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Astronomy Timeline
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30,000B.C. - 1200 A.D.
1265 A.D. - 1659 A.D.
1665 A.D. - 1842 A.D.
1846 A.D. - 1923 A.D.
1925 A.D. - 1965 A.D.
1967 A.D. - 2002 A.D.

c30,000 BC



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. p. 17, F OV 1.6.

c4000 BC



Mesopotamian ziggurats serve as observatories. Mesopotamian astronomers made careful observations from the tops of pyramid-like towers called ziggurats.

c2500 BC



Building of Stonehenge. The building of Stonehenge took place over many centuries. Alignments of the stones at Stonehenge mark the rising and setting points of the Sun at the solstices. p. 2.4, F 1.1, p. 32.

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. p. 33.

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. p. 37, F 1.15.

c1300 BC



Chinese begin centuries long series of obs of eclipses. Chinese astronomers recorded 900 solar and 600 lunar eclipses over a period of 2600 years. p. 46.

c700 BC



Babylonians predict lunar eclipses. The Babylonians used their long record of eclipses to see regular patterns of eclipses. They used these patterns to predict lunar eclipses.

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. p. 38, F 1.16.

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. p. 39.

c500 BC



Xenophanes concludes that the Earth is very old. Xenophanes reasoned that stratified rocks were laid down as layers of sediments on the ocean floor. Given the thickness of the rocks, he concluded that the Earth is ancient. p.160.

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. p.160.

413 BC



Lunar eclipse delays evacuation of Athenian army from Sicily. The Greeks regarded eclipses as uncertain omens. The delay doomed the Athenian army. p. 199.

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. p. 36, F 1.14.

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. p. 39-40, F 1.16.

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. p. 41-44, F 1.20, F 1.21, F1.22

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. p. 40-41, F 1.19

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. P. 28




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. p. 45-46, F 1.25 p. 365-366




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. p. 140-141, Box F 4.5




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. p. 309-310, F 10.8, F 10.9, F 10.10




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.




Roger Bacon advocates experimentation. Bacon was among the first to recommend experimentation as the best way to acquire scientific knowledge.




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.




Jean Buridan develops impetus theory of motion. Buridan's theory of motion was remarkable in that it contradicted the established ideas of Aristotle.




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. p. 52.




Ulugh Beg builds observatory at Samarkand. Beg also compiled a star catalog based on his own observations.




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.




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".




Copernicus publishes "De Revolutionibus". De Revolutionibus, Copernicus's description of his heliocentric model of the solar system, was published two months before his death. p. 46-47, F 1.25, F 1.26, A1.26




Tycho observes supernova. Tycho's book about his observations of the supernova established his reputation as an astronomer. p. 48-49




Tycho carries out 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. p. 48-49




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. p. 53




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. p. 169




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. p. 47




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.




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. p. 51




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. p. 52




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. p. 49-50, F 1.28, F 1.29




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. p. 52, F 1.29




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. p. 52




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. p. 162-163, F 5.11, F 5.12, F 5.13, F 5.14. A5.13




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. p. 80-81, F 2.1




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. p. 53.




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. p. 349, F 11.25




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. p. 160




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. p. 283, F 9.12



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. p. 274-275, F 9.1


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. p. 82.


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. p. 85, F 2.7


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. p. 376


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. p. 356-358, F 12.1, F 12.2


Christian Huygens proposes that light consists of waves. Huygens's ideas were disputed by Newton, who proposed that light was mae up of a stream of particles. p. 96-97. F. 3.1, A3.1


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. p. 309-310, F 10.8, 10.9, 10.10


Newton publishes "Principia". Newton's monumental work described his discoveries about gravity, motion and the orbits of the planets. p. 80-86, F 2.2, F 2.3, F 2.4, F 2.5, F 2.6, F 2.7, F 2.8


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. p. 463-466


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.


