![]() Vera Rubin: The astronomer who brought dark matter to lightĬlyde Tombaugh: Astronomer who discovered Pluto ![]() Though he made other contributions to mathematics and optics, Kepler's three laws made him a giant of astronomy. Kepler's third law, published a decade later, posited that the relationship between the length of two planets' orbits is related to their distances from the sun. Kepler's deduction that the planets travel elliptical orbits, with the sun at one of the foci formed his first planetary law, which he published in 1609 with the second law which stated that planets do not travel at the same rate throughout their orbits. Kepler defended and modified the Copernican view of the solar system with a radical reformation that established him as one of the great lights of the Scientific Revolution of the 16th-17th centuries. However, closed minds put Kepler's work at risk. In so doing, he calculated three laws involving the motions of planets that astronomers still use in calculations today. Using detailed measurements of the path of planets kept by Danish astronomer Tycho Brahe, Johannes Kepler (1571–1630) determined that planets traveled around the sun not in circles, as Copernicus had thought, but in ellipses. His ideas took almost a hundred years to gain credence, but Galileo's 1632 assertions that the Earth orbited the sun built upon the Polish astronomer's work, cementing the Copernican revolution. Copernicus published his book, De Revolutionibus Orbium Coelestium (" On the Revolutions of the Heavenly Spheres") when he was 70 and lay dying. The model wasn't completely correct, as astronomers of the time struggled with the backwards path Mars sometimes took, but it eventually changed the way many scientists viewed the solar system.Ĭopernicus, of Poland, felt the Ptolemaic view of the planets traveling in circular orbits around the Earth was over-complicated with many smaller circles, epicycles, needed to explain the intermittent retrograde motion of the planets (in which they appear to move in the opposite direction of the the stars). In 16th century Poland, astronomer Nicolaus Copernicus (1473–1543) proposed a model of the solar system that involved the Earth revolving around the sun, according to NASA. Persian astronomer Abd al-Rahman al-Sufi (903–986), known as Azophi to Westerners, made the first known observation of a group of stars outside of the Milky Way, the Andromeda galaxy. (Image credit: Bartolomeu Velho) Abd al-Rahman al-Sufi 29, 45 B.C.The Ptolemaic geocentric model of the universe, devised by the Greek scientist Claudius Ptolemy, had everything revolving around Earth. June 19, 1939: Dreaded ALS Gets a New Name - Lou Gehrig's Diseaseįeb. One reason is that Eratosthenes' very own library of Alexandria had been destroyed, and there was no complete backup of its data. But it was just not widely known among the masses in 15th-century Europe. Grade-school tales aside, it was thus known long before Columbus that the Earth was round and even how big it is, approximately. He suggested that calendars should have a leap day every fourth year, an idea taken up two centuries later by Julius Caesar. He also deduced the length of the year as 365¼ days. Pretty darn good for a guy without modern measurement tools.Įratosthenes went further and computed the tilt of the Earth's axis to within a degree. The accepted figure for equatorial circumference today is 24,902 miles. So how big is 252,000 stades? Depending on which classical source you trust, it's somewhere between 24,663 and 27,967 miles. (We know he knew it was just a rough estimate, because he adjusted his initial number of 250,000 upward by 2,000 - or 0.8 percent - to make it divisible by 60 or 360 for easy computation.) And we don't know today the exact size of the measurement unit Eratosthenes was using when he came up with the final figure of 252,000 stades.
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