Astronomy Timeline
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. 23, F 1.1, p. 30.
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. 30.
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. 34, F 1.15.
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. p. 42.
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. 34, F 1.14.
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. 35.
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.132.
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.132.
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. 196.
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. 32, F 1.12.
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. 35-36, 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. 37-39, F 1.19, F 1.20.
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. 3637, F 1.17
134 BC
c 0 AD
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 brightness of the stars.
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 brightness based partly on the earlier catalog of
Hipparchus. p. 41-42, p. 361-362, F 1.25
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. p. 167-168, F 5.19, F 5.20
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. p. 305-306, F 10.8, F 10.9, F 10.10
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.
Astronomy Timeline
c1265
1300
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.
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. p. 48.
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.
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, was published two months before his death. p. 42-44, F 1.26
1572
Tycho observes supernova. Tycho's book about his observations of the supernova established his
reputation as an astronomer. p. 44-45
c1580
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. 44-45
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. p. 49
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. p. 141
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. p.
44
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.
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. p. 48, F 1.30
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. p. 48
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. p. 45-46, F 1.28, F 1.29
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. p. 48, F 1.30
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. p. 48
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. p. 134-136, F 4.11, F 4.12, F 4.14
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. p. 78-79, F 2.1
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. p. 49.
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. p. 346, F 11.26
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. p. 132
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. p. 281, F 9.12
Astronomy Timeline
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. p. 273-275,
F 9.1
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. p. 80.
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. p. 83, F 2.7
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. p. 371
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 brightness. p. 353-355, F 12.2
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. p. 94-95. F. 31
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. p. 305-306, F 10.8, 10.9, 10.10
1686
Newton publishes "Principia". Newton's monumental work described his discoveries about gravity,
motion and the orbits of the planets. p. 79-84, F 2.2, F 2.3, F 2.4, F 2.5, F 2.6, F 2.7, F 2.8
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. p. 457460
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 brightness 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. 50, p. 475
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. p. 167, F 5.20, p. 475, F 16.1
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. p. 371
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. p. 366368, F 12.11
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. p. 441-442, F 15.4
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.
p. 424
1801
Giuseppe Piazzi discovers Ceres. Piazzi discovered Ceres, the first known asteroid, on January 1, 1801,
the first day of the 19th century. p. 300-302, F 10.4, F 10.5, F 10.6
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. p. 369, F 12.14
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. p. 109-110, F 3.15
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. p. 262
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. p. 262
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. p. 146-148, F 4.24, F
4.25
1837
η Carinae brightens to become second brightest star. η 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.
p. 353-355
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. p. 112, F
3.18
Astronomy Timeline
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. p. 288-289, F 9.23
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. p. 144
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. p. 363.
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. p. 262, F 8.24
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. p. 333
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. p. 262-263, F 8.25
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. p. 102.
1894
Wilhelm Wien finds how temperature affects the color of a blackbody. Wien found that a blackbody gets
bluer as its temperature increases. p. 102.
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. p. 102.
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. p. 95, F 3.2
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 HR) diagram. p. 372-373, F 12.17, F 12-18.
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. p. 442-443
1913
Henry Norris Russell independently invents H-R diagram. The H-R diagram became an important tool
for understanding the evolution of stars. p. 372-373, F 12.17, F 12-18
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 supercontinent that broke
apart about 200 million years ago. p. 134, F 4.13
1916
Albert Einstein's general theory of relativity. Einstein explained that matter curves space, causing bodies
to move in ways we attribute to gravity. p. 518
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. p. 424, F
14.12
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. p. 486, F 16.16
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. p. 442-443, F 25.6
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. p. 364-366, F 12.10
1920
M.N. Saha shows that the temperature of a star determines the appearance of its spectrum. Saha showed
that temperature differences rather than compositional differences are responsible for the existence of
different spectral classes of stars. p. 364
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. p.
333.
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. p. 475-479, F 16.1, F 16.3, F 16.4, F 16.5, F 16.6, F 16.7.
Astronomy Timeline
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. p. 364-365
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. p. 133, F 4.10
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. p. 517-518, F
17.12
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. p. 416
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. p. 292-293, F
9.27, 9.28, 9.29
1930
Robert Trumpler discovers diffuse interstellar dust. Trumpler found that unless he made a distance
correction for the dimming of light created by the dust (thereby placing clusters farther away than they
otherwise appeared) he ended up with the nonsensical result that the farther a cluster was from the Sun,
the larger it was. p. 452-454, F 15.13, F 15.14, F 15.15, F 15.16.
1931
Karl Jansky makes first radio astronomy observations. Jansky found that the Milky Way galaxy is a
source of radio emission. p. 463.
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. p. 164-165, F
15.16
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. p. 418-424, F 14.5, F 14.6, F 14.7, F
14.8, F 14.9, F 14.10, F 14.11
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. p. 456, F 15.19
1948
200" Palomar telescope completed. The Palomar telescope was the world's largest high quality optical
telescope for over 40 years. p. 163
1950
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. 222, F 7.4.
