Astronomy

• 1. Introduction

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  Astronomy is one of the oldest natural sciences, and its earliest practitioners had no telescopes at all. Before recorded history, people stacked great stones at sites such as Stonehenge to track the movements of the sky. They buried a bronze disc at Nebra inlaid with symbols read as a sun, a moon, and a cluster of seven stars. In Upper Egypt, builders at Nabta Playa aligned megalithic arrangements with the heliacal rising of Sirius, using the heavens to calibrate the calendar for the annual Nile flood. So how did this practical sky-watching grow into a science that measures the cosmos in billions of years? How did observers move from counting stars by eye to weighing galaxies they could not even see? And why, in a discipline this technical, do amateurs still find new comets? The answers run through clay tablets, polished lenses, and detectors buried deep underground.

• 2. A Name And A Boundary

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  Astronomy comes from the Greek astron, meaning star, and nomos, meaning law or rule, so the word itself promises the study of celestial objects. That study should not be confused with astrology, the belief system claiming that human affairs correlate with the positions of celestial objects. The two fields share a common origin but became distinct. Astronomy is supported by physics, while astrology is not. In modern usage, astronomy reaches further than its name suggests, covering everything that originates beyond Earth's atmosphere.

  Astrophysics sits so close to astronomy that the two words are now broadly synonymous. By dictionary definition, astronomy studies objects and matter outside the Earth's atmosphere and their physical and chemical properties. Astrophysics deals with the behavior, physical properties, and dynamic processes of celestial objects and phenomena. In the introductory textbook The Physical Universe by Frank Shu, astronomy means the qualitative study, while astrophysics is the physics-oriented version. A field such as astrometry counts as purely astronomy in that sense. Many professional astronomers hold physics degrees rather than astronomy degrees, and research departments choose their label partly by which physics department they descend from.

• 3. The Ancient Sky-Watchers

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  The Babylonians took the decisive early step, building mathematical and scientific astronomy that laid foundations for traditions in other civilizations. They discovered that lunar eclipses recurred in the saros cycle of 223 synodic months. Egypt, Mesopotamia, Greece, India, and China each raised observatories and shaped their own ideas about the nature of the Universe, often through cross-cultural influence.

  Heracleides Ponticus, in the 4th century BC, was the first to propose that the Earth rotates on its own axis. Aristarchus of Samos went further in the 3rd century BC, estimating the size and distance of the Moon and Sun and placing the Sun at the center in a heliocentric model. Hipparchus, in the 2nd century BC, calculated the size and distance of the Moon and built the earliest known astronomical devices, including the astrolabe. He noticed a small drift in the equinoxes and solstices against the fixed stars, now known to be caused by precession. He also catalogued 1020 stars, and most northern hemisphere constellations descend from Greek astronomy.

  The Antikythera mechanism, dated to roughly 150 to 80 BC, was an analog computer that calculated the location of the Sun, Moon, and planets for a given date. Nothing of similar complexity reappeared until mechanical astronomical clocks arrived in 14th-century Europe. That gap of more than a thousand years measures how singular the device was.

• 4. An Earth-Centered Millennium

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  Claudius Ptolemy fixed the Earth at the center of everything, and the Western world believed it for over a thousand years. His 13-volume work, known by its Arabic title the Almagest, became the primary astronomical reference. In this geocentric system the Sun, Moon, and stars rotated around a stationary Earth. The model was eventually discredited, yet it gave the most accurate predictions of planetary positions available at the time.

  Indian astronomy entered a new phase in the early centuries CE as Hellenistic models arrived through trade and cultural contact. Earlier traditions such as the Vedanga Jyotisa supplied calendrical foundations. Scholars including Aryabhata, Varahamihira, and Brahmagupta integrated Greek models, with Aryabhata improving methods for planetary motions and eclipses. Later, the Kerala school refined observational practice and sharpened planetary and eclipse calculations.

  The medieval Islamic world made astronomy flourish, with observatories established by the early 9th century. In 964, the Persian astronomer Abd al-Rahman al-Sufi described the Andromeda Galaxy in his Book of Fixed Stars. The SN 1006 supernova, the brightest stellar event of the last thousand years, was recorded by the Egyptian Arabic astronomer Ali ibn Ridwan and by Chinese astronomers in 1006. Al-Biruni observed that the Sun's apogee was mobile rather than fixed, contradicting Ptolemy.

  The ruins at Great Zimbabwe and Timbuktu may have housed observatories. In post-classical West Africa, astronomers charted the heavens and drew the orbits of the planets from complex calculations. The Songhai historian Mahmud Kati documented a meteor shower in 1583. In medieval Europe, Richard of Wallingford, who lived from 1292 to 1336, invented the first astronomical clock and built an equatorium called the Albion for lunar, solar, and planetary longitudes.

• 5. Breaking The Circle

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  Nicolaus Copernicus, during the Renaissance, moved the Sun back to the center with a heliocentric model. He kept circular orbits, yet his system could calculate the size of planetary orbits and their periods. Its appealing simplicity won astronomers over even before Galileo confirmed it telescopically in the 1600s.