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. p. 54, p. 480


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. p. 141, Box F 4.5, p. 480 (left out F 16.1)


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. p. 372


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. p. 371-373, F 12.10


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. p. 447-448, F 15.4


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. p. 431


Giuseppe Piazzi discovers Ceres. Piazzi discovered Ceres, the first known asteroid, on January 1, 1801, the first day of the 19th century. p. 304-307, F 10.4, F 10.5, F 10.6, A 10.6


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. p. 374, F 12.13, A12.13


William Wolleston 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. p. 111-112, F 3.15


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. p. 315-316


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. p. 315-316


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. p. 172-173, F 5.21, F 5.22


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.


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. p. 356-358


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. p. 114-115, F 3.18, A 3.18



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. p. 291-292, F 9.23


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. p. 147


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.


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. p. 366-367.


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. p. 264, F 8.24


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. p. 336


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. p. 264-265, F 8.25


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. p. 104-105.


Wilhelm Wien finds how temperature affects the color of a blackbody. Wien found that a blackbody gets bluer as its temperature increases. p. 104-105.


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 blakbody. p. 104-105.


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. 


Albert Einstein explains the photoelectric efect. Einstein explained that the emission of electrons only by light at short wavelengths occurs because light consistes of bundles of energy called photons. p. 97, F 3.2


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. p. 377-379, F 12.17, F 12-18.


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. p. 447-448


Henry Norris Russell independently invents H-R diagram. The H-R diagram became an important tool for understanding the evolution of stars. p. 377-379, F 12.17, F 12-18


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 supercontinent that broke apart about 200 million years ago. p. 162-163, F 5.13


Albert Einstein's general theory of relativity. Einstein explained that matter curves space, causing bodies to move in ways we attribute to gravity. p. 527


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. p. 431-432, F 14.12


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.


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. p. 492-493, F 16.16


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. p. 447-449, F 15.6


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. p. 368-370, F 12.9


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. p. 369


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. p. 336.


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. p. 481-485, F 16.1, F 16.3, F 16.4, F 16.5, F 16.6, F 16.7, F 16.8, F 16.9.


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. p. 369


Arthur Holmes proposes mantle convection drives continental drift. Holmes proposed that convenction currents in the layer Beneath the Earth's crust push the continents about. p. 161, F 5.10


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. p. 526-527, F 17.12


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. p. 422


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. p. 295, F 9.27, 9.28, 9.29


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. p. 458-460, F 15.13, F 15.14, F 15.15, F 15.16.


Karl Jansky makes first radio astronomy observations. Jansky found that the Milky Way galaxy is a source of radio emission. p. 469.


Grote Reber builds 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. p. 137-138, F 4.12


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. p. 424-431, F 14.5, F 14.6, F 14.7, F 14.8, F 14.9, F 14.10, F 14.11


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. p. 462-463, Box F 15.1


200" Palomar telescope completed. The Palomar telescope was the world's largest high quality optical telescope for over 40 years. p. 136


Jan Oort predicts existence of Oort Cloud of comets. Oort analyzed the orbits of comets entering the inner solar system for the first time. He proposed that these new comets originate in a cloud of comets tens of thousands of astronomical units from the Sun. p. 307, F 10.11.


Harold Ewen and Edward Purcell detect 21 cm line. Ewen and Purcell used a radio telescope to detect emission from interstellar hydrogen atoms. p. 462-463, Box F 15.1.


Gerard Kuiper proposes existence of Kuiper Belt of comets. Kuiper proposed that the comets with periods of less than 200 years originate in a flatted belt of comets whose inner edge lies just beyond the orbit of Neptune. p. 311-312, F 10.11.


James Van Allen discovers Van Allen radiation belts. Explorer 1, the first satellite launched by the United States, carried a Geiger counter built by Van Allen. The Geiger counter showed that there are zones of trapped energetic ions and electrons beyond Earth's atmosphere. p. 171-172, F 5.20.


Frank Drake uses radio telescope to search for interstellar signals. Drake searched at a wavelength of 21 cm for artificial signals from creatures on planets orbiting two nearby stars. No signals were detected. p. 462-463, Box F 15.1.


