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Ptolemy system


Armillary sphere

The invention of the armillary sphere (a model of a celestial sphere) around 255 B.C., is attributed to the Greek astronomer and philosopher Eratosthenes. For centuries, the instrument remained the primary tool for determining celestial positions. The photo shows an 18-century version of the device. Click to enlarge. Copyright © 2009 Anatoly Zak

Astrolabe 1500

Around 200 B.C. Greeks, (possibly Hipparchus) invented the astrolabe, which became a multipurpose tool used for many astronomical tasks, such as locating and predicting the positions of the Sun, Moon, and planets. The photo shows an astrolabe and its replaceable disks from southern Germany dating back to 1500. Click to enlarge. Copyright © 2009 Anatoly Zak


The quadrant was reportedly proposed by Ptolemy as an improved astrolabe to measure angles up to 90 degrees. The photo shows the 1685 quadrant by Carolus Graff and Michael Kauffer of Augsburg. Click to enlarge. Copyright © 2009 Anatoly Zak

Calendar 1375

The Solar System according to Ptolemy, as it was depicted in the 1375 Catalan Atlas. Click to enlarge. Copyright © 2009 Anatoly Zak


An astronomical and astrological "compendium" from 1489. Click to enlarge. Copyright © 2009 Anatoly Zak

Solar-lunar clock

Solar and lunar clock by Christoph Schisler of Augsburg, 1591. Click to enlarge. Copyright © 2009 Anatoly Zak


A reconstruction of the ecliptic instrument, used by Johannes Kepler around 1600. Click to enlarge. Copyright © 2009 Anatoly Zak


The proportional circinus, a calculation instrument designed by Heinrich Stolle in 1613. Click to enlarge. Copyright © 2009 Anatoly Zak


A 1681 astrolabe by Michael Kauffer of Augsburg. Click to enlarge. Copyright © 2009 Anatoly Zak


A "universal" astrolabe, Augsburg, 1685. Click to enlarge. Copyright © 2009 Anatoly Zak


A 1685 calendar by Michael Kauffer of Augsburg. Click to enlarge. Copyright © 2009 Anatoly Zak


Click to enlarge. Copyright © 2009 Anatoly Zak


Click to enlarge. Copyright © 2009 Anatoly Zak


The lunarium (top) and tellarium served as a mechanical representation of the Earth-Moon-Sun system that reproduced the relative movement of the three bodies. Such devices allowed to visualize the causes of night and day, of the seasons, of solar and lunar eclipses and of the phases of the Moon. The photos show the 1770 devices by Benjamin Martin of London and from a clock school in Furtwangen, Germany from 1900 (bottom). Click to enlarge. Copyright © 2009 Anatoly Zak


A mid-18th-century telescope. Click to enlarge. Copyright © 2009 Anatoly Zak


Swabish priest, inventor and mechanic Philipp Matthaus Hahn (1739-1790) wanted "to honor gods" with his model of the heavens. The great astronomical clock was built during 18 months in 1768-1769 for herzog Carl Eugen von Wurttenberg. It consisted of three parts: a central column with three clocks showing hours, weeks and months and centuries from the creation of the world until its end. The left part, illustrated the Solar System according to Copernicus complete with planets, their moons and the Sun. The right part, showed a map of the sky with the trajectories of the Sun, the Moon and the planets. (349) Click to enlarge. Copyright © 2009 Anatoly Zak


A calculating machine designed by Phillip Matthaus Hahn of Kornwestheim, Germany, between 1770 and 1774. Click to enlarge. Copyright © 2009 Anatoly Zak


A reflective quadrant, also known as an octant, by John Hadley from the end of the 18th century. Two men independently developed the octant around 1730: John Hadley (1682-1744), an English mathematician, and Thomas Godfrey (1704-1749), a glazier in Philadelphia. While both have a legitimate and equal claim to the invention, Hadley generally gets the greater share of the credit. This reflects the central role that London and the Royal Society played in the history of scientific instruments in the eighteenth century. Click to enlarge. Copyright © 2009 Anatoly Zak


Sky globe from 1837. Click to enlarge. Copyright © 2009 Anatoly Zak


A depleidoscope, circa 1850. This instrument was used to determine true noon with an accuracy of few seconds. Click to enlarge. Copyright © 2009 Anatoly Zak


A globe of Mars circa turn of the 20th century based on observations by the American astronomer Percival Lowell illustrates the popular and wildly inaccurate perception of the Red Planet as a world inhabited by an advanced civilization capable of constructing a collosal network of canals. Copyright © 2012 Anatoly Zak

Above: The geocentric system of the world, with the Earth at the center of the Universe, was as long lasting as it was flawed. Formulated in ancient Greece as a debatable theory, it was turned into a dogma by the church. Making the Earth the center of the Universe, the geocentric system had probably hampered the idea of space flight for 18 centuries.

