c = circa or approximately

c30,000BC			Bone carvings keep track of phases of Moon. Early people engraved patterns of lines on animal bones to keep track of the phases of the Moon.
c4000 BC			Mesopotamian astronomers made careful observations from the tops of pyramid-like towers called ziggurats.
c2500 BC			Building of Stonehenge. The building of Stonehenge apparently took place over many centuries. Alignments of the stones at Stonehenge mark the rising and setting points of the Sun at the solstices.
c2000 BC			Temple of Amen-Ra at Karnak. The Temple of Amen-Ra at Karnak, Egypt was built so that its main axis points to the sunset at the summer solstice.
c2000 BC			Lunar eclipse observed at Ur in Mesopotamia. The oldest known recording of a lunar eclipse took place at Ur more than 4000 years ago.
c1300 BC			Chinese begin centuries long series of observations of eclipses. Chinese astronomers recorded 900 solar and 600 lunar eclipses over a period of 2600 years.
c700 BC			Babylonians predict lunar eclipses. The Babylonians used their long record of eclipses to see regular patterns of eclipses. They also made the first calendar based on the Sun and moon movements.
c700 BC			Hesiod describes practical uses for astronomy. Hesiod's poem The Works and Days contains practical astronomical advice for navigation and for agricultural activities.
585 BC			Thales said to have predicted solar eclipse. The eclipse took place during a battle between the Lydians and the Persians. They were so stunned by the eclipse they ended the battle.
c580 BC			Anaximander describes model of Earth, Sun, Moon, stars. Anaximander's model was the forerunner of later Greek attempts to explain the heavens in non-mythological terms.
c560 BC			Anaximenes proposes model of cosmos. In Anaximenes model the stars are fixed to the inside of a solid vault surrounding the Earth. Later Greek astronomers develop this idea into the concept of the celestial sphere.
c550 BC			Pythagoras and students develop model of solar system. The model of Pythagoras used circular paths for the celestial bodies and assumed most celestial bodies are spheres.
c450 BC			Herodotus concludes Earth is at least thousands of years old. Herodotus reasoned that it would have taken millenia for the annual Nile flood to have produced the Nile delta.
413 BC			Lunar eclipse delays evacuation of Athenian army from Sicily. The Greeks regarded eclipses as uncertain omens. The delay doomed the Athenian army.
c400 BC			Eudoxus explains retrograde motion. Eudoxus's explanation involved the rotation of spheres in opposite directions. This geocentric model had the Earth at its center.
c350 BC			Aristotle argues celestial bodies are spheres. Aristotle used a number of proofs that the Earth is a sphere, including the observation that its shadow on the Moon during lunar eclipses is always a circle.
c280 BC			Aristarchus finds relative dimensions of solar system. Aristarchus concluded that the Earth was much smaller than the distances to the celestial bodies. He also invented a heliocentric (Sun-centered) model for the solar system.
c250 BC			Eratosthenes finds circumference of Earth. Eratosthenes uses observations of the altitude of the Sun to find the circumference of the Earth. His estimate may have been accurate to within a few percent.
134 BC			Hipparchus discovers precession, prepares stellar catalog. Hipparchus compared his own observations with earlier ones to discover precession, the slow change in the direction of the Earth's polar axis. He also made what was probably the first catalog of the positions and brightnesses of the stars.
c 0 AD			Building of Bighorn Medicine Wheel. Plains Indians of North America built medicine wheels, monuments made of piles of stones. Alignments in the medicine wheels often pointed toward the direction of sunrise at the winter solstice.
c140			Ptolemy "perfects" geocentric model of solar system. In Ptolemy's model the planets moved on circles (epicycles) that moved on other circles (deferents). The model could accurately predict the positions of the planets. Ptolemy also compiled a catalog of stellar brightnesses based partly on the earlier catalog of Hipparchus.
c165			Chinese Astronomers are the first to record sunspots.
1054			Chinese record supernova that produces Crab Nebula. Chinese astronomers observed a supernova that was visible in the daytime. The matter blasted outward by the supernova later became observable as the Crab Nebula. Also Known as Messier #1, or M1 the first nebula object recorded by Charles Messier.
