Timeline of astronomy

750 BC

Babylonian astronomers discover an 18.6-year cycle in the rising and setting of the Moon. From this they created the first almanacs – tables of the movements of the Sun, Moon and planets for the use in astrology. In 6th century BC Greece, this knowledge is used to predict eclipses.

585 BC

Thales predicts a solar eclipse.

467 BC

Anaxagoras produced a correct explanation for eclipses and then described the sun as a fiery mass larger than the Peloponnese , as well as attempting to explain rainbows and meteors . He was the first to explain that the moon shines due to reflected light from the sun.[1][2][3]

388 BC

Plato, a Greek philosopher, founds a school (the Platonic Academy) that will influence the next 2000 years. It promotes the idea that everything in the universe moves in harmony and that the Sun, Moon, and planets move around Earth in perfect circles.

270 BC

Aristarchus of Samos proposes heliocentrism as an alternative to the Earth-centered universe. His heliocentric model places the Sun at its center, with Earth as just one planet orbiting it. However, there were only a few people who took the theory seriously.

164 BC

The earliest recorded sighting of Halley's Comet is made by Babylonian astronomers. Their records of the comet's movement allow astronomers today to predict accurately how the comet's orbit changes over the centuries.

4 BC

The astronomer Shi Shen is believed to have cataloged 809 stars in 122 constellations, and he also made the earliest known observation of sunspots.

140 AD

Ptolemy publishes his star catalogue, listing 48 constellations and endorses the geocentric (Earth-centered) view of the universe. His views go unquestioned for nearly 1500 years in Europe, and are passed down to Arabic and medieval European astronomers in his book the Almagest.

400

The Hindu cosmological time cycles explained in the Surya Siddhanta, give the average length of the sidereal year (the length of the Earth's revolution around the Sun) as 365.2563627 days, which is only 1.4 seconds longer than the modern value of 365.256363004 days.[4] This remains the most accurate estimate for the length of the sidereal year anywhere in the world for over a thousand years.

499 CE

Indian mathematician-astronomer Aryabhata, in his Aryabhatiya, propounds a heliocentric solar system of gravitation, and an eccentric elliptical model of the planets, where the planets spin on their axes and follow elliptical orbits around the Sun. He also writes that the planets and the Moon do not have their own light but reflect the light of the Sun, and that the Earth rotates on its axis causing day and night and also round the sun causing year.

628

Indian mathematician-astronomer Brahmagupta, in his Brahma-Sphuta-Siddhanta, first recognizes gravity as a force of attraction, and briefly describes the second law of gravitation. He gives methods for calculations of the motions and places of various planets, their rising and setting, conjunctions, and calculations of the solar and lunar eclipses.

773

The Sanskrit works of Aryabhata and Brahmagupta, along with the Sanskrit text Surya Siddhanta, are translated into Arabic, introducing Arabic astronomers to Indian astronomy.

777

Muhammad al-Fazari and Yaqub ibn Tariq translate the Surya Siddhanta and Brahmasphutasiddhanta, and compile them as the Zij al-Sindhind, the first Zij treatise.[5]

830

The first major Arabic work of astronomy is the Zij al-Sindh by al-Khwarizimi. The work contains tables for the movements of the sun, the moon and the five planets known at the time. The work is significant as it introduced Ptolemaic concepts into Islamic sciences. This work also marks the turning point in Arabic astronomy. Hitherto, Arabic astronomers had adopted a primarily research approach to the field, translating works of others and learning already discovered knowledge. Al-Khwarizmi's work marked the beginning of nontraditional methods of study and calculations.[6]

850

al-Farghani wrote Kitab fi Jawani ("A compendium of the science of stars"). The book primarily gave a summary of Ptolemic cosmography. However, it also corrected Ptolemy based on findings of earlier Arab astronomers. Al-Farghani gave revised values for the obliquity of the ecliptic, the precessional movement of the apogees of the sun and the moon, and the circumference of the earth. The books were widely circulated through the Muslim world, and even translated into Latin.[7]

928

The earliest surviving astrolabe is constructed by Islamic mathematician-astronomer Mohammad al-Fazari. Astrolabes are the most advanced instruments of their time. The precise measurement of the positions of stars and planets allows Islamic astronomers to compile the most detailed almanacs and star atlases yet.