Robert Leighton discovers solar oscillations. Leighton found that the Sun vibrates at a variety of frequencies.


Harry Hess suggests mid-ocean ridges due to plate tectonics. Hess suggested that the mid-ocean ridges occur where the ocean floor splits apart due to plate tectonics and magma oozes out to form new ocean floor. p. 161-164, F 5.11


Horace Babcock proposes model for sunspot cycle. Babcock's model involved the twisting of solar magnetic fields lines because the rate of solar rotation varies with solar latitude. p. 346-347, F 11.23, 11.24, 11.25


Raymond Davis builds first solar neutrino telescope. Davis used a large tank of cleaning fluid a mile deep in a gold mine to detect neutrinos produced in nuclear reactions in the Sun's core. p. 339-340, F 11.12


Maarten Schmidt shows quasars have large redshifts. Schmidt found that previously unidentified lines in the the spectrum of the quasar 3C 273 were actually redshifted lines of hydrogen. This showed that quasars are moving rapidly away from us and are extremely distant. p. 500, Box F 16.2


Arno Penzias and Robert Wilson discoverd cosmic background radiation. Penzias and Wilson used a radio telescope to detect the highly redshifted radiation from the early stages in the expansion of the universe. p. 521-522, F 17.7, 17.8


C.-C. Lin and Frank Shu explain spiral arms of Milky Way. Lin and Shu explained that the spiral arms are the crests of density waves that rotate through the galaxy. p. 449-451, F 15.7, F 15.8.


Mariner 4 flies past Mars. Mariner 4 sent back pictures of Mars that showed a deal planet whose surface resembled that of the Moon. p. 257-258, F 8.16


Jocelyn Bell and Antony Hewish discover pulsar. Bell and Hewish discovered regular pulses of radio radiation from a point in the sky. The pulses were later attributed to beams of radiation emitted by a rotating neutron star. p. 425, F 14.6


Astronauts Edwin Aldrin and Neil Armstrong land on Moon. On July 20, 1969 Aldrin and Armstrong became the first people to land on the Moon.


Last Apollo mission to Moon. Apollo 17 concluded the series of lunar landings in which a dozen astronauts explored the Moon.


Voyagers 1 and 2 reach Jupiter. The Voyagers used gravitational boosts from their encounters with Jupiter to speed outward to encounters with other planets in the outer solar system. p. 278, F 9.5


Mariner 10 encounters Mercury. After passing Venus, Mariner 10 encountered Mercury four times, sending back pictures of Mercury's surface. Mariner 10 is the only spacecraft that has flows past Mercury.


Vikings land on Mars. The two Viking landers safely touched down on Mars's surface and, for several years, returned images of the surface, as well as meteorological and seismic data. The Vikings also carried life-detection experiments. p. 264-265.


International Ultraviolet Explorer (IUE) launched. IUE has been in operation for over 20 years, sending back ultraviolet spectra of celestial objects. p. 142-143, F 4.15


James Christy discovers Pluto's moon, Charon. Chrisy noticed that Pluto's image had a bump on it. The bump proved to be a satellite, Charon. p. 295-296, F 9.27


Very Large Array begins operations. The Very Large Array (VLA) is an array of 27 radio telescopes that work together to produce radio images that are comparable to those of traditional optical telescopes.


Alan Guth proposes early period of inflation of universe. Guth proposed that the a number of difficulties with the standard model of the expanding universe could be explained by an enormous expansion very early in the history of the universe. p. 532-533.


Fleet of space probes encounters Comet Halley. A fleet of five space probes flew past Comet Halley at distances as small as 600 km. Images sent back by the spacecraft showed that the nucleus of Halley is very dark and larger than anticipated. p. 310, F 10.10.


Kamiokande neutrino telescope begins operating. Shortly after it was completed, the Kamiokande telescope was one of two neutrino telescopes to detect neutrinos from a supernova in the Large Magellanic Cloud.


