Venusdurchgang am 8. Juni 2004
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The transit of Venus - Encyclopedia Britannica online (www.eb.com)

Transits of Venus can be seen without a telescope if the eyes are properly protected.
When the transit is central, it takes about eight hours. The phenomenon is rare and can happen only within a day or two of the dates when the Earth passes the nodes of Venus' orbit--that is, on June 7 and December 8. The transits occur in pairs, with an interval of eight years between members of a pair; between the pairs, more than 100 years elapse.
Transits of Venus are helpful in finding the parallax and from it the distance of the Sun, as first pointed out by the British astronomer Edmond Halley in 1679. Parallax is the apparent difference in direction of an object when observed from different positions. The transits of June 1761 and 1769 and those of December in 1874 and 1882 were, thus, extensively observed. The next pair of transits of Venus are expected on June 8, 2004, and June 6, 2012.
One kind of observation for determining parallax consists in fixing the times of the contacts of the disks of the planet and the Sun from different points on the Earth. The observers at past transits became aware of a few remarkable phenomena. When Venus was partially overlapping the disk of the Sun, the part of the limb of the planet that extended beyond the Sun was seen to be surrounded with a radiant aureole, which observers of the transit in 1761 ascribed to the presence of an atmosphere on Venus. A second phenomenon was seen just after second and before third contact, when Venus just touched the Sun's limb on the inside; this consisted in the development of a little dark connection--the so-called black drop--between Venus and the limb. Because of the black drop, the times of contact could not be sharply defined. Presumed causes of the black drop are diffraction, atmospheric agitation, and instrumental factors.
The amount of sunlight intercepted during a transit depends on the diameter of the planet, and measuring this amount of sunlight may be one of the most accurate ways of determining the planet's diameter.

Transits of Venus were extremely important events to 18th-century astronomy
Important early telescopic observations of Venus were conducted in the 1700s during the planet's solar transits. In a solar transit, a planet passes directly between the Sun and the Earth and is silhouetted briefly against the Sun's disk. Transits of Venus are rare events, occurring in pairs eight years apart with more than a century between pairs. They were extremely important events to 18th-century astronomy, since they provided at that time the most accurate method for determining the distance from the Earth to the Sun. (This distance, known as the astronomical unit, is one of the fundamental constants of astronomy.) Observations of the 1761 transit were only partially successful but did result in the first suggestion, by the Russian astronomer Mikhail V. Lomonosov*, that Venus has an atmosphere. The second transit of the pair, in 1769, was observed with somewhat greater success. Transits must be viewed from many points on Earth to yield accurate distances, and the transits of 1761 and, particularly, of 1769 prompted the launching of many scientific expeditions to remote parts of the globe. Among these was the first of the three voyages of exploration by Captain James Cook, who observed the 1769 transit from Tahiti. The transit observations of the 1700s provided not only an improved determination of the astronomical unit but also the impetus for many unrelated discoveries concerning the Earth's geography.
* Vladimiros Fotiadis (vfotiadi@chem.auth.gr - Thu, 29 May 2003) writes:
»I would like to inform you that Lomonosov was not russian astronomer, but russian chemist and physician. He accidentally had observed, that a Venus has an atmosphere, so many people think that he was an astronomer. I know that because I'm learning a biography of this scientist. It is my occupation in University.«

Aepinus, Franz Maria Ulrich Theodor Hoch
b. Dec. 13, 1724, Rostock, Mecklenberg-Schwerin
d. Aug. 10, 1802, Dorpat, Russia
Physicist whose Tentamen theoriae electricitatis et magnetismi (1759; "An Attempt at a Theory of Electricity and Magnetism") was the first work to apply mathematics to the theory of electricity and magnetism...
Aepinus improved the microscope, and his essay on the effects of parallax in the transit of a planet across the Sun's disk excited great general interest, for it was published in 1764, between the dates of two transits of Venus.

Boss, Lewis
b. Oct. 26, 1846, Providence, R.I., U.S.
d. Oct. 12, 1912, Albany, N.Y.
American astronomer best known for his compilation of star catalogs.
Boss worked for the U.S. government at Washington, D.C., and on a survey of the U.S.-Canadian border. In 1876 he became director of the Dudley Observatory at Albany, and in 1882 he led an expedition to Chile to observe a transit of Venus...

Cook, James
b. Oct. 27, 1728, Marton-in-Cleveland, Yorkshire, Eng.
d. Feb. 14, 1779, Kealakekua Bay, Hawaii
British naval captain, navigator, and explorer, who explored the seaways and coasts of Canada (1759, 1763-67) and conducted three expeditions to the Pacific Ocean (1768-71; 1772-75; 1776-79), ranging from the Antarctic ice fields to the Bering Strait and from the coasts of North America to Australia and New Zealand...
In 1768 the Royal Society, in conjunction with the Admiralty, was organizing the first scientific expedition to the Pacific, and the rather obscure 40-year-old James Cook was appointed commander of the expedition. Hurriedly commissioned as lieutenant, he was given a homely looking but extremely sturdy Whitby coal-hauling bark renamed Hms "Endeavour," then four years old, of just 368 tons, and less than 98 feet long. Cook's orders were to convey gentlemen of the Royal Society and their assistants to Tahiti to observe the transit of the planet Venus across the Sun. That done, on June 3, 1769, he was to find the southern continent, the so-called Terra Australis, which philosophers argued must exist to balance the landmasses of the Northern Hemisphere. The leader of the scientists was the rich and able Joseph Banks, aged 26, who was assisted by Daniel Solander, a Swedish botanist, as well as astronomers (Cook rating as one) and artists. Cook carried an early nautical almanac and brass sextants, but no chronometer on the first voyage...