1066			Comet Halley considered ill omen for King Harold. The appearance of Comet Halley in 1066 was considered an ill omen for Harold, King of England. Later that year Harold was killed in the Norman invasion of England.
c1200			Establishment of first universities in Europe. The development of astronomy was aided by the birth of universities at Bologna, Oxford, Paris, and a few other European cities.

1300			Dante describes medieval picture of universe in "Divine Comedy". Dante's picture of the universe has the Earth at its center, surrounded by the spheres of the Moon, Sun, planets, the fixed stars, a crystalline sphere and, finally, paradise.
c1330			Jean Buridan develops impetus theory of motion. Buridan's theory of motion was remarkable in that it contradicted the established ideas of Aristotle.
c1350			Oresme describes "Galilean" relativity. Oresme pointed out that we perceive only relative motion and that the daily motion of the stars could be explained either by the rotation of the Earth or the rotation of the celestial sphere.
c1420			Ulugh Beg builds observatory at Samarkand. Beg also compiled a star catalog based on his own observations.
c1465			Regiomontanus uses printing to produce astronomy books and tables. Regimontanus used the recently invented art of printing to produce books, almanacs, and tables of predictions of the positions of the Sun, Moon, and planets.
c1500s			Possibly the first reflecting telescope is developed by Leonard Digges.
1504			Columbus uses lunar eclipse prediction to influence Arawaks. From his almanac, Columbus knew that a lunar eclipse would occur on February 29. He impressed the Arawaks with his accurate prediction that the Moon would rise "inflamed with wrath".
1543			Copernicus publishes "De Revolutionibus". De Revolutionibus, Copernicus's description of his heliocentric model of the solar system, (Sun in center) was published two months before his death.
c1550			Leonard Digges makes Possibly the first refracting telescope.
1572			Tycho Brahe observes a supernova. Tycho's book about his observations of the supernova established his reputation as an astronomer.
c1580			Tycho carries out the best pre-telescopic observations ever. Tycho's observations, made using specially built instruments, were the most accurate ever made with the naked eye. Tycho also invented his own geocentric model of the solar system.
1596			Tycho Brahe completes the best star catalogue of pre-telescopic times.
c1600			Galileo begins experiments with falling and rolling bodies. Galileo's experiments led him to conclude that once something is set in motion it will remain in motion unless something stops it. This contradicted earlier ideas that said only rest was a natural state.
1600			William Gilbert proposes Earth has dipole magnetic field. Gilbert proposed that the Earth acts like a big magnet whose field aligns the small magnet used as a compass needle.
1600			Giordano Bruno burned at stake. Bruno was tried before the Inquisition and burned for heresy. Among his heresies was the idea that there were Earth-like, inhabited planets orbiting the infinitely many stars.
1604			Kepler observes supernova. The telescope was invented a few years after Kepler carried out extensive naked-eye observations of the supernova of 1604. Since the invention of the telescope no supernovas have been observed in the Milky Way.
1608			Dutch spectacle maker Hans Lippershay filed a patent application in Holland on October 2, 1608, requesting exclusive rights to make and distribute "an instrument for seeing at a distance", consisting of a weak positive objective lens and a strong negative eyepiece. Lippershay constructed a telescope on request and was tested successfully; he received an initial contract for construction of up to three instruments. Later objections by two other Dutch spectacle makers (Adriaanzoon and Jansen ) confuse the issue and Lippershay receives no patent.
1609			Galileo uses telescope for astronomical observations. Galileo didn't invent the telescope but he was among the first to use a telescope to examine the heavens. He carried out important observations of the Sun, Moon, Planets, and Stars.
1609			Galileo observes Moon. Galileo found that the Moon had mountains, valleys, and plains like the Earth. He called the dark regions of the Moon maria, the Latin word for seas.
c1610			Kepler discovers laws of planetary motion. Working with Tycho's observations, Kepler discovers the shapes of planetary orbits, how the speed of a planet varies as it orbits the Sun, and the relationship between orbital distance and orbital period.
1610			Galileo observes phases of Venus. Galileo's found that Venus shows all the phases from new to full. This observation was incompatible with the Ptolemaic model of the solar system.
1610			Galileo discovers four largest satellites of Jupiter. Galileo found that Jupiter is orbited by four large satellites, now called, collectively, the Galilean satellites. This proved that at least some celestial bodies didn't orbit the Earth.
