Photography is the art and practice of creating durable images by recording light or other electromagnetic radiation, either electronically by means of an image sensor, or chemically by means of a light-sensitive material such as photographic film. It is employed in many fields of science and business, as well as its more direct uses for art and video production, recreational purposes and mass communication. A lens is used to focus the light reflected or emitted from objects into a real image on the light-sensitive surface inside a camera during a timed exposure. With an electronic image sensor, this produces an electrical charge at each pixel, electronically processed and stored in a digital image file for subsequent display or processing; the result with photographic emulsion is an invisible latent image, chemically "developed" into a visible image, either negative or positive depending on the purpose of the photographic material and the method of processing. A negative image on film is traditionally used to photographically create a positive image on a paper base, known as a print, either by using an enlarger or by contact printing.
The word "photography" was created from the Greek roots φωτός, genitive of φῶς, "light" and γραφή "representation by means of lines" or "drawing", together meaning "drawing with light". Several people may have coined the same new term from these roots independently. Hercules Florence, a French painter and inventor living in Campinas, used the French form of the word, photographie, in private notes which a Brazilian historian believes were written in 1834; this claim is reported but has never been independently confirmed as beyond reasonable doubt. The German newspaper Vossische Zeitung of 25 February 1839 contained an article entitled Photographie, discussing several priority claims – Henry Fox Talbot's – regarding Daguerre's claim of invention; the article is the earliest known occurrence of the word in public print. It was signed "J. M.", believed to have been Berlin astronomer Johann von Maedler. The inventors Nicéphore Niépce, Henry Fox Talbot and Louis Daguerre seem not to have known or used the word "photography", but referred to their processes as "Heliography", "Photogenic Drawing"/"Talbotype"/"Calotype" and "Daguerreotype".
Photography is the result of combining several technical discoveries, relating to seeing an image and capturing the image. The discovery of the camera obscura that provides an image of a scene dates back to ancient China. Greek mathematicians Aristotle and Euclid independently described a pinhole camera in the 5th and 4th centuries BCE. In the 6th century CE, Byzantine mathematician Anthemius of Tralles used a type of camera obscura in his experiments; the Arab physicist Ibn al-Haytham invented a camera obscura and pinhole camera. Leonardo da Vinci mentions natural camera obscura that are formed by dark caves on the edge of a sunlit valley. A hole in the cave wall will act as a pinhole camera and project a laterally reversed, upside down image on a piece of paper. Renaissance painters used the camera obscura which, in fact, gives the optical rendering in color that dominates Western Art, it is a box with a hole in it which allows light to go through and create an image onto the piece of paper.
The birth of photography was concerned with inventing means to capture and keep the image produced by the camera obscura. Albertus Magnus discovered silver nitrate, Georg Fabricius discovered silver chloride, the techniques described in Ibn al-Haytham's Book of Optics are capable of producing primitive photographs using medieval materials. Daniele Barbaro described a diaphragm in 1566. Wilhelm Homberg described how light darkened some chemicals in 1694; the fiction book Giphantie, published in 1760, by French author Tiphaigne de la Roche, described what can be interpreted as photography. Around the year 1800, British inventor Thomas Wedgwood made the first known attempt to capture the image in a camera obscura by means of a light-sensitive substance, he used paper or white leather treated with silver nitrate. Although he succeeded in capturing the shadows of objects placed on the surface in direct sunlight, made shadow copies of paintings on glass, it was reported in 1802 that "the images formed by means of a camera obscura have been found too faint to produce, in any moderate time, an effect upon the nitrate of silver."
The shadow images darkened all over. The first permanent photoetching was an image produced in 1822 by the French inventor Nicéphore Niépce, but it was destroyed in a attempt to make prints from it. Niépce was successful again in 1825. In 1826 or 1827, he made the View from the Window at Le Gras, the earliest surviving photograph from nature; because Niépce's camera photographs required an long exposure, he sought to improve his bitumen process or replace it with one, more practical. In partnership with Louis Daguerre, he worked out post-exposure processing methods that produced visually superior results and replaced the bitumen with a more light-sensitive resin, but hours of exposure in the camera were still required. With an eye to eventual commercial exploitation, the partners opted for total secrecy. Niépce died in 1833 and Daguerre redirected the experiments toward the light-sensitive silver halides, which Niépce had abandoned many years earlier because of his inability to make the images he captured with them light-fast and permanent.
