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Black Drop Effect
The black drop effect is an optical phenomenon visible during a transit of Venus and, to a lesser extent, a transit of Mercury. Description Just after second contact, and again just before third contact during the transit, a small black "teardrop" appears to connect Venus's disc to the limb of the Sun, making it impossible to time the exact moment of second or third contact accurately. This led to the failure of the attempts during the 18th-century transits of Venus to establish a truly precise value for the astronomical unit. The black drop effect was long thought to be due to Venus's thick atmosphere, and indeed it was held to be the first real evidence that Venus had an atmosphere. However, it is now thought by many to be an optical effect caused by the combination of the extreme darkening of the Sun's disk near its apparent edge and the intrinsic imperfection of the viewing apparatus. Observing Mercury simultaneously during its transit in May 1832 with different instr ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Limb Darkening
Limb darkening is an optical effect seen in stars (including the Sun) and planets, where the central part of the disk appears brighter than the edge, or '' limb''. Its understanding offered early solar astronomers an opportunity to construct models with such gradients. This encouraged the development of the theory of radiative transfer. Basic theory Optical depth, a measure of the opacity of an object or part of an object, combines with effective temperature gradients inside the star to produce limb darkening. The light seen is approximately the integral of all emission along the line of sight modulated by the optical depth to the viewer (i.e. 1/e times the emission at 1 optical depth, 1/e2 times the emission at 2 optical depths, etc.). Near the center of the star, optical depth is effectively infinite, causing approximately constant brightness. However, the effective optical depth decreases with increasing radius due to lower gas density and a shorter line of sight distance ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Shadow Blister Effect
The shadow blister effect is a visual phenomenon in which a shadow bulges (or blisters) as it approaches another. The effect takes place when two objects are at varying distances between a non-point light source and a background upon which their shadows are cast. As the objects move transversely such that their shadows approach each other, the one nearest the light source begins blocking light from reaching the inside of the other object's Umbra, penumbra and antumbra, penumbra, thereby expanding its Umbra, penumbra and antumbra, umbra. This expansion of the further object's umbra continues until the umbras of both objects meet. This effect can be demonstrated and understood using Ray tracing (physics), ray theory. The shadow blister effect is commonly misconceived to be an illusion caused by the combining of the two object's penumbras, aided by factors such as diffraction, nonlinear response, and the eye's inability to differentiate between varying contrasts. See also *Bl ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Charles Green (astronomer)
Charles Green (baptised 26 December 1734 – 29 January 1771) was a British astronomer, noted for his assignment by the Royal Society in 1768 to the expedition sent to the Pacific Ocean in order to observe the transit of Venus aboard James Cook's '' Endeavour''. Early life and education Born sometime in December 1734, Green was the youngest son of Joshua Green, a prosperous farmer who lived near Swinton in South Yorkshire. His education, according to his future brother-in-law William Wales, was chiefly at a school near Denmark Street in Soho, London. This school was run by his eldest brother, the clergyman Rev. John Green, and the younger Green went on to become an assistant teacher there, continuing his studies in astronomy until he joined the staff of the Royal Greenwich Observatory in 1760. Career in astronomy Green was appointed as Assistant to the Astronomer Royal, James Bradley, succeeding the astronomer Charles Mason who left to join the expedition to the Cape of Go ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
James Cook
Captain (Royal Navy), Captain James Cook (7 November 1728 – 14 February 1779) was a British Royal Navy officer, explorer, and cartographer famous for his three voyages of exploration to the Pacific and Southern Oceans, conducted between 1768 and 1779. He completed the first recorded circumnavigation of the main islands of New Zealand and was the first known European to visit the eastern coastline of Australia and the Hawaiian Islands. Cook joined the British merchant navy as a teenager before enlisting in the Royal Navy in 1755. He served during the Seven Years' War, and subsequently surveyed and mapped much of the entrance to the St. Lawrence River during the Battle of the Plains of Abraham, siege of Quebec. In the 1760s, he mapped the coastline of Newfoundland (island), Newfoundland and made important astronomical observations which brought him to the attention of the Admiralty (United Kingdom), Admiralty and the Royal Society. This acclaim came at a crucial moment in Brit ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Torbern Bergman
Torbern Olof Bergman (''KVO'') (20 March 17358 July 1784) was a Swedish chemist and mineralogist noted for his 1775 ''Dissertation on Elective Attractions'', containing the largest chemical affinity tables ever published. Bergman was the first chemist to use the A, B, C, etc., system of notation for chemical species. Early life and education Torbern was born on 20 March 1735, the son of Barthold Bergman and Sara Hägg. He enrolled at the University of Uppsala at age 17. His father wished him to read either law or divinity, while he himself was anxious to study mathematics and natural science; in the effort to please both himself and his father, he overworked himself and harmed his health. During a period of enforced abstinence from study, he amused himself with field botany and entomology. He was able to send Linnaeus specimens of several new kinds of insects, and in 1756 he succeeded in proving that, contrary to the opinion of that naturalist, the so-called ''Coccus aquaticus'' ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Micrometer (device)