1620			Francis Bacon suggests that Earth's continents move about. Bacon noticed that the eastern and western shores of the Atlantic were parallel and could be fitted together.
c1630			Descartes develops concept of inertial motion. Descartes believed that all motion resulted from collision with particles called "corpuscles". In the absence of such collisions, a body remains at rest. An object in motion continues to move in the same direction at the same speed.
1632			Galileo publishes "The Dialogue". The Dialogue, although superficially a balanced debate about the merits of the geocentric and heliocentric models of the solar system, was in fact a powerful argument for the ideas of Copernicus. Galileo was brought before the Inquisition and spent the last nine years of his life under house arrest.
1640-1700			Maunder minimum. During the Maunder minimum almost no spots were seen on the Sun. This was also a time of cold climate in Western Europe and North America.
1654			Bishop Ussher uses Bible to calculate age of Earth. From the chronology of Biblical events, Ussher concluded that the Earth was created in 4004 BC.
1659			Christiaan Huygens realizes that the "appendages" of Saturn are rings. Huygens watched the appendages disappear in 1665 and reappear several years later. He realized that the appendages were really flat rings that disappeared when viewed edge on.

1665			Giovanni Cassini discovers Great Red Spot of Jupiter. Although its size and darkness have changed with time, the Great Red Spot has been continuously present since the time of Cassini's discovery.
1666			Robert Hooke shows that a central force leads to orbital motion. Hooke used a pendulum to demonstrate to the members of the Royal Society that in order to stay in orbit, the planets must be continually pulled toward the Sun.
1665-7			Newton discovers law of universal gravitation. When Cambridge University was closed by the plague, Newton spent most of the next two years at his family farm. During this period he made fundamental discoveries in optics, discovered the law of universal gravitation, and invented differential and integral calculus.
1667			G. Montanari notices brightness variations of Algol. Montanari saw that Algol occasionally dropped to a third of its normal brightness. Later, the drops in brightness were found to be caused by eclipses.
1668			James Gregory makes the first realistic estimate of the distances of the stars. Gregory assumed that the other stars were just as bright as the Sun and then calculated how distant they had to be to match their apparent brightnesses.
1678			Christian Huygens proposes that light consists of waves. Huygens's ideas were disputed by Newton, who proposed that light was made up of a stream of particles.
1682			Edmund Halley predicts return of Comet Halley. Halley noted that comets with similar orbits had appeared in 1456, 1531, 1607, and 1682. He proposed that these were all the same comet and that it would return in 1758 or 1759 - which it did.
1686			Newton publishes "Principia". Newton's monumental work described his discoveries about gravity, motion and the orbits of the planets.
1718			Edmund Halley discovers stars move through space. Halley found that the positions of stars change with time. He explained the changes in position as due to the individual motions of stars through space.
1729			James Bradley discovers the aberration of starlight. Bradley found that the positions of all the stars shift back and forth as part of an annual cycle caused by the motion of the Earth about the Sun.
1781			William Herschel discovers Uranus. While measuring the directions and brightnesses of stars, Herschel found a fuzzy spot that moved among the stars. This was Uranus, the first planet that was not known to the ancients.
1781			Charles Messier prepares list of nebulae. Like other comet hunters, Messier often mistook nebulae for comets. He compiled a list of 103 nebulae as an aid to other comet hunters. This was the first list of nebulae.
1783			John Goodricke discovers eclipses of Algol. Goodricke found that the brightness declines of Algol occur at regular intervals. He proposed that the brightness changes are due to eclipses of Algol by its binary companion.
1783			British astronomer John Mitchell is the first to propose black holes, although many did not believe him.
1783			William Herschel discovers speed and direction of Sun's motion. Herschel analyzed the motions of seven bright stars and showed that part of their motions was due to the motion of the Sun through space.
1785			William Herschel uses star counts to map Milky Way. Herschel assumed that the galaxy extended farther in directions in which he could see more stars. He found the galaxy to be flattened with the Sun near the middle. The next year he published his catalogue of nebulae, which was later expanded into the "New galactic catalogue."
1790			Pierre Simon Laplace proposes stars can produce black holes. Laplace proposed that if a star is so compact that its escape velocity exceeds the speed of light, then not even light can escape from the star.