Charting the Universe: achievements and setbacks

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Before humans could achieve space flight or even start dreaming about it, they had to discover its possibility. The idea of flight was probably obvious to the earliest humans, from observing birds and objects carried by the wind. However, drawing an even approximate picture of the Universe with conceivable travel destinations required centuries of overcoming counterintuitive concepts. According to the 20th century historians, to a scientifically unarmed inhabitant of the pre-historic world, the Earth looked flat. The night sky was peppered with bright dots of light and the lonely flat disk of the Moon. There were no hints of other worlds' existence, let alone their similarity to the Earth. Those living on the shores of great oceans could justifiably see them as the edge of the world extending endlessly into the abyss. Yet, the Sun had a mysterious capability to make a night-long trip "underground" to the opposite side of the world and reappear over the eastern horizon every morning. Along with the movement of the Moon and stars, it had to prompt questions about the dimensions and shape of the world.

Prehistoric structures like Stonehenge point to their architects' awareness of astronomical phenomena such as the movement of celestial bodies, however there is no sign of human investigation into the physical properties of the Moon, Sun or stars, until the rise of the Greek civilization.

The Greeks borrowed extensive astronomical knowledge from the Egyptians and Babylonians, however unlike their predecessors, they managed to combine observations with an active and persistent effort to explain the origin and the design of the Universe through science rather than myths. The result was a startling string of discoveries, which gave humanity its first chance to see the world as it is.

According to Greek historians, Thales of Miletus founded such sciences as mathematics and philosophy in the late 500s B.C. After a trip to Egypt, he founded the so-called Ionian school of Greek astronomy. However, despite his famous ability to predict a solar eclipse, he still pictured the Earth as a flat disk floating in the ocean, which was itself contained in a starry sphere. (348)

Another Greek thinker Anaximander (about 611-546 B.C.) is the first known challenger to the flat Earth theory. He speculated that the Earth was suspended freely at equal distance from all other heavenly bodies! Anaximander estimated the Moon's "orbit" to be 19 times the size of the Earth and the Sun's orbit 28 times the size of the Earth. He believed that the Sun is the size of the Earth. However, despite his brilliant estimates on the structure of the Earth and the Sun and their obvious globular shape, Anaximander described the Earth as a short cylinder, perhaps in the effort to explain the visible flatness of his home planet. Anaximander also made the first attempt to draw a map of the entire inhabited world.

Merely a generation later, Pythagoras (born 572 B.C.) declared the Earth a sphere, and it is safe to assume, he also attributed a spherical shape to the Moon, Sun and stars. (347) One of Pythagoras' followers, Philolaus made the first, though awkward attempt to draw a true picture of the Solar System, by suggesting that the sky, the planets, the Sun, the moon, the Earth and the "Counter earth" rotate around the Central Fire. It seems the picture of multiple worlds with a finite distance between them and the Earth as an ordinary body among them could now be imagined.

Even more remarkably, the debate about the possibility of other solar systems had also started in Greece. (148) With brilliant foresight, Democritus suggested the existence of multiple and diversely sized solar systems at different distances from each other! Despite an unproven believe by Aristotle about the impossibility of multiple worlds, Metrodotus echoed Democritus with a pointed metaphor: "it seems absurd that in a large field only one stalk should grow and in infinite space only one world exist." (213) Aristotle did make a good guess about layered nature of the Earth atmosphere, suggesting that only the lowest layer blanketing the Earth surface is suitable for life, while the next one was very cold and the top one was very hot.