Supernova detected in Large Magellanic Cloud. The supernova, which occurred in one of the nearest galaxies, was the first supernova in almost 400 years that could be seen without the aid of a telescope. p. 409.


Hipparcos satellite begins making observations. Hipparcos made highly accurate measurements of the positions and parallaxes of stars. p. 517.


Magellan begins radar mapping of Venus Magellan produced an almost complete radar map of the surface of Venus. p. 254-255, F 8.12, F 8.13


CCDs become the detector of choice in astronomy. CCDs are much more sensitive than photographic plates and allow astronomers to detect very faint objects.


Hubble Space Telescope launched. The HST has produced images of breathtaking clarity and has allowed astronomers to see light from more distant objects than ever before. p. 142-143, F 4.15, F 4.16


ROSAT launched. ROSAT produced X-ray images of hot, X ray emitting objects. p. 142, F 4.15


First Keck 10-m telescope completed. The Keck telescope, unlike most earlier large optical telescopes, has a mirror made of many hexagonal segments. There are now twin Keck telescopes on Mauna Kea. p. 125-126, p. 133, F 4.8


Hubble Space telescope launched.


Galileo obtains first up-close images of asteroid (Gaspra). The Galileo spacecraft passed Gaspra at a distance of only 1,600 km. Images from Galileo showed craters and groovelike cracks. p. 305, F 10.5


Very Long Baseline Array completed. The Very Long Baseline Array (VLBA) is an array of ten radio telescopes that work together to yield radio images with even better resolution than optical telescopes can achieve.


Fragments of Comet Shoemaker-Levy 9 strike Jupiter. Tides due to Jupiter broke Comet Shoemaker-Levy 9 into at least 18 fragments that later struck Jupiter, producing bright fireballs and new cloud features. p. 286


Galileo probe enters Jupiter's atmosphere. An entry probe detached from the Galileo spacecraft and parachuted into Jupiter's atmosphere. The probe sent back data for about an hour before it was destroyed by high pressure and temperature. p. 274, p. 281


Infrared Space Observatory (ISO) launched. ISO obtained high resolution infrared spectra and images of cool bodies and clouds of dust in the solar system, galaxy, and beyond. p. 142-143, F 4.15


Planets found orbiting stars like the Sun. Astronomers in the United States and Switzerland reported the first detections of Jupiter-like planets orbiting nearby Sun-like stars.


Claim of evidence for fossil life in Martian meteoroid. A meteorite blasted from Mars by an asteroidal impact was found to contain possible fossil traces of ancient biological activity and life-like structures. p. 265


Pathfinder lands on Mars, Mars Global Surveyor begins mapping of Mars. The Pathfinder lander and the Sojourner rover it carried landed in an ancient Martian riverbed. The Mars Global Surveyor is sending back pictures of Mars that have unprecedented clarity and detail. p. 257-264


Lunar Prospector obtains evidence of possible water on the Moon. p. 196


Japanese researchers find evidence that neutrinos (possible candidate as "dark matter") may have mass. p.495


Gamma ray bursts proven to originate in distant galaxies, not our own, thus deepening the mystery of their origin and power. p. 140.


Astronaut John Glenn returns to space aboard the Shuttle Discovery in October.


NASA received a double set-back when first the Mars Climate Orbiter and then the Mars Polar Lander space craft fail. P. 257 (Mars)


Lunar Prospector intentionally crashed into Moon in attempt to identify water. None is found. p. 188.


NEAR spacecraft orbits an asteroid (Eros) for the first time. p. 304.


Spacecraft NEAR-Shoemaker makes historic first landing on the asteroid (Eros). p. 304.


Leonid meteor storm on November 18. At times near the peak of shower, up to 5 or 6 meteors could be seen per second.


Sudbury neutrino detector in Canada demonstrates that neutrinos emitted from the Sun's core change their type as they travel to Earth. This resolves the many decade long mystery of the "missing solar neutrinos" and proves these tiny particles have mass.


Observations from a spacecraft orbiting Mars suggest large deposits of ice may lie below the Martian surface.

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