In astronomy, declination is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. Declination's angle is measured north or south of the celestial equator, along the hour circle passing through the point in question; the root of the word declination means "a bending away" or "a bending down". It comes from the same root as the words recline. In some 18th and 19th century astronomical texts, declination is given as North Pole Distance, equivalent to 90 -. For instance an object marked as declination -5 would have a NPD of 95, a declination of -90 would have a NPD of 180. Declination in astronomy is comparable to geographic latitude, projected onto the celestial sphere, hour angle is comparable to longitude. Points north of the celestial equator have positive declinations, while those south have negative declinations. Any units of angular measure can be used for declination, but it is customarily measured in the degrees and seconds of sexagesimal measure, with 90° equivalent to a quarter circle.
Declinations with magnitudes greater than 90° do not occur, because the poles are the northernmost and southernmost points of the celestial sphere. An object at the celestial equator has a declination of 0° north celestial pole has a declination of +90° south celestial pole has a declination of −90°The sign is customarily included whether positive or negative; the Earth's axis rotates westward about the poles of the ecliptic, completing one circuit in about 26,000 years. This effect, known as precession, causes the coordinates of stationary celestial objects to change continuously, if rather slowly. Therefore, equatorial coordinates are inherently relative to the year of their observation, astronomers specify them with reference to a particular year, known as an epoch. Coordinates from different epochs must be mathematically rotated to match each other, or to match a standard epoch; the used standard epoch is J2000.0, January 1, 2000 at 12:00 TT. The prefix "J" indicates. Prior to J2000.0, astronomers used the successive Besselian Epochs B1875.0, B1900.0, B1950.0.
A star's direction remains nearly fixed due to its vast distance, but its right ascension and declination do change due to precession of the equinoxes and proper motion, cyclically due to annual parallax. The declinations of Solar System objects change rapidly compared to those of stars, due to orbital motion and close proximity; as seen from locations in the Earth's Northern Hemisphere, celestial objects with declinations greater than 90° − φ appear to circle daily around the celestial pole without dipping below the horizon, are therefore called circumpolar stars. This occurs in the Southern Hemisphere for objects with declinations less than −90° − φ. An extreme example is the pole star which has a declination near to +90°, so is circumpolar as seen from anywhere in the Northern Hemisphere except close to the equator. Circumpolar stars never dip below the horizon. Conversely, there are other stars that never rise above the horizon, as seen from any given point on the Earth's surface. If a star whose declination is δ is circumpolar for some observer a star whose declination is −δ never rises above the horizon, as seen by the same observer.
If a star is circumpolar for an observer at latitude φ it never rises above the horizon as seen by an observer at latitude −φ. Neglecting atmospheric refraction, declination is always 0 ° at west points of the horizon. At the north point, it is 90° − |φ|, at the south point, −90° + |φ|. From the poles, declination is uniform around the entire horizon 0°. Non-circumpolar stars are visible only during certain seasons of the year; the Sun's declination varies with the seasons. As seen from arctic or antarctic latitudes, the Sun is circumpolar near the local summer solstice, leading to the phenomenon of it being above the horizon at midnight, called midnight sun. Near the local winter solstice, the Sun remains below the horizon all day, called polar night; when an object is directly overhead its declination is always within 0.01 degrees of the observer's latitude. The first complication applies to all celestial objects: the object's declination equals the observer's astronomic latitude, but the term "latitude" ordinarily means geodetic latitude, the latitude on maps and GPS devices.
In the continental United States and surrounding area, the difference is a few arcseconds but can be as great as 41 arcseconds. The second complication is that, assuming no deflection of the vertical, "overhead" means perpendicular to the ellipsoid at observer's location, but the perpendicular line does not pass through the center of the earth. For the moon this discrepancy can reach 0.003 degrees.