Finally, Aristarchus, working in Alexandria at the beginning of the third century B.C. essentially completed an accurate picture of the solar system by placing the Sun in the center and the Earth and other planets into orbits around it. (350) Aristarchus even attempted to measure the relative distances of the Sun and Moon from the Earth but apparently failed due to lack of technology. (213) In the meantime, an effort to measure the size of the Earth by Eratosthenes was a success for all practical purposes, despite some miscalculations.

Thus, thanks to unprecedented and unhindered effort by the Greeks to understand the world, a monumental stage for space travel had been set, at least in the imagination of some enlightened individuals. On the practical side, the Greeks' intellectual genius brought them to the brink of development of the steam engine, jet propulsion and even mechanical computation devices, even though economics and the technological level of the era could provide little stimulus for application of these inventions.

Unfortunately, even in classical Greece, a heliocentric (Sun-centered) system was rejected by at least two prominent thinkers, Hipparchus and Ptolemaeus (Ptolemy). Despite their scientific foresight, both still preferred to cling to what seemed so obvious in the sky -- the entire Universe rotating about the stationary Earth. Ptolemy solidified that misconception in his summary of Greek knowledge about the Universe. His book would probably have been lost along with countless other works by classical authors in the turmoil of Dark Ages, if not for the efforts of a single ruler of the Arab caliphate in the 9th century. Fascinated with the Greek intellectual legacy, Caliph al-Mamun made the surrender of Ptolemy's treatise one of the conditions for peace with the besieged Byzantine Empire. The Arab version of the book then made a long journey back to the West. With the advance of Dark Ages in Europe, astronomy flourished in India, China and the Islamic World.

Post-classical era


During the middle ages, the Christian church combined an ideologically convenient ptolemaic Earth-centered system, with the naive picture of creation found in the Bible. For hundreds of years, those who challenged this grotesque world view risked torture and death at the hands of the Inquisition. As a silver lining, Christian theologians relayed to future generations a number of otherwise lost Greek notions about the Universe... in the process of refuting them as "heresies."

Only in 1543, or almost 18 centuries after the Greeks first hypothesized a heliocentric universe, a true renaissance man and amateur astronomer, Mikolaj Kopernik (known in Latin as Nicolas Copernicus) finally dealt a mortal blow to Ptolemy's concept of the world with a scientifically solid proof of a Sun-centered solar system. Copernicus postponed the publication of his revolutionary work until the end of his life, possibly in fear of religious persecution. Initial condemnation of Copernicus' work by the church was somewhat muted and inconsistent, however as the heliocentric system's popularity grew, especially through the efforts of Galileo Galilei, reaction followed. In 1616, the church banned the original version of the Copernicus' book along with any writing which dared to defend the heliocentric system. The church would not relent on its official ban of the book until 1835. The chilling effect of the Inquisition essentially pushed further advances in astronomy to the Protestant world. (213)


With Copernicus' monumental discoveries, his misconceptions about planets rotating in small circles, while simultaneously moving about the sun looked really minor in retrospect. He also mistakenly placed the Sun away from the center of the circle about which planets were moving, in an effort to explain deviations in observational data. Yet, the true picture of the Solar System still had to be refined. Johannes Kepler achieved just that at the beginning of the 17th century. Ironically, despite his adherence to ideal shapes of the Universe, Kepler was the one who finally established that planets move around the Sun, not in perfect circles but on ellipses, with the Sun sitting asymmetrically at one of two "focuses" of the ellipse.

Kepler also proved, with Mars as the example, that planets accelerate as they approach the Sun and then slow down as they move toward the point of the ellipse farthest from the Sun. He also correctly assumed that the life-supporting atmosphere forms only a thin mantle around the Earth and beyond it extends a deadly abyss of vacuum.

Ironically, while these remarkable foresights greatly advanced human understanding of the world, they also erected seemingly unchallengeable barriers on the way of those who tried to dream of realistic ways for space travel.


The invention of a practical telescope in Holland in 1608 forever revolutionized astronomy and gave the ancient science its most powerful tool. On July 26, 1609, at 9 p.m., Thomas Harriot apparently made very first astronomical observations with a telescope pointing his new "Dutch trunke" to the Moon. Harriot's drawings of the five-day old Moon showed the terminator (day and night) line. The observations had never been published. Shortly thereafter, Galileo Galilei used his own version of the device to finally prove that planets are globular worlds of various sizes, like the Earth and the Moon. In 1610, Jupiter revealed the first four of its moons to the peering power of Galilei's telescope, further expanding a list of destinations in the Solar System.

