Astronomy For Dummies Cheat Sheet

A celestial timeline

People have been fascinated with the stars and planets since ancient times. Start your study of astronomy, which encompasses literally everything in the universe, by reviewing a selection of notable accomplishments in astronomical history.

• 129 B.C. In Greece, Hipparchus completes the first known star catalog; it included more than 850 entries.
• A.D. 150 Claudius Ptolemy (who wrote in Greek but was of uncertain nationality) publishes his theory of the Earth-centered universe.
• 1420 Ulugh Beg, a ruler of Samarkand (now part of Uzbekistan), builds a great observatory and prepares tables of planet and star data.
• 1543 Nikolaus Copernicus of Poland publishes his theory that planets — including Earth — orbit the Sun.
• 1572 Tycho Brahe of Denmark observes a “new star” (now known to have been a supernova) in the constellation Cassiopeia. Five years later, he demonstrates that comets lie not in Earth’s atmosphere but far beyond the Moon.
• 1609 Johannes Kepler of Germany, using Tycho Brahe’s unprecedentedly precise observations of planetary positions over many years, establishes that planets move not in circular orbits but in elliptical ones.
• 1610 Galileo Galilei of Italy publishes his discoveries of the moons of Jupiter, the phases of Venus, craters and mountains on the Moon, and the presence of innumerable stars in the Milky Way made with a refractor (lens-based telescope) that he built himself after hearing about a “spyglass” invented in the Netherlands.
• 1668 Isaac Newton in England builds the first astronomical reflector (mirror-based telescope), with a main mirror 2 inches in diameter.
• 1687  Newton publishes his laws of motion and theory of universal gravitation.
• 1781 In England, William Herschel (originally from Germany) finds the planet Uranus — the first such discovery since prehistory — while sweeping the sky with a large reflector he built himself.
• 1838 Friedrich Bessel of Germany first measures the distance to a star beyond the solar system, finding 61 Cygni to lie more than 10 light-years (60 trillion miles) away.
• 1839 John W. Draper publishes the first photograph of the Moon. A year later, he helps take the first photo of a star (Vega, in the constellation Lyra).
• 1846 Johann Galle and Heinrich d’Arrest in Germany discover the planet Neptune based on a prediction by Urbain Le Verrier in France.
• 1859 In Germany, Gustaf Kirchhoff and Robert Bunsen invent the spectroscope, an instrument that enables scientists to determine the compositions of celestial objects solely from their light.
• 1901 Annie Jump Cannon publishes the first catalog of stellar spectra based on a classification method she devised at Harvard College Observatory.
• 1908 Cannon’s colleague Henrietta Swan Leavitt discovers the first accurate method for measuring great distances in space. George Ellery Hale at Mount Wilson Observatory discovers that sunspots are magnetic.
• 1915 Albert Einstein introduces his general theory of relativity, describing gravity as the warping of space and time by massive objects.
• 1923 Edwin P. Hubble shows that the Andromeda “nebula” is actually a galaxy like our own Milky Way and that the universe is far larger than anyone knew.
• 1925 Cecilia Payne-Gaposchkin is the first to recognize that hydrogen is the most abundant chemical element in the stars and, by extension, the universe.
• 1929 Hubble discovers that the universe is expanding; more distant galaxies recede from the Milky Way faster than more nearby ones.
• 1930 Clyde Tombaugh, working at Lowell Observatory in Arizona, discovers Pluto, then called the ninth planet but now considered a dwarf planet and the brightest member of the Kuiper Belt of small icy bodies beyond Neptune.
• 1933 Karl Jansky of Bell Telephone Laboratories in New Jersey announces the discovery of cosmic radio waves. Fritz Zwicky at Caltech proposes that clusters of galaxies contain dark matter whose gravity keeps the galaxies from escaping.
• 1938 Hans Bethe of Cornell University shows that stars are powered by nuclear fusion at incredibly high pressures and temperatures.
• 1958 Using the USA’s first satellite, Explorer 1, James Van Allen discovers Earth’s radiation belts (magnetosphere).
• 1959 The Soviet Union’s Luna 3 probe is the first to photograph the Moon’s far side (the side that always faces away from Earth).