The Solar System is the gravitationally bound planetary system of the Sun and the objects that orbit it, either directly or indirectly. Of the objects that orbit the Sun directly, the largest are the eight planets, with the remainder being smaller objects, such as the five dwarf planets and small Solar System bodies. Of the objects that orbit the Sun indirectly—the moons—two are larger than the smallest planet, Mercury; the Solar System formed 4.6 billion years ago from the gravitational collapse of a giant interstellar molecular cloud. The vast majority of the system's mass is in the Sun, with the majority of the remaining mass contained in Jupiter; the four smaller inner planets, Venus and Mars, are terrestrial planets, being composed of rock and metal. The four outer planets are giant planets, being more massive than the terrestrials; the two largest and Saturn, are gas giants, being composed of hydrogen and helium. All eight planets have circular orbits that lie within a nearly flat disc called the ecliptic.
The Solar System contains smaller objects. The asteroid belt, which lies between the orbits of Mars and Jupiter contains objects composed, like the terrestrial planets, of rock and metal. Beyond Neptune's orbit lie the Kuiper belt and scattered disc, which are populations of trans-Neptunian objects composed of ices, beyond them a newly discovered population of sednoids. Within these populations are several dozen to tens of thousands of objects large enough that they have been rounded by their own gravity; such objects are categorized as dwarf planets. Identified dwarf planets include the trans-Neptunian objects Pluto and Eris. In addition to these two regions, various other small-body populations, including comets and interplanetary dust clouds travel between regions. Six of the planets, at least four of the dwarf planets, many of the smaller bodies are orbited by natural satellites termed "moons" after the Moon; each of the outer planets is encircled by planetary rings of dust and other small objects.
The solar wind, a stream of charged particles flowing outwards from the Sun, creates a bubble-like region in the interstellar medium known as the heliosphere. The heliopause is the point at which pressure from the solar wind is equal to the opposing pressure of the interstellar medium; the Oort cloud, thought to be the source for long-period comets, may exist at a distance a thousand times further than the heliosphere. The Solar System is located in the Orion Arm, 26,000 light-years from the center of the Milky Way galaxy. For most of history, humanity did not understand the concept of the Solar System. Most people up to the Late Middle Ages–Renaissance believed Earth to be stationary at the centre of the universe and categorically different from the divine or ethereal objects that moved through the sky. Although the Greek philosopher Aristarchus of Samos had speculated on a heliocentric reordering of the cosmos, Nicolaus Copernicus was the first to develop a mathematically predictive heliocentric system.
In the 17th century, Galileo discovered that the Sun was marked with sunspots, that Jupiter had four satellites in orbit around it. Christiaan Huygens followed on from Galileo's discoveries by discovering Saturn's moon Titan and the shape of the rings of Saturn. Edmond Halley realised in 1705 that repeated sightings of a comet were recording the same object, returning once every 75–76 years; this was the first evidence that anything other than the planets orbited the Sun. Around this time, the term "Solar System" first appeared in English. In 1838, Friedrich Bessel measured a stellar parallax, an apparent shift in the position of a star created by Earth's motion around the Sun, providing the first direct, experimental proof of heliocentrism. Improvements in observational astronomy and the use of unmanned spacecraft have since enabled the detailed investigation of other bodies orbiting the Sun; the principal component of the Solar System is the Sun, a G2 main-sequence star that contains 99.86% of the system's known mass and dominates it gravitationally.
The Sun's four largest orbiting bodies, the giant planets, account for 99% of the remaining mass, with Jupiter and Saturn together comprising more than 90%. The remaining objects of the Solar System together comprise less than 0.002% of the Solar System's total mass. Most large objects in orbit around the Sun lie near the plane of Earth's orbit, known as the ecliptic; the planets are close to the ecliptic, whereas comets and Kuiper belt objects are at greater angles to it. All the planets, most other objects, orbit the Sun in the same direction that the Sun is rotating. There are exceptions, such as Halley's Comet; the overall structure of the charted regions of the Solar System consists of the Sun, four small inner planets surrounded by a belt of rocky asteroids, four giant planets surrounded by the Kuiper belt of icy objects. Astronomers sometimes informally divide this structure into separate regions; the inner Solar System includes the asteroid belt. The outer Solar System is including the four giant planets.