Within four decades, Johannes Hevelius published a detailed topographical study of the Moon based on telescope observations, dethroning it as the sister of the Earth. Instead Hevelius presented an essentially modern image of the Moon as a lifeless, waterless body with strange ring-shaped mountain ranges.

During the same century, calculations by Giovanni Cassini and other astronomers "expanded" the solar system to an astounding 80 million miles across (against an actual 93 million miles), essentially doubling the stage for potential exploration in comparison to previous estimates.

Despite the Moon lost appeal as an inhabitable world, Mars and Venus were proven to have atmospheres and thus presented intriguing mysteries. At the same time, a new understanding of distances devoid of air between those worlds, solidly relegated the idea of space travel to the realm of fiction.


In 1687, Isaac Newton postulated the law of universal gravitation, essentially discovering the most important mechanism for both the ancient science of astronomy and yet-to-be achieved space exploration. The concept of artificial satellite of the Earth was finally born. It took another century for Laplace to publish Celestial Mechanics, which used Newtonian principles to provide a detailed picture of the Solar System. Astronomers acquired the ability to predict the existence of unknown planets by detecting their gravitational influence on visible planetary bodies. The method was instrumental in predicting the existence of Neptune by Urbain Leverrier in 1846 and for the discovery of Pluto by Clyde Tombaugh in 1930.

Following the discovery of Uranus in 1781 by William Herschel, the list of objects in the Solar System started growing dramatically during the 19th century, as the "vast gap" between the orbits of Mars and Jupiter was being "filled" with newly discovered asteroids.

By the end of the 19th century, the true picture of the Solar System was finally established, just in time for the machine age to provide practical means of space travel. Still, countless ideas from the most imaginative fiction writers and the most advanced scientists were collapsing before the towering challenges of overcoming the Earth's gravity and surviving in the vacuum of space. Ironically, the dawn of the space age would ultimately rely on an invention which had been around for hundreds of years.

Next chapter: Origins of rocket technology

Milestones in astronomy:

3,000 B.C.: The Babylonians record the movement of celestial bodies. (348)

3,000 B.C.: A neolithic temple in Ireland carries a depiction of the Moon with lunar maria. (399)

2,000 B.C.: The existence and movement of planets is recognized in Babylon.

1921-1901 B.C.: Babylonian tables registering movements of Venus, Jupiter and Mars. (348)

1,500 B.C.: The egyptians use water clocks. (347)

1,259 B.C.: Chinese astronomers Hi and Ho are condemned to death "in accordance with the law" for their failure to predict a solar eclipse. (347)

1,000 B.C.: The Babylonians track the movements of the Sun, the Moon and the planets and determine time of day with a sundial.

1921-1901 B.C.: Tables recording the movement of Venus appear; Mars and Jupiter are also observed. (347)

1300 B.C.: An Egyptian star catalog lists 43 constellations. Five planets: Mercury, Venus, Mars, Jupiter and Saturn are also known. (347)

747 B.C.: During the reign of Nabonassar, the Babylonians start continuous observations of celestial bodies, according to Ptolemy. (347)

~650 B.C.: A Babylonian astronomer teaches on Kos, an island off the cost of Asia Minor, a region of active interaction between the Greeks and eastern cultures at the time.

624-547 B.C.: Thales, a Greek astronomer and philosopher lives in Miletus.

585 May 28 B.C.: Thales predicts a solar eclipse at a battle of Lydians and Medes.

572 B.C.: Pythagoras is born in Samos.

540-500 B.C.: Pythagoreans postulate the existence of "counter earth," a clone of the Earth, not visible from Greece, but which moves with the Earth. (148)

~466 B.C., early June - late August: A possible sighting of Comet Halley in Greece.

384 B.C.: Aristotle rejects plurality of planets, negatively impacting later thinkers. (148)

~360 B.C.: Heraclides of Pontus, a disciple of Plato, concludes that the Sun is the center of revolution for Mercury and Venus, but that the Sun itself revolves around the Earth.

