining the suitability of a planet for hosting life. Besides exoplanets, there are rogue planets, which do not orbit any star; these tend to be considered as a separate category if they are gas giants, in which case they are counted as sub-brown dwarfs, like WISE 0855−0714. The rogue planets in the Milky Way number in the billions; the convention for designating exoplanets is an extension of the system used for designating multiple-star systems as adopted by the International Astronomical Union. For exoplanets orbiting a single star, the designation is formed by taking the name or, more designation of its parent star and adding a lower case letter; the first planet discovered in a system is given the designation "b" and planets are given subsequent letters. If several planets in the same system are discovered at the same time, the closest one to the star gets the next letter, followed by the other planets in order of orbital size.
A provisional IAU-sanctioned standard exists to accommodate the designation of circumbinary planets. A limited number of exoplanets have IAU-sanctioned proper names. Other naming systems exist. For centuries scientists and science fiction writers suspected that extrasolar planets existed, but there was no way of detecting them or of knowing their frequency or how similar they might be to the planets of the Solar System. Various detection claims made in the nineteenth century were rejected by astronomers; the first evidence of an exoplanet was not recognized as such. The first suspected scientific detection of an exoplanet occurred in 1988. Shortly afterwards, the first confirmed detection came in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12; the first confirmation of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi. Some exoplanets have been imaged directly by telescopes, but the vast majority have been detected through indirect methods, such as the transit method and the radial-velocity method.
In February 2018, researchers using the Chandra X-ray Observatory, combined with a planet detection technique called microlensing, found evidence of planets in a distant galaxy, stating "Some of these exoplanets are as small as the moon, while others are as massive as Jupiter. Unlike Earth, most of the exoplanets are not bound to stars, so they're wandering through space or loosely orbiting between stars. We can estimate. In the sixteenth century the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are accompanied by planets. In the eighteenth century the same possibility was mentioned by Isaac Newton in the "General Scholium" that concludes his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One."In 1952, more than 40 years before the first hot Jupiter was discovere
Caladenia dimidia known as the chameleon orchid is a species of orchid endemic to the south-west of Western Australia. It has two yellow, cream-coloured or pinkish flowers, it is a variable species, similar to the Joseph's spider orchid but has a more northerly distribution and smaller flowers. Caladenia dimidia is a terrestrial, deciduous, herb with an underground tuber and which grows as a solitary plant or in small clumps, it has a single, linear, hairy leaf, 7–15 cm long and 2–3 mm wide. The leaf is pale green and has purple-red blotches near its base. One or two yellow to cream-coloured, sometimes pinkish flowers, with dark maroon markings are borne on a stalk 15–30 cm tall; the flowers are 7 -- 4 -- 9 cm wide. The bases of the sepals and petals are linear to lance-shaped and held stiffly for about one-third suddenly narrow to a dark brown, densely glandular thread-like section; the dorsal sepal is erect, linear to lance-shaped, 4.5–7 cm long, 2–3 mm wide at the base and has its edges turned inwards.
The lateral sepals are spreading and downcurved, 5–7.5 cm long and 2–3 mm wide at the base and are inclined downwards. The petals are 4.6–6.5 cm long and 2–3 mm wide at the base, spread near the base incline downwards. The labellum is white or pale yellow with maroon stripes and blotches, it is comparatively small, diamond-shaped, 7–11 mm long and 6–8 mm wide and curves downward at the front. The sides of the labellum curve upwards and have a fringe of small teeth decreasing in size towards the front of the labellum. There are 6 to 14 creamy-white or pale pink, narrow anvil-shaped calli in two rows in the centre of the labellum for at least half of its length. Flowering occurs from July to late September; this species is similar to Caladenia polychroma but its flowers its labellum is smaller. Caladenia dimidia was first formally described by Stephen Hopper and Andrew Brown in 2001 from a specimen collected by Hopper south-east of Hyden; the description was published in Nuytsia. The specific epithet is Latin word meaning "half" referring to the characters of this species being half-way between those of C. polychroma and C. paradoxa.
The chameleon orchid is a common orchid in the eastern wheatbelt from near Paynes Find to near Ravensthorpe in the Avon Wheatbelt, Esperance Plains, Geraldton Sandplains, Jarrah Forest, Mallee biogeographic regions where it grows in a range of habitats that are wet in winter. Caladenia dimidia is classified as "not threatened" by the Western Australian Government Department of Parks and Wildlife