Since the discovery of the Kuiper belt, the outermost parts of the Solar Sys
The visible spectrum is the portion of the electromagnetic spectrum, visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or light. A typical human eye will respond to wavelengths from about 380 to 740 nanometers. In terms of frequency, this corresponds to a band in the vicinity of 430–770 THz; the spectrum does not contain all the colors. Unsaturated colors such as pink, or purple variations like magenta, for example, are absent because they can only be made from a mix of multiple wavelengths. Colors containing only one wavelength are called pure colors or spectral colors. Visible wavelengths pass unattenuated through the Earth's atmosphere via the "optical window" region of the electromagnetic spectrum. An example of this phenomenon is when clean air scatters blue light more than red light, so the midday sky appears blue; the optical window is referred to as the "visible window" because it overlaps the human visible response spectrum. The near infrared window lies just out of the human vision, as well as the medium wavelength infrared window, the long wavelength or far infrared window, although other animals may experience them.
In the 13th century, Roger Bacon theorized that rainbows were produced by a similar process to the passage of light through glass or crystal. In the 17th century, Isaac Newton discovered that prisms could disassemble and reassemble white light, described the phenomenon in his book Opticks, he was the first to use the word spectrum in this sense in print in 1671 in describing his experiments in optics. Newton observed that, when a narrow beam of sunlight strikes the face of a glass prism at an angle, some is reflected and some of the beam passes into and through the glass, emerging as different-colored bands. Newton hypothesized light to be made up of "corpuscles" of different colors, with the different colors of light moving at different speeds in transparent matter, red light moving more than violet in glass; the result is that red light is bent less than violet as it passes through the prism, creating a spectrum of colors. Newton divided the spectrum into six named colors: red, yellow, green and violet.
He added indigo as the seventh color since he believed that seven was a perfect number as derived from the ancient Greek sophists, of there being a connection between the colors, the musical notes, the known objects in the solar system, the days of the week. The human eye is insensitive to indigo's frequencies, some people who have otherwise-good vision cannot distinguish indigo from blue and violet. For this reason, some commentators, including Isaac Asimov, have suggested that indigo should not be regarded as a color in its own right but as a shade of blue or violet. Evidence indicates that what Newton meant by "indigo" and "blue" does not correspond to the modern meanings of those color words. Comparing Newton's observation of prismatic colors to a color image of the visible light spectrum shows that "indigo" corresponds to what is today called blue, whereas "blue" corresponds to cyan. In the 18th century, Johann Wolfgang von Goethe wrote about optical spectra in his Theory of Colours. Goethe used the word spectrum to designate a ghostly optical afterimage, as did Schopenhauer in On Vision and Colors.
Goethe argued. Where Newton narrowed the beam of light to isolate the phenomenon, Goethe observed that a wider aperture produces not a spectrum but rather reddish-yellow and blue-cyan edges with white between them; the spectrum appears only. In the early 19th century, the concept of the visible spectrum became more definite, as light outside the visible range was discovered and characterized by William Herschel and Johann Wilhelm Ritter, Thomas Young, Thomas Johann Seebeck, others. Young was the first to measure the wavelengths of different colors of light, in 1802; the connection between the visible spectrum and color vision was explored by Thomas Young and Hermann von Helmholtz in the early 19th century. Their theory of color vision proposed that the eye uses three distinct receptors to perceive color. Many species can see light within frequencies outside the human "visible spectrum". Bees and many other insects can detect ultraviolet light. Plant species that depend on insect pollination may owe reproductive success to their appearance in ultraviolet light rather than how colorful they appear to humans.
Birds, can see into the ultraviolet, some have sex-dependent markings on their plumage that are visible only in the ultraviolet range. Many animals that can see into the ultraviolet range cannot see red light or any other reddish wavelengths. Bees' visible spectrum ends at about 590 nm. Birds can see some red wavelengths; the popular belief that the common goldfish is the only animal that can see both infrared and ultraviolet light is incorrect, because goldfish cannot see infrared light. Dogs are thought to be color blind but they have been shown to be sensitive to colors, though not as many as humans; some snakes can "see" radiant heat at wavelengths between 5 and 30 μm to a degree of accuracy such that a blind rattlesnake can target vulnerable body parts of the prey at which it strikes, other snakes with the organ may detect warm bodies from a meter away. It may be used in thermoregulation and predator detection. (See
Minute and second of arc
A minute of arc, arc minute, or minute arc is a unit of angular measurement equal to 1/60 of one degree. Since one degree is 1/360 of a turn, one minute of arc is 1/21600 of a turn – it is for this reason that the Earth's circumference is exactly 21,600 nautical miles. A minute of arc is π/10800 of a radian. A second of arc, arcsecond, or arc second is 1/60 of an arcminute, 1/3600 of a degree, 1/1296000 of a turn, π/648000 of a radian; these units originated in Babylonian astronomy as sexagesimal subdivisions of the degree. To express smaller angles, standard SI prefixes can be employed; the number of square arcminutes in a complete sphere is 4 π 2 = 466 560 000 π ≈ 148510660 square arcminutes. The names "minute" and "second" have nothing to do with the identically named units of time "minute" or "second"; the identical names reflect the ancient Babylonian number system, based on the number 60. The standard symbol for marking the arcminute is the prime, though a single quote is used where only ASCII characters are permitted.