1951
Harold Ewen and Edward Purcell detect 21 cm line. Ewen and Purcell used a radio telescope to detect
emission from interstellar hydrogen atoms. p. 457-458.
1951
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. 307-308, F 10.11.
1958
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. 143-144, F 4.20.
1960
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.
457-458.
1960
Robert Leighton discovers solar oscillations. Leighton found that the Sun vibrates at a variety of
frequencies.
c1960
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. 133-136, F 4.11
1961
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. 343-346, F 11.24,
11.25, 11.26
1963
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. 336-337, F 11.12
1963
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. 492, F 16.23
1964
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. 513-514, F 17.7, 17.8
1964
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. 444-445, F 15.7.
1965
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. 254-255, F 8.16
Astronomy Timeline
1967
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. 419, F 14.6
1969
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.
1972
Last Apollo mission to Moon. Apollo 17 concluded the series of lunar landings in which a dozen
astronauts explored the Moon.
1973
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. 276, F 9.5
1974
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.
1976
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. 262.
1978
International Ultraviolet Explorer (IUE) launched. IUE has been in operation for over 20 years, sending
back ultraviolet spectra of celestial objects. p. 168-169, F 5.21
1978
James Christy discovers Pluto's moon, Charon. Christy noticed that Pluto's image had a bump on it. The
bump proved to be a satellite, Charon. p. 292-293, F 9.27
1981
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.
1981
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. 522-523.
1986
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. 306, F 10.10.
1986
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.
1987
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.
404.
1989
Hipparcos satellite begins making observations. Hipparcos made highly accurate measurements of the
positions and parallaxes of stars. p. 509.
1990
Magellan begins radar mapping of Venus Magellan produced an almost complete radar map of the
surface of Venus. p. 250-251, F 8.12, F 8.13
c1990
CCDs become the detector of choice in astronomy. CCDs are much more sensitive than photographic
plates and allow astronomers to detect very faint objects.
1990
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. 168-170, F 5.21, F 5.22
1990
ROSAT launched. ROSAT produced X-ray images of hot, X ray emitting objects. p. 168
1990
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.
154
1991
Gamma Ray Observatory (GRO) launched. The GRO is much more sensitive than earlier gamma ray
telescopes.
1991
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 groove-like cracks. p. 301, F 10.5
1992
First Kuiper belt object discovered. Astronomers now know of hundreds of these icy objects, of which
Pluto is the largest.
1993
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.
1993
Taylor and Hulse win the Nobel Prize in Physics for their discovery of evidence that a binary pulsar (two
neutron stars orbiting one another) were emitting Gravitational Radiation. Einstein had predicted such
waves in his theory of General relativity.
1994
Fragments of Comet Shoemaker-Levy 9 strike Jupiter. Tides due to Jupiter broke Comet ShoemakerLevy 9 into at least 18 fragments that later struck Jupiter, producing bright fireballs and new cloud
features. p. 283
1995
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. 272, p. 279
1995
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. 168-169, F 5.21
1995
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. Many dozen are now known.
1996
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 Subsequent studies cast doubt on whether the microscopic structures are really biologically formed
or are merely mineral deposits.
1997
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. 535
1998
Gamma ray bursts proven to originate in distant galaxies, not our own, resolving their mystery of where
they come from, but leaving unanswered what creates them. p. 140.
1998
Two groups of astronomers independently discover evidence from distant supernova explosions that the
expansion of the Universe is accelerating. If effect is real, it implies existence of a cosmological constant
and, perhaps, dark energy.
1998
The joint European Southern Observatory Very Large Telescope (VLT) - first of four linked telescopes begins observing.
1999
NASA received a double set-back when first the Mars Climate Orbiter and then the Mars Polar Lander
space craft fail. P. 257 (Mars)
1999
NASA Successfully launches the Chandra X-Ray satellite. The satellite is named for Subrahmanyan
Chandrasekhar, a theoretical astrophysicist whose contributions include working out the maximum mass
of a white dwarf star. He was one of three astronomers awarded the Nobel Prize in physics in 1983.
1999
Lunar Prospector intentionally crashed into Moon in attempt to identify water. None is found. p. 188.
1999
NEAR spacecraft orbits an asteroid (Eros) for the first time. p. 304.
2000
Observations made with BOOMERANG (a balloon born telescope above Antarctica) detect small
fluctuations in the cosmic microwave background radiation. From the size distribution of these
fluctuations astronomers deduce that the Universe is flat and that roughly only a few percent of the mass
of the Universe is matter as we know it. The rest is dark matter or dark energy.
2001
Leonid meteor storm on November 18. At times near the peak of shower, up to 5 or 6 meteors could be
seen per second.
2001
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.
2002
Observations from a spacecraft orbiting Mars suggest large deposits of ice may lie below the Martian
surface.