Draper, Henry
b. March 7, 1837, Prince Edward County, Va., U.S.
d. Nov. 20, 1882, New York City
American physician and amateur astronomer who made the first photograph of the spectrum of a star (Vega), in 1872. He was also the first to photograph a nebula, the Orion Nebula, in 1880. His father, John William Draper, in 1840 had made the first photograph of the Moon...
For his photography of the transit of Venus in 1874, Congress ordered a gold medal struck in his honour.

Encke, Johann Franz
b. Sept. 23, 1791, Hamburg
d. Aug. 26, 1865, Spandau, Ger.
German astronomer who in 1819 established the period of the comet now known by his name (see Encke's Comet).
Encke was educated at Hamburg and the University of Göttingen, where he worked under the direction of Carl Friedrich Gauss...
From observations of the transits of Venus recorded in 1761 and 1769, he derived a value for the solar parallax (in effect, for the Sun's distance from the Earth) that, at 8.57, is close to the presently accepted figure.

Gill, Sir David
b. June 12, 1843, Aberdeen, Aberdeen, Scot.
d. Jan. 24, 1914, London, Eng.
Scottish astronomer known for his measurements of solar and stellar parallax, showing the distances of the Sun and other stars from Earth, and for his early use of photography in mapping the heavens. To determine the parallaxes, he perfected the use of the heliometer, a telescope that uses a split image to measure the angular separation of celestial bodies...
Gill was educated at the University of Aberdeen, and in 1872 he became director of James Ludovic Lindsay's private observatory near Aberdeen. From there he undertook expeditions to Mauritius in 1874, to observe the transit of Venus, and to Ascension Island in 1877, when Mars was in opposition...

Halley, Edmond
b. Nov. 8, 1656, Haggerston, Shoreditch, near London
d. Jan. 14, 1742, Greenwich, near London
Edmond also spelled EDMUND, English astronomer and mathematician who was the first to calculate the orbit of a comet later named after him. He is also noted for his role in the publication of Newton's Philosophiae Naturalis Principia Mathematica...
In 1716 he devised a method for observing transits of Venus across the disk of the Sun, predicted for 1761 and 1769, in order to determine accurately, by solar parallax, the distance of the Earth from the Sun...
Halley's concern with practical applications of science, such as problems of navigation, reflects the influence on the Royal Society of Francis Bacon, who held that science should be for the "relief of man's estate." Though wide ranging in his interests, Halley displayed a high degree of professional competence that foreshadowed scientific specialization. His wise assessment of Newton's work and his persistence in guiding it to completion earned for him an important place in the emergence of Western thought.

Horrocks, Jeremiah
b. c. 1617,, Toxteth Park, near Liverpool [now in Merseyside], Eng.
d. Jan. 3, 1641, Toxteth Park
Horrocks also spelled HORROX, British astronomer and clergyman who applied Johannes Kepler's laws of planetary motion to the Moon and whose observations of a transit of Venus (1639) are the first recorded...

Lalande, Jérôme
b. July 11, 1732, Bourg-en-Bresse, France
d. April 4, 1807, Paris
In full JOSEPH-JÉRÔME LEFRANÇAIS DE LALANDE, Lefrançais also spelled LE FRANÇAIS, LEFRANÇOIS, OR LE FRANÇOIS, French astronomer whose tables of the planetary positions were considered the best available until the end of the 18th century...
He helped organize international collaboration in observing the transits of Venus in 1761 and 1769; the data obtained made possible the accurate calculation of the distance between the Sun and the Earth...

Maskelyne, Nevil
b. Oct. 6, 1732, London
d. Feb. 9, 1811, Greenwich, London
British astronomer noted for his contribution to the science of navigation...
Maskelyne was ordained a minister in 1755, but his interest in astronomy had been aroused by the eclipse of July 25, 1748. In 1758 he was admitted to the Royal Society of London, which in 1761 sent him to the island of St. Helena to observe a transit of Venus...

Rittenhouse, David
b. April 8, 1732, Germantown, Pa. [U.S.]
d. June 26, 1796, Philadelphia, Pa., U.S.
American astronomer and inventor who was an early observer of the atmosphere of Venus.
A clockmaker by trade, Rittenhouse built mathematical instruments and, it is believed, the first telescope in the United States...
Rittenhouse was elected to the American Philosophical Society in 1768, and in 1769 he observed the transit of Venus across the face of the Sun. During this transit he observed that Venus has an atmosphere. His findings were similar to those of the Russian scientist Mikhail Vasilyevich Lomonosov, who had identified Venus' atmosphere during a transit in 1761. Though both had written about their observations, neither report was published or publicized for more than a century...

© 4. April 2001 (Upd. 29. 05. 2003)  by Josef Gräf,   Venusdurchgang am 8. Juni 2004