One arcminute is thus written 1′. It is abbreviated as arcmin or amin or, less the prime with a circumflex over it; the standard symbol for the arcsecond is the double prime, though a double quote is used where only ASCII characters are permitted. One arcsecond is thus written 1″, it is abbreviated as arcsec or asec. In celestial navigation, seconds of arc are used in calculations, the preference being for degrees and decimals of a minute, for example, written as 42° 25.32′ or 42° 25.322′. This notation has been carried over into marine GPS receivers, which display latitude and longitude in the latter format by default; the full moon's average apparent size is about 31 arcminutes. An arcminute is the resolution of the human eye. An arcsecond is the angle subtended by a U. S. dime coin at a distance of 4 kilometres. An arcsecond is the angle subtended by an object of diameter 725.27 km at a distance of one astronomical unit, an object of diameter 45866916 km at one light-year, an object of diameter one astronomical unit at a distance of one parsec, by definition.
A milliarcsecond is about the size of a dime atop the Eiffel Tower. A microarcsecond is about the size of a period at the end of a sentence in the Apollo mission manuals left on the Moon as seen from Earth. A nanoarcsecond is about the size of a penny on Neptune's moon Triton as observed from Earth. Notable examples of size in arcseconds are: Hubble Space Telescope has calculational resolution of 0.05 arcseconds and actual resolution of 0.1 arcseconds, close to the diffraction limit. Crescent Venus measures between 66 seconds of arc. Since antiquity the arcminute and arcsecond have been used in astronomy. In the ecliptic coordinate system and longitude; the principal exception is right ascension in equatorial coordinates, measured in time units of hours and seconds. The arcsecond is often used to describe small astronomical angles such as the angular diameters of planets, the proper motion of stars, the separation of components of binary star systems, parallax, the small change of position of a star in the course of a year or of a solar system body as the Earth rotates.
These small angles may be written in milliarcseconds, or thousandths of an arcsecond. The unit of distance, the parsec, named from the parallax of one arc second, was developed for such parallax measurements, it is the distance at which the mean radius of the Earth's orbit would subtend an angle of one arcsecond. The ESA astrometric space probe Gaia, launched in 2013, can approximate star positions to 7 microarcseconds. Apart from the Sun, the star with the largest angular diameter from Earth is R Doradus, a red giant with a diameter of 0.05 arcsecond. Because of the effects of atmospheric seeing, ground-based telescopes will smear the image of a star to an angular diameter of about 0.5 arcsecond. The dwarf planet Pluto has proven difficult to resolve because its angular diameter is about 0.1 arcsecond. Space telescopes are diffraction limited. For example, the Hubble Space Telescope can reach an angular size of stars down to about 0.1″. Techniques exist for improving seeing on the ground. Adaptive optics, for example, can produce images around 0.05 arcsecond on a 10 m class telescope.
Minutes and seconds of arc are used in cartography and navigation. At sea level one minute of arc
Earth's rotation is the rotation of Planet Earth around its own axis. Earth rotates eastward, in prograde motion; as viewed from the north pole star Polaris, Earth turns counter clockwise. The North Pole known as the Geographic North Pole or Terrestrial North Pole, is the point in the Northern Hemisphere where Earth's axis of rotation meets its surface; this point is distinct from Earth's North Magnetic Pole. The South Pole is the other point where Earth's axis of rotation intersects its surface, in Antarctica. Earth rotates once in about 24 hours with respect to the Sun, but once every 23 hours, 56 minutes, 4 seconds with respect to other, stars. Earth's rotation is slowing with time; this is due to the tidal effects. Atomic clocks show that a modern day is longer by about 1.7 milliseconds than a century ago increasing the rate at which UTC is adjusted by leap seconds. Analysis of historical astronomical records shows a slowing trend of about 2.3 milliseconds per century since the 8th century BCE.