1030

Abū Rayḥān al-Bīrūnī discussed the Indian heliocentric theories of Aryabhata, Brahmagupta and Varahamihira in his Ta'rikh al-Hind (Indica in Latin). Biruni stated that the followers of Aryabhata consider the Earth to be at the center. In fact, Biruni casually stated that this does not create any mathematical problems.[8]

1031

Abu Said Sinjari, a contemporary of Abu Rayhan Biruni, suggested the possible heliocentric movement of the Earth around the Sun.[9]

1054

Chinese astronomers record the sudden appearance of a bright star. Native-American rock carvings also show the brilliant star close to the Moon. This star is the Crab supernova exploding.

1070

Abu Ubayd al-Juzjani published the Tarik al-Aflak. In his work, he indicated the so-called "equant" problem of the Ptolemic model. Al-Juzjani even proposed a solution for the problem. In al-Andalus, the anonymous work al-Istidrak ala Batlamyus (meaning "Recapitulation regarding Ptolemy"), included a list of objections to the Ptolemic astronomy.

One of the most important works in the period was Al-Shuku ala Batlamyus ("Doubts on Ptolemy"). In this, the author summed up the inconsistencies of the Ptolemic models. Many astronomers took up the challenge posed in this work, namely to develop alternate models that evaded such errors.

1126

Islamic and Indian astronomical works (including Aryabhatiya and Brahma-Sphuta-Siddhanta) are translated into Latin in Córdoba, Spain in 1126, introducing European astronomers to Islamic and Indian astronomy.

1150

Indian mathematician-astronomer Bhāskara II, in his Siddhanta Shiromani, calculates the longitudes and latitudes of the planets, lunar and solar eclipses, risings and settings, the Moon's lunar crescent, syzygies, and conjunctions of the planets with each other and with the fixed stars, and explains the three problems of diurnal rotation. He also calculates the planetary mean motion, ellipses, first visibilities of the planets, the lunar crescent, the seasons, and the length of the Earth's revolution around the Sun to 9 decimal places.

1250

Mo'ayyeduddin Urdi develops the Urdi lemma, which is later used in the Copernican heliocentric model.

Nasir al-Din al-Tusi resolved significant problems in the Ptolemaic system by developing the Tusi-couple as an alternative to the physically problematic equant introduced by Ptolemy.[10] His Tusi-couple is later used in the Copernican model.

Tusi's student Qutb al-Din al-Shirazi, in his The Limit of Accomplishment concerning Knowledge of the Heavens, discusses the possibility of heliocentrism.

Najm al-Din al-Qazwini al-Katibi, who also worked at the Maraghah observatory, in his Hikmat al-'Ain, wrote an argument for a heliocentric model, though he later abandoned the idea.[9]

1350

Ibn al-Shatir (1304–1375), in his A Final Inquiry Concerning the Rectification of Planetary Theory, eliminated the need for an equant by introducing an extra epicycle, departing from the Ptolemaic system in a way very similar to what Copernicus later also did. Ibn al-Shatir proposed a system that was only approximately geocentric, rather than exactly so, having demonstrated trigonometrically that the Earth was not the exact center of the universe. His rectification is later used in the Copernican model.

1543

Nicolaus Copernicus publishes De revolutionibus orbium coelestium containing his theory that Earth travels around the Sun. However, he complicates his theory by retaining Plato's perfect circular orbits of the planets.

1573

A brilliant supernova (SN 1572 - thought, at the time, to be a comet) is observed by Tycho Brahe, who proves that it is traveling beyond Earth's atmosphere and therefore provides the first evidence that the heavens can change.

1608

Dutch eyeglass maker Hans Lippershey invents the refracting telescope. The invention spreads rapidly across Europe, as scientists make their own instruments. Their discoveries begin a revolution in astronomy.

1609

Johannes Kepler publishes his New Astronomy. In this and later works, he announces his three laws of planetary motion, replacing the circular orbits of Plato with elliptical ones. Almanacs based on his laws prove to be highly accurate.

1610

Galileo Galilei publishes Sidereus Nuncius describing the findings of his observations with the telescope he built. These include spots on the Sun, craters on the Moon, and four satellites of Jupiter. Proving that not everything orbits Earth, he promotes the Copernican view of a Sun-centered universe.

1655

As the power and the quality of the telescopes increase, Christiaan Huygens studies Saturn and discovers its largest satellite, Titan. He also explains Saturn's appearance, suggesting the planet is surrounded by a thin ring.