A micrometer, sometimes known as a micrometer screw gauge (MSG), is a device incorporating a calibrated screw for Accuracy and precision, accurate measurement of the size of components. It widely used in mechanical engineering, machining, metrology as well as most mechanical trades, along with other dimensional instruments such as Caliper#Dial caliper, dial, Caliper#Vernier caliper, vernier, and Caliper#Digital caliper, digital calipers. Micrometers are usually, but not always, in the form of calipers (opposing ends joined by a frame). The spindle is a very accurately machined screw and the object to be measured is placed between the spindle and the anvil. The spindle is moved by turning the ratchet knob or thimble until the object to be measured is lightly touched by both the spindle and the anvil. History The word ''micrometer'' is a classical compound, neoclassical coinage from and . According to the Webster's Dictionary#Merriam-Webster Collegiate Dictionary, ''Merriam-Web ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Royal Observatory, Greenwich
The Royal Observatory, Greenwich (ROG; known as the Old Royal Observatory from 1957 to 1998, when the working Royal Greenwich Observatory, RGO, temporarily moved south from Greenwich to Herstmonceux) is an observatory situated on a hill in Greenwich Park in south east London, overlooking the River Thames to the north. It played a major role in the history of astronomy and navigation, and because the Prime meridian (Greenwich), Prime Meridian passed through it, it gave its name to Greenwich Mean Time, the precursor to today's Coordinated Universal Time (UTC). The ROG has the IAU observatory code of 000, the first in the list. ROG, the National Maritime Museum, the Queen's House and the clipper ship ''Cutty Sark'' are collectively designated Royal Museums Greenwich. The observatory was commissioned in 1675 by Charles II of England, King Charles II, with the foundation stone being laid on 10 August. The old hilltop site of Greenwich Castle was chosen by Sir Christopher Wren, a for ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Shuckburgh Telescope
The Shuckburgh telescope or Shuckburgh equatorial refracting telescope was a diameter aperture telescope on an equatorial mount completed in 1791 for Sir George Shuckburgh (1751–1804) in Warwickshire, England, and built by British instrument maker Jesse Ramsden (1735–1800)... The Shuckburgh telescope is described op. 99 It was transferred to the Royal Observatory, Greenwich in 1811 and the London Science Museum in 1929. Even though it has sometimes not been regarded as particularly successful, its design was influential. It was one of the larger achromatic doublet telescopes at the time, and one of the largest to have an equatorial mount. It was also known as the eastern equatorial for its location. It was pictured in the Rees Cyclopedia of the early 1800s. It was early pictured in ''Philosophical Transactions'', published in 1793. At the Royal Observatory in Greenwich, it was for a time installed in the North Dome, although this had a Sky view partially obscured by th ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Mercury (planet)
Mercury is the first planet from the Sun. It is a rocky planet with a trace atmosphere. While it is the List of Solar System objects by size, smallest and least massive planet of the Solar System, its surface gravity is slightly higher than that of Mars. The surface of Mercury is similar to Earth's Moon, heavily Impact crater, cratered, with expansive rupes system, generated from thrust faults, and bright ray systems, formed by ejecta. Its largest crater, Caloris Planitia, has a diameter of , which is about one-third the diameter of the planet (). Being the most inferior planet, inferior orbiting planet it appears in Earth's sky, always close to the Sun, either as a "morning star" or an "evening star". It stays most of the time the closest to all other planets and is the planet with the highest delta-v needed to travel to from all other planets of the Solar System. Mercury's sidereal year (88.0 Earth days) and sidereal day (58.65 Earth days) are in a 3:2 ratio. This relation ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Transit Of Venus
A transit of Venus takes place when Venus passes directly between the Sun and the Earth (or any other superior planet), becoming visible against (and hence obscuring a small portion of) the solar disk. During a transit, Venus is visible as a small black circle moving across the face of the Sun. Transits of Venus reoccur periodically. A pair of transits takes place eight years apart in December (Gregorian calendar) followed by a gap of 121.5 years, before another pair occurs eight years apart in June, followed by another gap, of 105.5 years. The dates advance by about two days per 243-year cycle. The periodicity is a reflection of the fact that the orbital periods of Earth and Venus are close to 8:13 and 243:395 commensurabilities. The last pairs of transits occurred on 8 June 2004 and 5–6 June 2012. The next pair of transits will occur on 10–11 December 2117 and 8 December 2125. Transits of Venus were in the past used to determine the size of the Solar System. The ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Friedrich Wilhelm Argelander
Friedrich Wilhelm August Argelander (22 March 1799 – 17 February 1875) was a German astronomer. He is known for his determinations of stellar brightnesses, positions, and distances. Life and work Argelander was born in Memel in the Kingdom of Prussia (now Klaipėda in Lithuania), the son of a father of Finnish descent, Johann Gottlieb Argelander, and German (Prussian) mother, Dorothea Wilhelmina Grünhagen. He studied with Friedrich Bessel, whose assistant he became in 1820, and obtained his Ph.D. in 1822 at University of Königsberg. From 1823 until 1837, Argelander was the head of the Finnish observatory, first in Turku and then in Helsinki. He then moved to Bonn, Germany. There he designed and built a new observatory at the University of Bonn with funding approved directly by King Frederick William IV whom Argelander had become friends with in his childhood. This lifelong friendship had started when the then crown prince temporarily lived in Argelander's parents hou ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
