2000 BCE
Mesopotamian and Egyptian astronomers begin to map the heavens. They create the 365-day year.
340 BCE
Aristotle reasons that the Earth is a sphere.
140 CE
Ptolemy maps the positions of approximately 1,000 stars.
1054 CE
Chinese astronomers record and describe a supernova.
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1543
Nicolaus Copernicus is the first European to claim that the Earth is not the center of the universe.
1608
Hans Lippershey invents the telescope.
1609
Galileo Galilei, using a telescope to make astronomical observations, discovers the phases of Venus, four moons of Jupiter, and mountains on the Moon.
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1633
Galileo, brought before the Inquisition, recants his sun-centered model of the universe.
1668
Isaac Newton builds the first reflecting telescope.
1775
Sir William Herschel constructs the largest reflecting telescope and discovers star clusters and nebulae.
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1847
A 15-inch refracting telescope is built in Cambridge, MA.
1897
A 40-inch refracting telescope is built at the University of Chicago. It is still the largest telescope of its type in the world.
1915
Einstein publishes his General Theory of Relativity.
1922
Russian physicist/ mathematician Alexander Freedmann predicts that the universe is expanding.
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1924
Edwin Hubble determines that spiral nebulae are distant galaxies outside the Milky Way. He categorizes the galaxies by type.
1929
Hubble observes that all galaxies are receding from each other at speeds proportional to their distances and concludes that the universe is expanding.
1952
Allan Sandage at Hale Observatories estimates the age of the universe at 20 billion years.
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1965
American physicists Arno Penzias and Robert W. Wilson use a radio telescope to discover background radiation from the big bang.
1974
An X-ray source is discovered in the constellation Cygnus, postulated to be a black hole.
1981
NASA launches the first space shuttle.
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