The first people who kept astronomical records were the Akkadians who lived in northern Babylonia. Babylonian priest were able to use their data to predict the course of wandering objects in the sky. The Chinese also kept records, and there is evidence they witnessed a close alignment of the planets around 2500 B.C. They divided the stars into constellations, and the first atlas of comets, "The Book of Silk", was written by them. The Chinese also created one of the first known calanders. Even in ancient times, there were seven day weeks to match each quarter phase of the Moon, and twelve months for twelve complete cycles of the Moon.
The Egyptions were also great calander-makers. They studied the star Sirius closley for its rising signaled the annual flood of the Nile River. They also built the pyramids, which were used to study the motion of the Sun. The Greeks were the first to change astronomy from a mystical cult to a science. Thales of Miletus, the first of the great Greek philosophers, studied the records of the Babylonians and thought the Earth was flat and floated on water like a log. Aristotle, the greatest of the Greek philosophers, put foward three proofs to show the Earth was round. He also believed the Earth was the center of the universe and that the Sun, Moon, planets, and a sphere of fixed stars revolved around it. Ptolemy of Alexandria studied centuries of Babylonian observations of the motions of the planets to perfect his theory of an Earth-centerd universe. He published his ideas in the work "Almagest" in 140 A.D., and this would rule astronomy for 1,500 years.
EARLY EUROPEAN ASTRONOMY
The first to challenge the theory of an Earth-centered universe was Mikolaj Kopernik, better known as Copernicus. His theory was simple: The Sun is the center of the Solar System, and the Earth and the other planets orbit it in perfect circles.Copernicus knew he would meet hostility from the Church, so he waited until just before his death to publish his theory in "On the Revolution of the Celestial Spheres". Copernican theory was condemned as hersey by the Church. Another early European astronomer was Tycho Brahe, who published the most accurate star catalouges at the times. He also made detailed observations on the posistions of the planets, particulary Mars.
Johannes Kepler, a student of Tycho Brahe, used Brahe's observations to prove that the Earth is not the center of everything. He also used them to formulate his famous laws of planetary motion. Sir Isaac Newton published his laws of motion in "Principia" in 1687. These two theories combined marked the true beginning of the modern phase of astronomy. Its study advanced further when Galileo Galilei built a telescope that magnified objects 30 times and turned it to the heavens. Galileo studied the Moon, the phases pf Venus, and thousandsof stars in the Milky Way. He also discovered spots on the Sun and the four largest moons of Jupiter.
THE AGE OF THE GREAT TELESCOPES
The first great telescope was built by William Herschel in 1789. It had a 49 inch mirror, but was poorly made and practically useless. Herschel's important discoveries, such as the planet Uranus in 1781, came with much smaller instruments. In 1845, the Earl of Rosse built a large 72 inch reflecting telescope, which was used to discover the "spiral nebulae", which were determined later to be other galaxies. The Rosse telescope remained the world's largest until 1917 when the 100 inch reflector at Mount Wilson, California was completed.
In the late 19th century, several large refracting telescopes were built. The largest refractor is the 40 inch telescope at the Yerkes Observatory in Wisconsin. The Yerkes refractor was built in 1897. The second largest is the Lick Observatory in California, with a 36 inch refractor completed in 1888. Others include the Potsdam refractor (31 inches), the Meudon refractor (33 inches), and the 29 inch refractor of the Nice Observatory in France. All of these refractors were built in the 1880's and 1890's.
The greatest discovery of the 19th century was the planet Neptune. in 1846. The discovery came from the efforts of four astronomers from three different nations. U.J.J. LeVerrier of France and J.C. Adams of England "discovered" the planet on paper by calculating mathmatically possible posistions of Neptune. Johann Galle and Heinrich D'Arrest, both of Germany, were the first to view the planet, using the posistion calculations of Adams and LeVerrier as a guide. Uranus, Neptune, and later Pluto are the only planets to be discovered in modern times.
THE TWENTIETH CENTURY
Albert Einstein made the first major contribution to twentieth century astronomy. In 1905, he published his special theory of realitivity and his general theory of realtivity in 1916. These theories answered many of the fundimental questions of the universe. Edwin Hubble made his mark in the 1920's. In 1923, he proved that the "spiral nebulae" were actually galaxies outside of our own Milky Way. In 1929, Hubble showed that the universe is expanding; now called the "Hubble constant". The third planet in modern times, Pluto, was discovered by Clyde Tombaugh at the Lowell Observatory in 1930.
In the 1930's and 40's, a new type of astronomy was born and took shape: radio astronomy. In 1931, the first radio waves were detected from space. Six years later, the first radio waves from the center of the galaxy were heard. In 1942, it was discovered that the Sun sends out waves, and in 1946, the most powerful radio source in the universe, Cygnus A, was found. Today, radio astronomers listen to the sounds of space, hoping to hear a pattern in the noise that could indicate a transmission from other intelligent beings.
The era known as the space age was born in late 1957 when the Soviet union launched the first artificial Earth satellite, Sputnik 1. The Soviets progressed further by launching the first human, Yuri Gagarin into orbit in 1961. From that time on, what was known as"the space race" was carried out between the United States and the Soviet Union. Throughout the 1960's, the were spacewalks, space dockings, satellites launched, as each nation would match what the other had accomplished. The United States claimed vixtory in the space race when Neil Armstrong and Edwin "Buzz" Aldrin became the first men to walk on the Moon in 1969.
Exploration of the planets by space probes began in ernest when Mariner 9 visited Mars in 1971 and 1972. Pioneer 10 made the first fly-by of Jupiter in 1973, and the first detailed pictures of Mercury were taken by Mariner 10 in 1974. In 1979, Pioneer 10 became the first spacecraft to fly by Saturn. By far the most successful space exploration missions are those of the Voyager spacecraft. Both visited Jupiter in 1979, and Saturn in 1981 and 1982. As Voyager 1 flew out of the Solar System, Voyager 2 proceeded to Uranus in 1986 and Neptune in 1989. The first spacecraft to visit a comet was Giotto, which visited Comet Halley in 1986. Two planets, Venus and Mars, have actually had landed on. Venera 9 was the first to land on Venus in 1773, and Mars was visited by the Viking spacecraft in 1974 and 1975.
More spacecraft visit the planets today.Recent missions include Magellan, which used radar to map the entire surface of Venus, the Pathfinder landing on Mars, the mapping of the Moon by Clemintine, and the Galileo probe to Jupiter. Future missions include the mars Polar Lander, Cassini, which will drop a lander onto the surface of Titan, the largest moon of Saturn, Stardust, which will return a sample of a comet to Earth, and the Pluto Express, which will visit Pluto, which remains the only planet to yet have been visited by a spacecraft.
Great observatories were built during this century, too. In 1917, the 100 inch reflector at Mount Wilson, California was built. Years later, in 1948, the Mount Palomar 200 inch reflector was comissioned. The 150 inch telescope at Kitt Peak, Hawaii, opened in 1973. The largest radio telescope ever built, the Very Large Array (VLA) began to listen to the sky in 1980. Last is the crown jewel of telescopes, the Hubble Space Telescope. Placed in Earth orbit in 1990, and repaired in 1993 to correct some flaws, the hubble has allowed us to see deeper into space than ever before- almost 13 billion light-years into the past, which is near the beginning of time itself.
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