  Galileo Galilei observed phases on Venus by 1610, resembling those of the Moon and supporting the heliocentric view. The telescope had been invented sometime around 1608. Johannes Kepler, working through two decades of careful observations by Tycho Brahe, organized the heliocentric model quantitatively and replaced uniform circular motion with elliptical motion. Kepler wrote down the laws but never found the theory behind them.

  Isaac Newton supplied that theory, inventing celestial dynamics and the law of gravitation to explain the planets' motions. He built the reflecting telescope and, with Richard Bentley, proposed that stars are like the Sun, only much further away. During the 18th and 19th centuries, the three-body problem studied by Leonhard Euler, Alexis Claude Clairaut, and Jean le Rond d'Alembert sharpened predictions of the Moon and planets. Joseph-Louis Lagrange and Pierre Simon Laplace refined this further, letting the masses of planets and moons be estimated from their perturbations.

• 6. Counting The Galaxies

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  John Flamsteed, Britain's first Astronomer Royal, catalogued over 3000 stars, though the data were published against his wishes in 1712. James Gregory had earlier, in 1668, compared the luminosity of Jupiter to Sirius to estimate its distance at over 83,000 AU. William Herschel built a detailed catalog of nebulosity and clusters and in 1781 discovered Uranus, the first new planet found. Friedrich Bessel developed stellar parallax in 1838, but the technique was so hard to apply that only about 100 stars had been measured by 1900.

  New instruments reshaped the questions. Joseph von Fraunhofer discovered some 574 dark lines in the spectrum of the Sun and other stars in 1814 to 1815. Gustav Kirchhoff, in 1859, ascribed those lines to the presence of different elements. Herschel had mapped the distribution of stars in the late 1700s and concluded the Sun sat near the center of a disk, the Milky Way. After John Michell showed stars differ in intrinsic luminosity, astronomers began to wonder whether some fuzzy spiral nebulae were distant island Universes.

  Henrietta Leavitt cracked the distance problem in 1912 by finding Cepheid variable stars whose periodic luminosity changes reveal their true brightness. Harlow Shapley used Cepheids to build the first accurate map of the Milky Way. Edwin Hubble, using the Hooker Telescope, identified Cepheids in several spiral nebulae and proved in 1922 to 1923 that Andromeda and Triangulum were galaxies outside our own. The universe held a multitude of galaxies.

• 7. The Expanding Universe

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  Albert Einstein's 1917 publication of general relativity opened the modern era of theoretical models of the universe as a whole. Alexander Friedman published simplified models in 1922 showing static, expanding, and contracting solutions. Then in 1929 Hubble reported that galaxies are all moving away from Earth with a velocity proportional to distance, the relation now called Hubble's law. Such a relation is exactly what an expanding universe would produce.

  Georges Lemaitre had expounded the Big Bang concept in 1927, the idea that the universe was once very dense and hot. No experimental evidence supported it at first. From the 1940s onward, study of nuclear reaction rates under high density built a successful model of big bang nucleosynthesis by the late 1940s and early 1950s. The discovery of cosmic microwave background radiation in 1965 settled the evidence for the Big Bang.

  The Big Bang now anchors physical cosmology, the study of the large-scale structure of the Universe. By this account the cosmos began extremely dense and hot, then expanded over 13.8 billion years. Dark matter and dark energy are thought to form 96 percent of the Universe's mass. Their physics remains a central object of effort, since the visible matter is only a sliver of the whole.

• 8. Light, And What Lies Beyond It

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  Radio astronomy works at long wavelengths, mainly between 1 millimeter and 15 meters, far outside the visible range. Hydrogen, otherwise invisible, produces a spectral line at 21 centimeters, observable at radio wavelengths and revealing interstellar gas, pulsars, fast radio bursts, and active galactic nuclei. The pioneer of amateur radio astronomy, Karl Jansky, discovered a radio source at the centre of the Milky Way.

  Infrared astronomy reaches objects too cold to glow in visible light, such as planets and dust-shrouded nebulae. Its longer wavelengths pierce clouds of dust to reveal young stars embedded in molecular clouds. The James Webb Space Telescope senses infrared to detect very distant galaxies, whose visible light, emitted billions of years ago, was stretched into the infrared by the expanding universe. Optical astronomy, the oldest form, moved from hand-drawn images to photographic plates and then to charge-coupled devices.

  The most violent processes show up at the extremes of the spectrum. X-ray astronomy, demanding observations from balloons, rockets, or satellites, captures X-ray binaries, supernova remnants, and the hot solar corona. Gamma ray astronomy works at the shortest wavelengths and highest energies, with gamma-ray bursts ranking as the brightest phenomena in the universe.

  Some signals arrive without any light at all. In neutrino astronomy, shielded underground facilities such as SAGE, GALLEX, and Kamioka II/III catch neutrinos, and 24 of them were detected from supernova 1987A. Gravitational-wave astronomy reads the tremor of distant massive objects. LIGO made its first detection on the 14th of September 2015, observing gravitational waves from a binary black hole, with a second following on the 26th of December 2015. Combining light, neutrinos, and gravitational waves into one picture is known as multi-messenger astronomy, and it leaves astronomers still asking what dark matter is, why cosmic lithium runs four times lower than the standard model predicts, and whether other intelligent life exists.