1663

Scottish astronomer James Gregory describes his "gregorian" reflecting telescope, using parabolic mirrors instead of lenses to reduce chromatic aberration and spherical aberration, but is unable to build one.

1668

Isaac Newton builds the first reflecting telescope, his Newtonian telescope.

1687

Isaac Newton publishes his first copy of the book Philosophiae Naturalis Principia Mathematica, establishing the theory of gravitation and laws of motion. The Principia explains Kepler's laws of planetary motion and allows astronomers to understand the forces acting between the Sun, the planets, and their moons.

1705

Edmond Halley calculates that the comets recorded at 76-year intervals from 1456 to 1682 are one and the same. He predicts that the comet will return again in 1758. When it reappears as expected, the comet is named in his honor.

1750

French astronomer Nicolas de Lacaille sails to southern oceans and begins work compiling a catalog of more than 10000 stars in the southern sky. Although Halley and others have observed from the Southern Hemisphere before, Lacaille's star catalog is the first comprehensive one of the southern sky.

1781

Amateur astronomer William Herschel discovers the planet Uranus, although he at first mistakes it for a comet. Uranus is the first planet to be discovered beyond Saturn, which was thought to be the most distant planet in ancient times.

1784

Charles Messier publishes his catalog of star clusters and nebulas. Messier draws up the list to prevent these objects from being identified as comets. However, it soon becomes a standard reference for the study of star clusters and nebulars and is still in use today.

1800

William Herschel splits sunlight through a prism and with a thermometer, measures the energy given out by different colours. He notices a sudden increase in energy beyond the red end of the spectrum, discovering invisible infrared and laying the foundations of spectroscopy.

1801

Italian astronomer Giuseppe Piazzi discovers what appears to be a new planet orbiting between Mars and Jupiter, and names it Ceres. William Herschel proves it is a very small object, calculating it to be only 320 km in diameter, and not a planet. He proposes the name asteroid, and soon other similar bodies are being found. We now know that Ceres is 932 km in diameter, and is now considered to be a dwarf planet.

1814

Joseph von Fraunhofer builds the first accurate spectrometer and uses it to study the spectrum of the Sun's light. He discovers and maps hundreds of fine dark lines crossing the solar spectrum. In 1859 these lines are linked to chemical elements in the Sun's atmosphere. Spectroscopy becomes a method for studying what stars are made of.

1838

Friedrich Bessel successfully uses the method of stellar parallax, the effect of Earth's annual movement around the Sun, to calculate the distance to 61 Cygni, the first star other than the Sun to have its distance from Earth measured. Bessel's is a truly accurate measurement of stellar positions, and the parallax technique establishes a framework for measuring the scale of the universe.

1843

German Amateur astronomer Heinrich Schwabe, who has been studying the Sun for the past 17 years, announces his discovery of a regular cycle in sunspot numbers - the first clue to the Sun's internal structure.

1845

Irish astronomer William Parsons, 3rd Earl of Rosse completes the first of the world's great telescopes, with a 180-cm mirror. He uses it to study and draw the structure of nebulas, and within a few months discovers the spiral structure of the Whirlpool Galaxy.

French physicists Jean Foucault and Armand Fizeau take the first detailed photographs of the Sun's surface through a telescope - the birth of scientific astrophotography. Within five years, astronomers produce the first detailed photographs of the Moon. Early film is not sensitive enough to image stars.

1846

A new planet, Neptune, is identified by German astronomer Johann Gottfried Galle while searching in the position suggested by Urbain Le Verrier. Le Verrier has calculated the position and size of the planet from the effects of its gravitational pull on the orbit of Uranus. An English mathematician, John Couch Adams, also made a similar calculation a year earlier.

1868

Astronomers notice a new bright emission line in the spectrum of the Sun's atmosphere during an eclipse. The emission line is caused by an element's giving out light, and British astronomer Norman Lockyer concludes that it is an element unknown on Earth. He calls it helium, from the Greek word for the Sun. Nearly 30 years later, helium is found on Earth.

1872

An American astronomer Henry Draper takes the first photograph of the spectrum of a star (Vega), showing absorption lines that reveal its chemical makeup. Astronomers begin to see that spectroscopy is the key to understanding how stars evolve. William Huggins uses absorption lines to measure the redshifts of stars, which give the first indication of how fast stars are moving.