Among the ancient Greeks, several of the Pythagorean school believed in the rotation of Earth rather than the apparent diurnal rotation of the heavens. The first was Philolaus, though his system was complicated, including a counter-earth rotating daily about a central fire. A more conventional picture was that supported by Hicetas and Ecphantus in the fourth century BCE who assumed that Earth rotated but did not suggest that Earth revolved about the Sun. In the third century BCE, Aristarchus of Samos suggested the Sun's central place. However, Aristotle in the fourth century BCE criticized the ideas of Philolaus as being based on theory rather than observation, he established the idea of a sphere of fixed stars. This was accepted by most of those who came after, in particular Claudius Ptolemy, who thought Earth would be devastated by gales if it rotated. In 499 CE, the Indian astronomer Aryabhata wrote that the spherical Earth rotates about its axis daily, that the apparent movement of the stars is a relative motion caused by the rotation of Earth.
He provided the following analogy: "Just as a man in a boat going in one direction sees the stationary things on the bank as moving in the opposite direction, in the same way to a man at Lanka the fixed stars appear to be going westward."In the 10th century, some Muslim astronomers accepted that Earth rotates around its axis. According to al-Biruni, Abu Sa'id al-Sijzi invented an astrolabe called al-zūraqī based on the idea believed by some of his contemporaries "that the motion we see is due to the Earth's movement and not to that of the sky." The prevalence of this view is further confirmed by a reference from the 13th century which states: "According to the geometers, the Earth is in constant circular motion, what appears to be the motion of the heavens is due to the motion of the Earth and not the stars." Treatises were written to discuss its possibility, either as refutations or expressing doubts about Ptolemy's arguments against it. At the Maragha and Samarkand observatories, Earth's rotation was discussed by Qushji.
In medieval Europe, Thomas Aquinas accepted Aristotle's view and so, did John Buridan and Nicole Oresme in the fourteenth century. Not until Nicolaus Copernicus in 1543 adopted a heliocentric world system did the contemporary understanding of Earth's rotation begin to be established. Copernicus pointed out that if the movement of Earth is violent the movement of the stars must be much more so, he pointed to examples of relative motion. For Copernicus this was the first step in establishing the simpler pattern of planets circling a central Sun. Tycho Brahe, who produced accurate observations on which Kepler based his laws, used Copernicus's work as the basis of a system assuming a stationary Earth. In 1600, William Gilbert supported Earth's rotation in his treatise on Earth's magnetism and thereby influenced many of his contemporaries; those like Gilbert who did not support or reject the motion of Earth about the Sun are called "semi-Copernicans". A century after Copernicus, Riccioli disputed the model of a rotating Earth due to the lack of then-observable eastward deflections in falling bodies.
However, the contributions of Kepler and Newton gathered support for the theory of the rotation of Earth. Earth's rotation implies that the geographical poles are flattened. In his Principia, Newton predicted this flattening would occur in the ratio of 1:230, pointed to the pendulum measurements taken by Richer in 1673 as corroboration of the change in gravity, but initial measurements of meridian lengths by Picard and Cassini at the end of the 17th century suggested the opposite. However, measurements by Maupertuis and the French Geodesic Mission in the 1730s established the oblateness of Earth, thus confirming the positions of both Newton and Copernicus. In Earth's rotating frame of reference, a moving body follows an apparent path that deviates from the one it would follow in a fixed frame of reference; because of the Coriolis effect, falling bodies veer eastward from the vertical plumb line below their point of release, projectiles veer right in the Northern Hemisphere from the direction in which they are shot.
