Astronomy
The Nebra sky disc, found near a possibly astronomical complex in Germany, serves as an ancient calendar defining a year as twelve lunar months of 354 days. This artifact from the Bronze Age includes symbols interpreted as a sun, moon, and stars including a cluster of seven stars known as the Pleiades. Megalithic structures at Nabta Playa in Upper Egypt featured astronomy arrangements aligned with the heliacal rising of Sirius to support calibration for the annual Nile flood. These practices have been linked with the emergence of cosmology in Old Kingdom Egypt around 2600 BCE. Babylonian planispheres from the 7th century BCE used sexagesimals like 12, 24, 60, and 360 which are still being used today through having been broadly adopted for timekeeping and astrometry. The Babylonians discovered that lunar eclipses recurred in the saros cycle of 223 synodic months. Greek astronomers made significant advances starting in the 4th century BC when Heracleides Ponticus proposed that the Earth rotates on its own axis. Aristarchus of Samos estimated the size and distance of the Moon and Sun in the 3rd century BC while proposing a model where the Earth and planets rotated around the Sun. Hipparchus calculated the size and distance of the Moon and invented the earliest known astronomical devices such as the astrolabe in the 2nd century BC. He also observed the small drift in the positions of the equinoxes and solstices with respect to the fixed stars that we now know is caused by precession. Hipparchus created a catalog of 1020 stars and most of the constellations of the northern hemisphere derive from Greek astronomy. The Antikythera mechanism dating to 80 BC was an early analog computer designed to calculate the location of the Sun, Moon, and planets for a given date.
Nicolaus Copernicus proposed a heliocentric model of the solar system during the Renaissance which maintained circular orbits but was sufficient to calculate the size of planetary orbits and their period. Galileo Galilei observed phases on the planet Venus similar to those of the Moon in 1610 supporting the heliocentric model. Johannes Kepler organized the heliocentric model quantitatively around the same time analyzing two decades of careful observations by Tycho Brahe. Kepler devised a system that described the details of the motion of the planets around the Sun while discarding the uniform circular motion of Copernicus in favor of elliptical motion. Isaac Newton developed celestial dynamics and his law of gravitation which finally explained the motions of the planets. Newton also developed the reflecting telescope. In 1610 Galileo discovered that the band of light crossing the sky at night called the Milky Way was composed of numerous stars. James Gregory compared the luminosity of Jupiter to Sirius in 1668 to estimate its distance at over 83,000 AU. The English astronomer John Flamsteed catalogued over 3000 stars but the data were published against his wishes in 1712. William Herschel made a detailed catalog of nebulosity and clusters and in 1781 discovered the planet Uranus the first new planet found. Friedrich Bessel developed the technique of stellar parallax in 1838 but it was so difficult to apply that only about 100 stars were measured by 1900. During the 18th and 19th centuries the study of the three-body problem by Leonhard Euler Alexis Claude Clairaut and Jean le Rond d'Alembert led to more accurate predictions about the motions of the Moon and planets.
Albert Einstein's 1917 publication of general relativity began the modern era of theoretical models of the universe as a whole. Alexander Friedman published simplified models for the universe showing static expanding and contracting solutions in 1922. Hubble published observations in 1929 that the galaxies are all moving away from Earth with a velocity proportional to distance known as Hubble's law. This relation is expected if the universe is expanding. The consequence that the universe was once very dense and hot a Big Bang concept expounded by Georges Lemaître in 1927 was discussed but no experimental evidence was available to support it. From the 1940s on nuclear reaction rates under high density conditions were studied leading to the development of a successful model of big bang nucleosynthesis in the late 1940s and early 1950s. Then in 1965 cosmic microwave background radiation was discovered cementing the evidence for the Big Bang. Henrietta Leavitt discovered Cepheid variable stars with well-defined periodic luminosity changes which can be used to fix the star's true luminosity in 1912. Using Cepheid variable stars Harlow Shapley constructed the first accurate map of the Milky Way. Edwin Hubble identified Cepheid variables in several spiral nebulae and in 1922, 1923 proved conclusively that Andromeda Nebula and Triangulum among others were entire galaxies outside our own thus proving that the universe consists of a multitude of galaxies.