1895

Konstantin Tsiolkovsky publishes his first article on the possibility of space flight. His greatest discovery is that a rocket, unlike other forms of propulsion, will work in a vacuum. He also outlines the principle of a multistage launch vehicle.

1901

A comprehensive survey of stars, the Henry Draper Catalog, is published. In the catalog, Annie Jump Cannon proposes a sequence of classifying stars by the absorption lines in their spectra, which is still in use today.

1906

Ejnar Hertzsprung establishes the standard for measuring the true brightness of a star. He shows that there is a relationship between color and absolute magnitude for 90% of the stars in the Milky Way Galaxy. In 1913, Henry Norris Russell publishes a diagram that shows this relationship. Although astronomers agree that the diagram shows the sequence in which stars evolve, they argue about which way the sequence progresses. Arthur Eddington finally settles the controversy in 1924.

1916

German physicist Karl Schwarzschild uses Albert Einstein's theory of general relativity to lay the groundwork for black hole theory. He suggests that if any star collapse to a certain size or smaller, its gravity will be so strong that no form of radiation will escape from it.

1923

Edwin Hubble discovers a Cepheid variable star in the "Andromeda Nebula" and proves that Andromeda and other nebulas are galaxies far beyond our own. By 1925, he produces a classification system for galaxies.

1926

Robert Goddard launches the first rocket powered by liquid fuel. He also demonstrates that a rocket can work in a vacuum. His later rockets break the sound barrier for the first time.

1929

Edwin Hubble discovered that the universe is expanding and that the farther away a galaxy is, the faster it is moving away from us. Two years later, Georges Lemaître suggests that the expansion can be traced to an initial "Big Bang".

1930

By applying new ideas from subatomic physics, Subrahmanyan Chandrasekhar predicts that the atoms in a white dwarf star of more than 1.44 solar masses will disintegrate, causing the star to collapse violently. In 1933, Walter Baade and Fritz Zwicky describe the neutron star that results from this collapse, causing a supernova explosion.

Clyde Tombaugh discovers the dwarf planet Pluto at the Lowell Observatory in Flagstaff, Arizona. The object is so faint and moving so slowly that he has to compare photos taken several nights apart.

1932

Karl Jansky detects the first radio waves coming from space. In 1942, radio waves from the Sun are detected. Seven years later radio astronomers identify the first distant source - the Crab Nebula, and the galaxies Centaurus A and M87.

1938

German physicist Hans Bethe explains how stars generate energy. He outlines a series of nuclear fusion reactions that turn hydrogen into helium and release enormous amounts of energy in a star's core. These reactions use the star's hydrogen very slowly, allowing it to burn for billions of years.

1944

A team of German scientists led by Wernher von Braun develops the V-2, the first rocket-powered ballistic missile. Scientists and engineers from Braun's team were captured at the end of World War II and drafted into the American and Russian rocket programs.

1948

The largest telescope in the world, with a 5.08m (200 in) mirror, is completed at Palomar Mountain in California. At the time, the telescope pushes single-mirror telescope technology to its limits - large mirrors tend to bend under their own weight.

1957

Russia launches the first artificial satellite, Sputnik 1, into orbit, beginning the space age. The US launches its first satellite, Explorer 1, four months later.

1958

(July 29) Beginning of the NASA (National Aeronautics and Space Administration), agency newly created by the United States to catch up with Soviet space technologies. It absorbs all research centers and staffs of the NACA (National Advisory Committee for Aeronautics), an organization founded in 1915.

1959

Russia and the US both launch probes to the Moon, but NASA's Pioneer probes all failed. The Russian Luna program was more successful. Luna 2 crash-lands on the Moon's surface in September, and Luna 3 returns the first pictures of the Moon's farside in October.

1961

Russia takes the lead in the space race as Yuri Gagarin becomes the first person to orbit Earth in April. NASA astronaut Alan Shepard becomes the first American in space a month later, but does not go into orbit, although he is the first person to land with himself still inside his spacecraft and thus technically completing the first complete human spaceflight.[11] John Glenn achieves orbit in early 1962.

1962

Mariner 2 becomes the first probe to reach another planet, flying past Venus in December. NASA follows this with the successful Mariner 4 mission to Mars in 1965, both the US and Russia send many more probes to planets through the rest of the 1960s and 1970s.