1801			Giuseppe Piazzi discovers Ceres. Piazzi discovered Ceres, the first known asteroid, on January 1, 1801, the first day of the 19th century.
1801			William Herschel shows many double stars are binaries. Herschel found that for many pairs of stars, the orientation of the two stars changes with time. The changes are a result of orbital motion.
1802			William Wollaston sees dark lines in solar spectrum. Wollaston passed sunlight through a prism and noticed that there were numerous dark bands and lines in the spectrum.
1803			Meteorite shower at l'Aigle convinces scientists that meteorites have an extraterrestrial origin. A careful investigation of the shower by Edouard Biot convinced most skeptics that meteorites really do fall from the sky.
1820			The British "Royal Astronomical Society" is established.
1824			The first telescope to be mounted equatorially with clock drive is made by Joseph von Fraunhofer.
1833			Denison Olmstead discovers shower meteors come from a common point in the sky. Olmstead realized that the meteors seem to diverge from a point in the sky because they originate in a swarm of meteoroids moving on parallel paths through space.
1835			Gaspar de Coriolis discovers Coriolis effect. The Coriolis effect, the apparent deflection of moving bodies due to Earth's rotation, explained many atmospheric circulation patterns.
1837			h Carinae brightens to become second brightest star. h Carinae is normally too faint to be seen, but between 1837 and 1860 it was the brightest star in the sky.
1838			Friedrich Bessel, Wilhelm Struve, Thomas Henderson measure the distances of stars. Bessel, Struve, and Henderson, working independently, almost simultaneously measured the parallaxes, and hence the distances, of nearby stars. These were the first measurements, rather than estimates, of stellar distances.
1842			Christian Johann Doppler describes the Doppler effect. Doppler discovered that the wavelength and frequency of a wave change if the source of the wave moves toward or away from the observer.
1844			The first official observatory in the U.S.A. is established at Harvard.

1846			Calculations of Adams and Leverrier lead to discovery of Neptune. Adams and Leverrier independently calculated the location of the unknown planet, Neptune, that was required to explain discrepancies in the orbit of Uranus.
1851			Jean Foucault uses pendulum to demonstrate Earth rotates. Foucault showed that the pendulum swung in the same plane but the Earth rotated under it, causing an apparent change in the direction of the pendulum's swing.
1859			James Clerk Maxwell discovers velocity distribution function for a gas. Maxwell showed that the distribution of velocities of the atoms or molecules in a gas depends on temperature and the mass of the molecules.
1862			William Huggins identifies chemical elements in stars. Huggins studied the spectra of bright stars and found that the dark lines in their spectra matched the wavelengths of atoms measured in terrestrial laboratories. The companion star to Sirius is found by the American telescope maker Alyan Clark. This is the first white dwarf to be discovered.
1871			Asaph Hall discovers Phobos and Deimos. Remarkably, the prediction that Mars had two small satellites was made by Jonathan Swift 151 years before Hall's discovery.
c1875			Lord Kelvin and Hermann von Helmholtz estimate the age of the Sun. Kelvin and Helmholtz independently calculated the length of time it would have taken for the Sun to have shrunk to its present size. This time, called the Kelvin-Helmholtz time, is about 20 million years.
1877			Giovanni Schiaparelli discovers "canals" of Mars. Schiaparelli reported thin, dark lines crisscrossing the surface of Mars. The discovery raised the possibility of intelligent life on Mars.
1879			Joseph Stefan finds the rate at which a blackbody emits energy. Stefan found that the energy emitted by a blackbody increases as the fourth power of temperature.
1894			Wilhelm Wien finds how temperature affects the color of a blackbody. Wien found that a blackbody gets bluer as its temperature increases.
1900			Max Planck presents formula of spectral distribution of a blackbody. Planck found the way that the energy emitted by a blackbody depends on its wavelength and the temperature of the blackbody.
1904			J. Hartmann finds interstellar absorption lines. Hartmann found a very narrow line of calcium that didn't change in wavelength as the spectral lines from the two stars in a binary star system shifted back and forth.
1905			Albert Einstein explains the photoelectric effect. Einstein explained that the emission of electrons only by light at short wavelengths occurs because light consists of bundles of energy called photons.