The Coriolis effect is observable at a meteorological scale, where it is responsible for the opposite directions of cyclone rotation in th
Hipparcos was a scientific satellite of the European Space Agency, launched in 1989 and operated until 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial objects on the sky; this permitted the accurate determination of proper motions and parallaxes of stars, allowing a determination of their distance and tangential velocity. When combined with radial velocity measurements from spectroscopy, this pinpointed all six quantities needed to determine the motion of stars; the resulting Hipparcos Catalogue, a high-precision catalogue of more than 118,200 stars, was published in 1997. The lower-precision Tycho Catalogue of more than a million stars was published at the same time, while the enhanced Tycho-2 Catalogue of 2.5 million stars was published in 2000. Hipparcos' follow-up mission, was launched in 2013; the word "Hipparcos" is an acronym for HIgh Precision PARallax COllecting Satellite and a reference to the ancient Greek astronomer Hipparchus of Nicaea, noted for applications of trigonometry to astronomy and his discovery of the precession of the equinoxes.
By the second half of the 20th century, the accurate measurement of star positions from the ground was running into insurmountable barriers to improvements in accuracy for large-angle measurements and systematic terms. Problems were dominated by the effects of the Earth's atmosphere, but were compounded by complex optical terms and gravitational instrument flexures, the absence of all-sky visibility. A formal proposal to make these exacting observations from space was first put forward in 1967. Although proposed to the French space agency CNES, it was considered too complex and expensive for a single national programme, its acceptance within the European Space Agency's scientific programme, in 1980, was the result of a lengthy process of study and lobbying. The underlying scientific motivation was to determine the physical properties of the stars through the measurement of their distances and space motions, thus to place theoretical studies of stellar structure and evolution, studies of galactic structure and kinematics, on a more secure empirical basis.
Observationally, the objective was to provide the positions and annual proper motions for some 100,000 stars with an unprecedented accuracy of 0.002 arcseconds, a target in practice surpassed by a factor of two. The name of the space telescope, "Hipparcos" was an acronym for High Precision Parallax Collecting Satellite, it reflected the name of the ancient Greek astronomer Hipparchus, considered the founder of trigonometry and the discoverer of the precession of the equinoxes; the spacecraft carried a single all-reflective, eccentric Schmidt telescope, with an aperture of 29 cm. A special beam-combining mirror superimposed two fields of view, 58 degrees apart, into the common focal plane; this complex mirror consisted of two mirrors tilted in opposite directions, each occupying half of the rectangular entrance pupil, providing an unvignetted field of view of about 1°×1°. The telescope used a system of grids, at the focal surface, composed of 2688 alternate opaque and transparent bands, with a period of 1.208 arc-sec.
Behind this grid system, an image dissector tube with a sensitive field of view of about 38-arc-sec diameter converted the modulated light into a sequence of photon counts from which the phase of the entire pulse train from a star could be derived. The apparent angle between two stars in the combined fields of view, modulo the grid period, was obtained from the phase difference of the two star pulse trains. Targeting the observation of some 100,000 stars, with an astrometric accuracy of about 0.002 arc-sec, the final Hipparcos Catalogue comprised nearly 120,000 stars with a median accuracy of better than 0.001 arc-sec. An additional photomultiplier system viewed a beam splitter in the optical path and was used as a star mapper, its purpose was to monitor and determine the satellite attitude, in the process, to gather photometric and astrometric data of all stars down to about 11th magnitude. These measurements were made in two broad bands corresponding to B and V in the UBV photometric system.
The positions of these latter stars were to be determined to a precision of 0.03 arc-sec, a factor of 25 less than the main mission stars. Targeting the observation of around 400,000 stars, the resulting Tycho Catalogue comprised just over 1 million stars, with a subsequent analysis extending this to the Tycho-2 Catalogue of about 2.5 million stars. The attitude of the spacecraft about its center of gravity was controlled to scan the celestial sphere in a regular precessional motion maintaining a constant inclination between the spin axis and the direction to the Sun; the spacecraft spun around its Z-axis at the rate of 11.25 revolutions/day at an angle of 43° to the Sun. The Z-axis rotated about the sun-satellite line at 6.4 revolutions/year. The spacecraft consisted of two platforms and six vertical panels, all made of aluminum honeycomb; the solar array consisted of three deployable sections. Two S-band antennas were located on the top and bottom of the spacecraft, providing an omni-directional downlink data rate of 24 kbit/s.
An attitude and orbit-control subsystem ensured correct dynamic attitude control and determination during the operational lifetim