331 B.C.: Soon after the Macedonians' capture of Babylon, Callipus, the nephew of Aristotle, delivers Babylonian astronomical records to Greece. (347)

~280 B.C.: Aristarchus suggests that the Earth revolves around the Sun. He provides the first estimation of the Earth-Sun distance. (213)

240 B.C.: Chinese astronomers record a sighting of what is later known as Comet Halley.

~240 B.C.: Eratosthenes measures the circumference of the Earth with surprising accuracy. He also invents a system of latitude and longitude, and a calendar that has leap years.

129 B.C.: Greek astronomer Hipparchus of Nicaea catalogues the relative positions and brightness of around one thousand stars with an accuracy of about one degree thanks to the use of gnomons, astrolabes, and armillary spheres. He also created the magnitude system for describing the brightness of stars in use until today.


1st century A.D.: First known magnetic compass invented in China.

46-120 (127): Plutarch believes the Moon is a solid, smaller Earth, inhabited by demons. (148)

140-150: Ptolemy suggests a geocentric theory of the universe in his main work entitled Megale Syntaxis (Great Collection).

Post-classical world

774-775: A powerful gamma-ray burst strikes Earth, possibly, as a result of a collision of a pair of black holes or neutron stars in our galaxy or a Sun flare.

~830: Along with a number of Greek writings, Ptolemy's Great Collection is translated into Arabic under the name Almagest for the Caliph al-Mamun.

1054: A supernova explodes in Crab Nebula.

1120-1125: One of the earliest observatories of Mediaeval Islam operates in Cairo until its patron is condemned to death for communication with Saturn and the facility is destroyed.

1145: The Christian church bans debate on the plurality of the worlds as heresy. (213)

1259: Hulagu, a Mongolian ruler, sponsors the construction of the Maragha observatory led by the Persian astronomer Nasir al-Din al-Tusi to satisfy the ruler's belief in astrology. The facility includes a big library and numerous outdoor instruments.

1277: Étienne Tempier, the bishop of Paris, with the blessing of Pope John XXI, officially rejects the church's previous view about the impossibility of more than one world on the grounds that "God's Plentitude and creative power could not be restricted." (213)

1428: Ulugh Beg (1394-1449), ruler of Samarkand, initiates the construction of an observatory in the city. The center includes a circular three-floor building and many large-scale, custom-built instruments. Among them is a quadrant so large that part of the ground has to be removed to allow it to fit in the observatory. There is also a marble sextant, a triquetram and an armillary sphere.

1437: Ulugh Beg publishes a catalogue of 992 stars and brings the accuracy of the year length to 365 days, 5 hours, 49 minutes and 15 seconds.

1543: "De revolutionibus orbium coelestium" (On the Revolutions of the Celestial Spheres) by Copernicus reviving the heliocentric theory of the Universe once and for all is published.

1572, early November: Observers on Earth witness the appearance of a "new star" in the constellation Cassiopeia, an event later recognized as the brightest naked-eye-visible supernova in more than 400 years. Tycho Brahe, a Danish astronomer, proved that the new object was among the stars, the sphere previously thought to be unchangeable.

1582: Pope Gregory XIII introduces the Gregorian calendar.

1598: Tycho Brahe completes a catalogue with the positions of about 1,000 stars thanks to the use of sextants and quadrants.

1603: In Augsburg, Germany, Johann Bayer publishes Uranometria star atlas, which introduces a new system of star designation, using Greek letters. It is still in use today and bears his name.

1604 Oct. 17: Johannes Kepler begins systematically observing a new, very bright star that had abruptly appeared in the constellation Ophiuchus. It was later identified as a supernova.

1608: In Netherlands, Hans Lippershey and Jacob Metius independently invent the telescope within weeks of each other.

1609 July 26: Outside London, Thomas Harriot makes first astronomical observations with a telescope.

1609: In Italy, Galileo uses a telescope for astronomical purposes. He observes the Moon's craters and the Milky Way galaxy.

1609: Johannes Kepler publishes "Astronomia Nova," containing a revolutionary explanation of planetary motions.

1609 Dec. 29: In Auerbach, Germany, Simon Marius possibly observes moons of Jupiter with a telescope.

1610 Jan. 7: In Padua, Itlay, Galileo makes observations with a telescope discovering moons of Jupiter.

1616: The Church officially outlaws Copernicus' book on heliocentric theory.

