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The transit of Venus - Encyclopedia Britannica online (www.eb.com)
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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.
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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.«
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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.
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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...
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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...
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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.
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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.
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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...
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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.
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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...
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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...
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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...
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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...
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