1963

Dutch-American astronomer Maarten Schmidt measures the spectra of quasars, the mysterious star-like radio sources discovered in 1960. He establishes that quasars are active galaxies, and among the most distant objects in the universe.

1965

Arno Penzias and Robert Wilson announce the discovery of a weak radio signal coming from all parts of the sky. Scientists figure out that this must be emitted by an object at a temperature of -270 °C. Soon it is recognized as the remnant of the very hot radiation from the Big Bang that created the universe 13 billion years ago. This radio signal is emitted by the electron in hydrogen flipping from pointing up or down and is approximated to happen once in a million years for every particle. Hydrogen is present in interstellar space gas throughout the entire universe and most dense in nebulae which is where the signals originate. Even though the electron of hydrogen only flips once every million years the mere quantity of hydrogen in space gas makes the presence of these radio waves prominent.

1966

Russian Luna 9 probe makes the first successful soft landing on the Moon in January, while the US lands the far more complex Surveyor missions, which follows up to NASA's Ranger series of crash-landers, scout sites for possible manned landings.

1967

Jocelyn Bell Burnell and Antony Hewish detected the first pulsar, an object emitting regular pulses of radio waves. Pulsars are eventually recognized as rapidly spinning neutron stars with intense magnetic fields - the remains of a supernova explosion.

1968

NASA's Apollo 8 mission becomes the first human spaceflight mission to travel beyond Earth's gravitational influence and to orbit another celestial body.

1969

The US wins the race for the Moon, as Neil Armstrong steps onto the lunar surface on July 20. Apollo 11 is followed by five further landing missions, three carrying a sophisticated lunar rover vehicle.

1970

The Uhuru satellite, designed to map the sky at X-ray wavelengths, is launched by NASA. The existence of X rays from the Sun and a few other stars has already been found using rocket-launched experiments, but Uhuru charts more than 300 X-ray sources, including several possible black holes.

1971

Russia launches its first space station, Salyut 1, into orbit. It is followed by a series of stations, culminating with Mir in 1986. A permanent platform in orbit allows cosmonauts to carry out serious research and to set a series of new duration records for spaceflight.

1972

Charles Thomas Bolton was the first astronomer to present irrefutable evidence of the existence of a black hole.

1975

The Russian probe Venera 9 lands on the surface of Venus and sends back the first picture of its surface. The first probe to land on another planet, Venera 7 in 1970, had no camera. Both break down within an hour in the hostile atmosphere.

1976

Two NASA probes arrive at Mars. Each Viking mission consists of an orbiter, which photographs the planet from above, and a lander, which touches down on the surface, analyzes the rocks, and searches unsuccessfully for life.

1977

(August 20) The Voyager 2 space probe launched by NASA to study the Jovian system, Saturnian system, Uranian system, Neptunian system, the Kuiper belt, the heliosphere and the interstellar space.

(September 5) The Voyager 1 space probe launched by NASA to study the Jovian system, Saturnian system and the interstellar medium.

1981

Columbia, the first of NASA's reusable space shuttles, makes its maiden flight, ten years in development, the shuttle will make space travel routine and eventually open the path for a new International Space Station.

1983

The first infrared astronomy satellite, IRAS, is launched. It must be cooled to extremely low temperatures with liquid helium, and it operates for only 300 days before the supply of helium is exhausted. During this time it completes an infrared survey of 98% of the sky.

1986

NASA's spaceflight program comes to a halt when Space Shuttle Challenger explodes shortly after launch. A thorough inquiry and modifications to the rest of the fleet kept the shuttles on the ground for nearly three years.

The returning Halley's comet is met by a fleet of five probes from Russia, Japan, and Europe. The most ambitious is the European Space Agency's Giotto spacecraft, which flies through the comet's coma and photographs the nucleus.

1990

The Magellan probe, launched by NASA, arrives at Venus and spends three years mapping the planet with radar. Magellan is the first in a new wave of probes that include Galileo, which arrives at Jupiter in 1995, and Cassini which arrives at Saturn in 2004.

The Hubble Space Telescope, the first large optical telescope in orbit, is launched using the Space Shuttle, but astronomers soon discovered that it is crippled by a problem with its mirror. A complex repair mission in 1993 allows the telescope to start producing spectacular images of distant stars, nebulae, and galaxies.

1992

The Cosmic Background Explorer satellite produces a detailed map of the background radiation remaining from the Big Bang. The map shows "ripples", caused by slight variations in the density of the early universe – the seeds of galaxies and galaxy clusters.