Radio astronomy uses radiation with long wavelengths mainly between 1 millimeter and 15 meters frequencies from 20 MHz to 300 GHz far outside the visible range. Hydrogen otherwise an invisible gas produces a spectral line at 21 cm 1420 MHz which is observable at radio wavelengths. Objects observable at radio wavelengths include interstellar gas pulsars fast radio bursts supernovae and active galactic nuclei. Infrared astronomy detects infrared radiation with wavelengths longer than red visible light outside the range of our vision. The infrared spectrum is useful for studying objects that are too cold to radiate visible light such as planets circumstellar disks or nebulae whose light is blocked by dust. Observations from the Wide-field Infrared Survey Explorer WISE have been particularly effective at unveiling numerous galactic protostars and their host star clusters. The James Webb Space Telescope senses infrared radiation to detect very distant galaxies. Visible light from these galaxies was emitted billions of years ago and the expansion of the universe shifted the light in to the infrared range. Ultraviolet astronomy employs ultraviolet wavelengths which are absorbed by the Earth's atmosphere requiring observations from the upper atmosphere or from space. X-ray astronomy uses X-radiation produced by extremely hot and high-energy processes since X-rays are absorbed by the Earth's atmosphere observations must be performed at high altitude such as from balloons rockets or specialized satellites. Gamma ray astronomy observes astronomical objects at the shortest wavelengths highest energy of the electromagnetic spectrum.
Some events originating from great distances may be observed from the Earth using systems that do not rely on electromagnetic radiation. In neutrino astronomy astronomers use heavily shielded underground facilities such as SAGE GALLEX and Kamioka II/III for the detection of neutrinos. The vast majority of the neutrinos streaming through the Earth originate from the Sun but 24 neutrinos were also detected from supernova 1987A. Cosmic rays which consist of very high energy particles atomic nuclei that can decay or be absorbed when they enter the Earth's atmosphere result in a cascade of secondary particles which can be detected by current observatories. Gravitational-wave astronomy employs gravitational-wave detectors to collect observational data about distant massive objects. A few observatories have been constructed such as the Laser Interferometer Gravitational Observatory LIGO. LIGO made its first detection on the 14th of September 2015 observing gravitational waves from a binary black hole. A second gravitational wave was detected on the 26th of December 2015 and additional observations should continue but gravitational waves require extremely sensitive instruments. The combination of observations made using electromagnetic radiation neutrinos or gravitational waves and other complementary information is known as multi-messenger astronomy.
Astronomy is one of the sciences to which amateurs can contribute the most. Collectively amateur astronomers observe celestial objects and phenomena sometimes with consumer-level equipment or equipment that they build themselves. Common targets include the Sun the Moon planets stars comets meteor showers and deep-sky objects such as star clusters galaxies and nebulae. Astronomy clubs throughout the world have programs to help their members set up and run observational programs such as to observe all the objects in the Messier catalog containing 110 objects or Herschel 400 catalogues. Most amateurs work at visible wavelengths but some have experimented with wavelengths outside the visible spectrum. The pioneer of amateur radio astronomy Karl Jansky discovered a radio source at the centre of the Milky Way. Some amateur astronomers use homemade telescopes or radio telescopes originally built for astronomy research such as the One-Mile Telescope. Amateurs can make occultation measurements to refine the orbits of minor planets. They can discover comets and perform regular observations of variable stars. Improvements in digital technology have allowed amateurs to make advances in astrophotography. Amateur astronomers hold star parties and gatherings such as Stellafane where they share knowledge and equipment.
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Common questions
What is the Nebra sky disc and what does it represent?
The Nebra sky disc serves as an ancient calendar defining a year as twelve lunar months of 354 days. This artifact from the Bronze Age includes symbols interpreted as a sun, moon, and stars including a cluster of seven stars known as the Pleiades.
When did Babylonian planispheres use sexagesimals for timekeeping?
Babylonian planispheres from the 7th century BCE used sexagesimals like 12, 24, 60, and 360 which are still being used today through having been broadly adopted for timekeeping and astrometry. The Babylonians discovered that lunar eclipses recurred in the saros cycle of 223 synodic months.
Who proposed that the Earth rotates on its own axis during the 4th century BC?
Greek astronomers made significant advances starting in the 4th century BC when Heracleides Ponticus proposed that the Earth rotates on its own axis. Aristarchus of Samos estimated the size and distance of the Moon and Sun in the 3rd century BC while proposing a model where the Earth and planets rotated around the Sun.
What was the result of Hubble's observations published in 1929 regarding galaxies?
Hubble published observations in 1929 that the galaxies are all moving away from Earth with a velocity proportional to distance known as Hubble's law. This relation is expected if the universe is expanding.
When did LIGO make its first detection of gravitational waves?
LIGO made its first detection on the 14th of September 2015 observing gravitational waves from a binary black hole. A second gravitational wave was detected on the 26th of December 2015 and additional observations should continue but gravitational waves require extremely sensitive instruments.