1619: Kepler introduces the 3rd Law of Planetary Motion in his work "Harmonice Mundi" (Harmony of the World).

1627: Tycho Brahe's catalogue with the positions of about 1,000 stars is published, greately improving known accuracy of star positions.

1633 April 12: The Church starts a trial against Galilei forcing him to recant his views. The astronomer will spend the rest of his life under house arrest.

1638: Bishop John Wilkins publishes his work about the Moon and the possibility of flying to it. (148)

1647: Johannes Hevelius (Michel Florent van Langren) publishes "Selenographia," a topographic review of the Moon, providing evidence that the Earth's natural satellite likely lacks air or water.

1651: An Italian jesuit priest Giovanni Battista Riccioli names many prominent features of the Moon, which became accepted. (399)

1655 March 25: In the Netherlands, Christian Huygens discovers Titan, Saturn's fourth satellite.

1656: Christian Huygens discovers Saturn's rings.

1659: Huygens notes markings on Mars.

1664: Robert Hooke observes a huge oval feature in the atmopshere of Jupiter.

1664 December - 1665: Two bright comets are observed across Europe.

1666: Cassini detects Martian polar ice caps.

1668: In England, Newton builds the first reflecting telescope.

1669: In Italy, Geminiano Montanari discovers that the star Algol is not steady in brightness, thus recognizing the first variable star.

1672: Astronomers use a close encounter with Mars to calculate the size of the Solar System. (213)

1675: Danish astronomer Ole Romer working in Paris measures the speed of light through observations of moons of Jupiter.

1675: Cassini discovers that Saturn's rings are split into two parts, separated by what is later known as the "Cassini Division".

1680 November: German astronomer Gottfried Kirch makes first discovery of a comet with the help of a telescope. Kirch comet becomes briefly visible in daylight on December 18.

1687: In England, Newton publishes his theory of universal gravitation in the work "Philosophiae Naturalis Principia Mathematica." It is later regarded as the beginning of modern astronomy.

1705: In England, Halley correctly predicts the return of a comet (later named Halley's comet) in 1758.

1713 March 20: The first planetarium arrives from Europe to St. Petersburg as a present to Peter the Great.

1758: In Germany, Johann Palitzsch observes Halley's comet as predicted by Halley in 1705.

1772: French mathematician Joseph-Louis Lagrange discovers locations in space where gravitational forces and the orbital motion of a body balance each other. They became known as Lagrangian or L points and centuries later would play an important role in space exploration.

1781: In England, William Herschel discovers Uranus.

1781: In France, Charles Messier catalogs 103 deep-space objects (later called Messier objects), including galaxies, nebula and star clusters, in an effort to distinguish them from comets and other transient bodies.

1789: William Herschel discovers two satellites of Saturn in addition to five moons already known. (Enceladus was discovered on Aug. 28.)

1801: In Italy, Giuseppe Piazzi discovers the first asteroid, Ceres.

1802: A second asteroid, Pallas, is discovered.

1835: The religious ban of Copernicus' book on the heliocentric theory ends.

1837: Madler and Beer publish a highly accurate map of the Moon.

1838 June: Johann Galle observes that in the ring of Saturn a veil extends across half the dark space separating it from the planet.

1839 Jan. 2: Louis Daguerre takes the first photograph of the Moon.

1842: In Austria, Christian Doppler, physicist and mathematician, discovers the phenomenon later known as 'Doppler Effect'.

1843: In Germany, Samuel Heinrich Schwabe suggests the existence of an approximately 10-year cycle in the Sun spot activity.

1846: In Germany, Johann Galle observes and discovers Neptune. His observations were prompted by mathematical calculations by the French astronomer Joseph Leverrier and the English astronomer John Couch Adams.

1846 oct. 10: In England, William Lassell discovers Triton, the Moon of Neptune with his 24-inch reflector telescope.

1848: Edward Roche suggests that rings of Saturn are probably made of smal particles.

1851 July 28: A Russian daguerrotypist named Berkowski takes the first photograph of a solar eclipse, using an 84-second exposure.

1852: Italian astronomer Annibale de Gasparis discovers 16 Psyche, proven to be a unique metal asteroid.

