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Astronomy & Cosmology I
A reference
to Aristotle is essential in a history of science, though impossible in
a few sentences without being banal. This greatest of all Greek philosophers,
who flourished in the 4th century BC, exerted an all-encompassing influence
on the development of western thought. He introduced the systematic study
of logic which he applied to his teachings. In his biological writings
he presented a classification system of animals that was not fully replaced
until the time of Linnaeus. All branches of knowledge were classified: physics, methaphysics,
rhetoric, poetics. His picture of a perfect, unchanging, spherical universe
centered on the earth placed a long-lasting damper on western understanding
of the workings of the physical world, particularly the motions of the
stars and planets. To the four supposed elements making up the universe,
earth, water, air, and fire, he added a fifth, invisible element, the
ether. This ether, made necessary because "nature abhorrs a vacuum", was
an inpalpable fixture for over 2000 years until its existence was disproved
by the Michelson-Morley experiment. His teachings, lost to western Europe during the dark ages,
were reintroduced after contacts with the Arab world increased. The Scholastics
reconciled Christian dogma with Aristotle's ideas, which virtually became
dogma in secular learning. This discouraged independent scientific observation,
inquiry, and experiment, even though Aristotle himself had been an interested
observer of the natural world.
A 32 cm bronze disc with inlays of gold was found in 1999 near Nebra in Germany. It dates from approximately 1600 BC and is the oldest representation of the cosmos known today, showing the sun, a crescent moon, and stars, including the Pleiades. It is thought to be an astronomical clock designed to help synchronize the lunar calendar with the solar calendar. The twelve month lunar calendar is shorter than the solar calendar by 11 days, and every two or three years a month has to be inserted to stay in step with the solar year. According to Babylonian calculations, a thirteenth month should be inserted when a crescent moon and the Pleiades take on positions as shown on this disc, a fact known and used by the Bronze Age people as well.
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Aristarchus of Samos (3rd century BC) considered the sizes and distances of the sun and the moon, and was the first to try to calculate the distances of these bodies geometrically . Furthermore, he advanced the theory that the sun was at rest at the center of the sphere of fixed stars, and that the earth and planets revolved around the sun. The apparent motion of the stars was due to the daily rotation of the earth. Copernicus was familiar with Aristarchus' theory of the universe, which at the time, however, did not find favor with the ancient philosophers. |
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Hipparchus (2nd century BC), the greatest astronomer of the ancient world, is thought to have invented the astrolabe. He continued the work of Aristarchus in calculating the distance of the moon by measuring its parallax against the sphere of fixed stars. His result of 30 earth diameters is correct. From his observations he produced the first star catalogue, grading the stars by brightness. He also discovered the precession of the equinoxes. |
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Zhang Heng (78-139) was a Chinese astronomer, geographer, and mathematician. He constructed a celestial globe, believing that the world was round, "The sky is like a hen's egg, and is as round as a crossbow pellet; the Earth is like the yolk of the egg, lying alone at the centre. The sky is large and the Earth small." He also created a primitive, but very fanciful seismograph. His approximation of pi was the square root of 10. |
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Ptolemy (2nd century AD) was a famous astronomer, mathematician and geographer whose geocentric view of the universe was dogma in western thought until 1200 years later, when Copernicus was able to offer a competing heliocentric model which accounted just as well for many of its inconsistencies with observations. Ptolemy synthesized and extended the star catalogue of Hipparchus and much of Greek astronomy in his Almagest. |
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A Chinese astronomer and Buddhist monk of the Tang dynasty, Zhang Sui (683-727), was the first to describe proper stellar motion, or the apparent motion of stars across the plane of the sky relative to more distant stars. In Western astronomy, Edmond Halley is credited with this discovery in 1718 for some stars from Ptolemy's catalogue. |
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Ulugh Beg (1394-1449) was the grandson of Tamerlane, the Turkish conqueror of a large empire stretching from the Mediterranean to India. While acting as a ruler under his grandfather and father, the prince was an astronomer and builder of observatories in their capital city of Samarkand, where a stellar catalog was produced, the first since Ptolemy's and before Tycho Brahe. Permanent astronomical instruments of huge dimensions were constructed, since telescopes had not yet been invented. The most direct influence of the Samarkand Observatory was some centuries hence on the Indian Maharajah Jai Singh (1686-1743) who constructed five observatories, or Jantar Mantars, as shown on the Jaipur stamp below: |
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Nicolaus Copernicus (1473-1543), a Polish astronomer,
proposed a heliocentric theory of the universe in which the planets orbited
the sun, rather than the earth. It marked the end of the world with man
and the earth at its center. Ironically, Copernicus appears here on a
Vatican stamp for a theory that flew in the face of established religion
in his time. Although known portraits of Copernicus are full face, one
modern artist's conception shows a thoughtful man in profile. The French
stamp shows a picture of the universe as Copernicus envisioned it -- the
six known planets circling the sun. Silhouettes of the Polish churches
from whose towers Copernicus made his observations are in the foreground.
The souvenir sheet on the left shows the Copernican world view from a beautiful celestial atlas made by Andreas Cellarius, 
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 This colorful Chinese stamp approximates our current view of the solar system, even showing the asteroid belt between the orbits of Mars and Jupiter. It was issued in 1982 to mark a rare alignment of the planets twice during that year, in in March and May. The nine planets clustered into a relatively small fan-shaped area on the same side of the sun, in arcs of 96 and 105 degrees. The average cycle for such an event is 179 years, and the next one is estimated to happen in 2357. This clustering, or conjunction, is also called syzygy, a word worth remembering for a game of Scrabble. |
  A conjunction of planets is said to occur at their closest approach where the planes of their orbits intersect. When earth and either of the interior planets, Mercury or Venus, are in conjunction and are aligned with the sun, it is possible see the interior planet move across the disc of the sun and a transit takes place. During a normal conjunction the interior planet is seen to move above or below the sun. The mechanism is the same as for a solar eclipse, when the moon moves between the earth and the sun, but since it is nearer to the earth than Venus or Mercury it subtends a much larger angle and obscures the face of the sun entirely. The transits of Venus occur regularly in pairs separated by eight years and repeat the cycle over 243 years. The most recent observed transit was in 2004, the next will be in 2012. Measurements of the solar parallax at widely separated locations during transit can be used to calculate the distance from earth to sun, and in 18th century Europe expeditions were mounted to observe the rare phenomena of the transits of 1761 and 1769. Captain James Cook was dispatched to the South Pacific Ocean by the Admiralty in Great Britain for this purpose and his success in 1769 is noted on the New Zealand stamp. Maximilian Hell (1720-1792), a Jesuit astronomer based in Vienna, observed this transit in Lapland (now in Norway), following an invitation from the king of Denmark. |
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