1905			Einar Hertzsprung plots absolute magnitude versus spectral type. Hertzsprung found that the stars are concentrated in a few regions of such a diagram, which became known as an Hertzsprung-Russell (or H-R) diagram.
1905			Jacobus Kapteyn uses star counts to map Milky Way. Using star counts, Kapteyn determined that the Sun lay 2000 pc from the center of a flattened galaxy.
1913			Henry Norris Russell independently invents H-R diagram. The H-R diagram became an important tool for understanding the evolution of stars.
1915			Alfred Wegener proposes continental drift. Wegener noted the similarity of rocks on opposite sides of the Atlantic Ocean and proposed that the present continents had been part of a super continent that broke apart about 200 million years ago.
1916			Albert Einstein's general theory of relativity. Einstein explained that matter curves space, causing bodies to move in ways we attribute to gravity.
1916			Karl Schwarzschild calculates geometry of black hole. Schwarzschild found that if a massive body is compressed to a very small size it curves space around it so severely that it forms a black hole.
1917			100" Mt. Wilson telescope completed. The Mt. Wilson telescope was the largest optical telescope in the world for 31 years until the Palomar reflector was completed.
1917			V. M. Slipher obtains radial velocities for 25 galaxies. Slipher found that 21 of the 25 galaxies had red shifted spectral lines, indicating that they are moving away from the us.
1918			Harlow Shapley find the size and shape of the Milky Way. Shapley assumed that globular clusters are distributed uniformly about the center of the Milky Way. From this, he found that the Earth is located 15,000 pc from the center of the Milky Way.
1918			Annie J. Cannon and co-workers present thousands of stellar spectral classifications in the Henry Draper Catalogue. Cannon and her co-workers devised a classification system for stellar spectra and used it to produce a catalog of spectra classifications for about 225,000 stars.
1920			M.N. Saha shows that the temperature of a star determines the appearance of its spectrum. Saha showed that temperature differences rather are responsible for the existence of different spectral classes of stars.
1920			A. S. Eddington proposes that fusion powers the Sun. Eddington suggested that the fusion of hydrogen, the most common element in the Sun, into helium provides the enormous energy output of the Sun.
1923			Edwin Hubble shows spiral nebulae are galaxies. Hubble identified individual Cepheid variable stars in spiral nebulae and used them to show that the spiral nebulae are huge collections of stars far from the Milky Way.

1925			Cecilia Payne shows stars of different classes have essentially the same chemical composition. Payne determined the chemical composition of a number of stars of different spectral classes and found that they were nearly the same.
1928			Arthur Holmes proposes mantle convection drives continental drift. Holmes proposed that convection currents in the layer Beneath the Earth's crust push the continents about.
1929			Edwin Hubble discovers universe is expanding. Hubble found that the speed of recession of galaxies increases with distance. He explained that this is due to the expansion of the universe.
1930			S. Chandrasekhar shows white dwarf stars are made of degenerate electrons. Chandrasekhar also showed that the more massive a white dwarf, the smaller it is and that there is a maximum mass, the Chandrasekhar limit, that a white dwarf can have.
1930			Clyde Tombaugh discovers Pluto. Tombaugh discovered Pluto by comparing photographic plates taken of the same region of the sky about a week apart. Pluto's image moved among the stars.
1930			Robert Trumpler discovers diffuse interstellar dust. Trumpler found that distant star clusters were bigger than nearby star clusters. He reasoned that this was because interstellar dust made distant clusters look fainter and, hence, more distant.
1931			Karl Jansky makes first radio astronomy observations. Jansky found that the Milky Way galaxy is a source of radio emission.
1937			Grote Reber builts first radio telescope. Reber built the first antenna specifically designed for radio astronomy. Using the radio telescope, Reber made the first map of cosmic radio emission.
1939			Robert Oppenheimer and George Volkoff calculate properties of neutron stars. Oppenheimer and Volkoff calculated that a neutron star would be only about 10 km in radius.
1944			H.C. van de Hulst predicts 21 cm line of interstellar hydrogen. van de Hulst calculated that interstellar hydrogen atoms emit a spectral line at a wavelength of 21 cm in the radio part of the spectrum. He suggested that it would be possible to detect the 21 cm line using radio telescopes.