The 10 meter Keck telescope on Mauna Kea, Hawaii, is completed. The first revolutionary new wave of telescopes, the Keck's main mirror is made of 36 six-sided segments, with computers to control their alignment. New optical telescopes also make use of interferometry – improving resolution by combining images from separate telescopes.

1998

Construction work on a huge new space station named ISS has begun. A joint venture between many countries, including former space rivals Russia and the US.

2005

Mike Brown and his team discovered a large body in the outer solar system.[12] It was temporarily named as (2003) UB313. Initially, it appeared larger than Pluto, and was called tenth planet.[13]

2006

International Astronomical Union (IAU) adopted a new definition of planet. A new distinct class of objects called dwarf planets was also decided. Pluto was redefined as a dwarf planet along with Ceres and Eris, formerly known as (2003) UB313. Eris was named after the IAU General Assembly in 2006.[14][15]

2012

(May 2) First visual proof of existence of black holes is published. Suvi Gezari's team in Johns Hopkins University, using the Hawaiian telescope Pan-STARRS 1, record images of a supermassive black hole 2.7 million light-years away that is swallowing a red giant.[16]

2013

In October 2013, the first extrasolar asteroid is detected around white dwarf star GD 61. It is also the first detected extrasolar body which contains water in liquid or solid form.[17][18][19]

2015

In July 14, with the successful encounter of Pluto by NASA's New Horizons spacecraft, the United States became the first nation to explore all of 9 major planets recognized in 1981. Later in September 14, LIGO was the first to directly detect gravitational waves.[20]

References

  1. ↑ Russell, Bertrand (2008) [1st. pub. 1945]. History of Western Philosophy. Simon and Schuster. p. 213. ISBN 0-6712-0158-1. Retrieved 1 June 2015.
  2. ↑ Tassoul, Jean-Louis; Tassoul, Monique (2014). A Concise History of Solar and Stellar Physics. Princeton University Press. p. 8. ISBN 0-6911-6592-0. Retrieved 1 June 2015.
  3. ↑ Burnet J. (1892) Early Greek Philosophy A. & C. Black, London, OCLC 4365382, and subsequent editions, 2003 edition published by Kessinger, Whitefish, Montana, ISBN 0-7661-2826-1
  4. ↑ White, M.J. "Sidereal, tropical, and anomalistic years" (PDF). Retrieved 2016-05-16.
  5. ↑ Kennedy, Edward S. (1956), "A Survey of Islamic Astronomical Tables", Transactions of the American Philosophical Society, 46 (2): 123, doi:10.2307/1005726
  6. ↑ Dallal (1999), pg. 163
  7. ↑ Dallal (1999), pg. 164
  8. ↑ Saliba (1999).
  9. 1 2 A. Baker, L. Chapter (2002)
  10. ↑ M. Gill (2005). Was Muslim Astronomy the Harbinger of Copernicanism?
  11. ↑ "Geek Trivia: A leap of fakes". Retrieved 18 August 2016.
  12. ↑ "The Discovery of Eris". Space Daily. 2 March 2007. Retrieved 6 June 2015.
  13. ↑ "Object Bigger than Pluto Discovered, Called 10th Planet". Space.com. 29 July 2005. Retrieved 6 June 2015.
  14. ↑ "Pluto and the Developing Landscape of Our Solar System". International Astronomical Union. Retrieved 6 June 2015.
  15. ↑ "Planet community in a spin". ABC. 25 August 2006. Retrieved 6 June 2015.
  16. ↑ Scientific American - Big Gulp: Flaring Galaxy Marks the Messy Demise of a Star in a Supermassive Black Hole
  17. ↑ "Scientists discover water-rich asteroid orbiting dead star GD 61 outside our solar system". Pentagonpost.com. Retrieved 2013-10-12.
  18. ↑ "Watery asteroid discovered in dying star points to habitable exoplanets". Phys.org. Retrieved 2013-10-12.
  19. ↑ Mack, Eric (2011-10-17). "Newly spotted wet asteroids point to far-flung Earth-like planets | Crave - CNET". News.cnet.com. Retrieved 2013-10-12.
  20. ↑ Castelvecchi, Davide; Witze, Witze (February 11, 2016). "Einstein's gravitational waves found at last". Nature News. doi:10.1038/nature.2016.19361. Retrieved 2016-02-11.
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