1857: A Scottish physicist James Clerk Maxwell proves mathematically that the rings of Saturn consist of particles orbiting Saturn.

1860-63: In England, William Huggins begins the spectral analysis of stars.

1868: Pierre Jansen and Joseph Lockyer observe solar prominence, detecting helium, the first chemical element to be discovered in space before it was found on Earth.

1872: In the US, Henry Draper takes the first photograph of the stellar spectrum from the star Vega.

1877 August: In the US, Asaph Hall discovers Deimos and Phobos, the moons of Mars.

1877: In Italy, Schiaparelli observes features on Mars, later sensationally described as "canals."

1892: E. E. Barnard discovers a fifth moon of Jupiter.

1905: Mount Wilson Observatory is established in California to study the Sun.

1905: Albert Einstein introduces the special Theory of Relativity in the paper "Electrodynamics of Moving Bodies."

1908: In Denmark, Ejnar Hertzsprung describes giant and dwarf stars.

1911-14: Ejnar Hertzsprung and Henry Norris Russel introduce the H-R diagram that shows how the characteristics of stars are related.

1915: Albert Einstein introduces the General Theory of Relativity.

1923: In the US, Edwin Hubble shows that galaxies exist outside the Milky Way.

1924: In the US, Frank Schlesinger publishes a catalogue with the positions of almost 2,000 stars to a precision of about one hundredth of an arc second.

1927: In Netherlands, Jan Oort shows that the center of the Milky Way galaxy is in Sagittarius.

1930: In the US, Clyde Tombaugh discovers Pluto.

1931: In the US, Karl Jansky discovers cosmic radio waves.

1937: In the US, the first radio telescope built by Grote Reber.

1949: Physicist Enrico Fermi suggests that the highest-energy cosmic rays are accelerated in the magnetic fields of interstellar gas clouds. In the decades that followed, astronomers showed that supernova remnants may be the galaxy's best candidate sites for this process.

1958 Nov. 13: Nikolay Kozyrev, sees a brightening at the central peak in the crater Alphonsus on the Moon. He photographs its spectrum, which shows carbon-vapour emissions.

1959: The Soviet Luna-3 probe delivers the first images of the far side of the Moon.

1965 July 14-15: The first close-up images of Mars delivered by Mariner-4 reveal a Moon-like cratered surface.

1966: The Soviet Luna-9 probe conducts a soft landing on the surface of the Moon and sends the first images from the surface.

1974: The U.S. Mariner-10 probe transmits the first image of Mercury.

1976: A pair of NASA's Viking probes lands on Mars.

1977: Uranus' rings are discovered.

1978 June 22: In the US, James Christy and Robert Harrington discover Charon, the moon of Pluto.

1980: NASA's Voyager 1 transmits the first images of Saturn and its rings.

1983: IRAS infrared telescope is launched.

1985: Charon starts eclipsing Pluto enabling the establishment of the size of the planet as 2,300 kilometers.

1986: The Soviet Vega and European Giotto spacecraft deliver the first close-up images of a comet.

1986: NASA's Voyager 2 flies by Uranus.

1990: The Hubble Space Telescope is launched.

1991: NASA launches the Gamma Ray Observatory, GRO, later known as Compton.

1992: A special Vatican comission established by Pope John Paul II rehabilitates Galileo, previously convicted of heresy for his support of the Copernicus' theory.

1994: Comet Shoemaker Levy crashes into Jupiter.

1999: NASA launches the Chandra X-ray Observatory.

2000: The NEAR spacecraft reaches asteroid Eros.

2000: NASA publishes evidence of recent water on Mars found in photos from Martian orbit.

2006: Russia launches the European Corot observatory to search for planets outside the Solar System.

2011: An exo-planet located in a potentially habitable zone around its host star is detected by NASA's Kepler space telescope.

2011 September: Observations made by Magellan telescope allowed to detect two tiny moons of Jupiter, bringing a number of the planet's natural satellites to 66. A pair of one-kilometer bodies make one orbit around its host planet in 580 and 726 days.

Writing and photography by Anatoly Zak; Last update: January 5, 2017

Page editor: Alain Chabot; Last edit: October 20, 2009

All rights reserved