• 1960 Frank Drake begins the modern era of the search for extraterrestrial intelligence (SETI) at the National Radio Astronomy Observatory in Green Bank, West Virginia.
• 1962 Riccardo Giacconi detects X-rays from beyond the Sun for the first time using an instrument aboard an Aerobee sounding rocket; the source, Sco X-1 in Scorpius, is a binary system containing a neutron star. NASA’s Mariner 2 becomes the first probe to successfully fly past another planet (Venus).
• 1965 Arno Penzias and Robert W. Wilson discover the cosmic microwave background, the “echo” of the Big Bang, at Bell Telephone Laboratories.
• 1967 Jocelyn Bell Burnell and Antony Hewish discover pulsars at the University of Cambridge. The Vela 3 and 4 satellites detect the first cosmic gamma-ray bursts.
• 1971 Mariner 9 is the first spacecraft to orbit another planet (Mars). Richard Tousey of the U.S. Naval Research Laboratory discovers the Sun’s coronal mass ejections with the Orbiting Solar Observatory 7.
• 1973 NASA’s Pioneers 10 and 11 are the first spacecraft to visit Jupiter (and later, Saturn).
• 1976 Vikings 1 and 2 become the first spacecraft to land on Mars; E. Margaret Burbidge, a leading expert on galaxies and quasars, becomes the first female president of the American Astronomical Society.
• 1979 Using pictures from Voyager 1, Linda Morabito at the Jet Propulsion Laboratory discovers erupting volcanoes on Jupiter’s moon Io.
• 1980 Vera C. Rubin and Kent Ford of the Carnegie Institution investigate the rotation of galaxies and detect strong evidence of dark matter.
• 1984 Astronauts of the Space Shuttle Challenger make the first repairs in space of an unmanned satellite, the Solar Maximum Mission.
• 1987 Canadian Ian Shelton and Chilean Oscar Duhalde discover Supernova 1987A, the first supernova since 1604 that was plainly visible to the naked eye, at Las Campanas Observatory, Chile.
• 1991 Polish astronomer Alexander Wolszczan discovers planets orbiting a pulsar — the first known planets outside the solar system.
• 1992 Jane Luu and David C. Jewitt find Albion, the first object (other than Pluto and its large moon Charon) recognized to be orbiting the Sun in the Kuiper Belt beyond Neptune.
• 1994 Multiple fragments of Comet Shoemaker-Levy 9, discovered a year earlier by Carolyn and Eugene Shoemaker and David H. Levy, crash into Jupiter, leaving scars visible in backyard telescopes.
• 1995 Swiss astronomers Michel Mayor and Didier Queloz discover 51 Pegasi b, the first known planet orbiting a normal star beyond the Sun, from the Haute-Provence Observatory in France.
• 1998 Two teams of astronomers independently discover that the expansion of the universe is getting faster, apparently due to a mysterious “dark energy” associated with the vacuum of space.
• 1999 Mars Global Surveyor finds that the Red Planet may have had an ocean at one time.
• 2001 A team led by Canadian-American astronomer Wendy Freedman of Carnegie Observatories publishes the most precise value of the Hubble constant (the cosmic expansion rate) to date based on observations from the Hubble Space Telescope.
• 2003 NASA’s Wilkinson Microwave Anisotropy Probe finds that the universe is 13.7 billion years old; later in the same decade, Europe’s Planck satellite refines this to 13.8 billion years.
• 2012 The Kepler spacecraft finds that there probably are billions of planets in orbit around stars in our galaxy, and the rover Curiosity lands on Mars.
• 2013 The European Space Agency’s Gaia spacecraft is launched for observations to create a three-dimensional catalog of 1 billion stars.
• 2015 The New Horizons probe explores Pluto and its moons and then heads outward in the Kuiper Belt; a team of physicists detect gravitational waves for the first time using the Laser Interferometric Gravitational-wave Observatory (LIGO) in the U.S. states of Louisiana and Washington.
• 2019 The globe-spanning Event Horizon Telescope captures the first image of a black hole, the supermassive object at the center of galaxy M87 in Virgo.
• 2020 American Andrea Ghez and Germany’s Reinhard Genzel, whose teams independently proved the existence of a supermassive black hole at the center of the Milky Way galaxy, share the 2020 Nobel Prize in Physics.
• 2022 NASA’s DART spacecraft slams into an asteroid to change its orbit to show how we might someday nudge an asteroid off of a collision course with our planet.
• 2023 Early data from the James Webb Space Telescope suggest that large galaxies formed much earlier in cosmic history than current theory predicts.