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The Astronomy Portal

Astronomy portal

A man sitting on a chair mounted to a moving platform, staring through a large telescope.

Astronomy is a natural science that is the study of celestial objects (such as moons, planets, stars, nebulae, and galaxies), the physics, chemistry, and evolution of such objects, and phenomena that originate outside the atmosphere of Earth, including supernovae explosions, gamma ray bursts, and cosmic background radiation.

Astronomy is one of the oldest sciences. Prehistoric cultures have left astronomical artifacts such as the Egyptian monuments and Nubian monuments, and early civilizations such as the Babylonians, Greeks, Chinese, Indians, Iranians and Maya performed methodical observations of the night sky. However, the invention of the telescope was required before astronomy was able to develop into a modern science. Historically, astronomy has included disciplines as diverse as astrometry, celestial navigation, observational astronomy, and the making of calendars, but professional astronomy is nowadays often considered to be synonymous with astrophysics.

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Photograph of the crescent planet Neptune (top) and its moon Triton (center), taken by Voyager 2 during its 1989 flyby
The definition of planet has included a wide range of celestial bodies. Early use of the term was never strict and its meaning has blurred to include or exclude a variety of objects. By the end of the 19th century, the word planet had a more firm definition: it applied only to objects in the Solar System, a number small enough that any differences could be dealt with on an individual basis. After 1992 however, astronomers began to discover many additional objects beyond the orbit of Neptune, as well as hundreds of extrasolar planets. These discoveries not only increased the number of potential planets, but expanded their variety and peculiarity. The issue of a clear definition for planet came to a head in 2005 with the discovery of the trans-Neptunian object Eris, a body larger than the smallest then-accepted planet, Pluto. In its 2006 response to the discovery, the International Astronomical Union released its decision on the matter. Its definition, which applies only to the Solar System, states that a planet is a body that orbits the Sun, is massive enough for its own gravity to make it round, and has "cleared its neighbourhood" of smaller objects around its orbit. Under this new definition, Pluto and the other trans-Neptunian objects do not qualify as planets. (more...)

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M82 HST ACS 2006-14-a-large web.jpg
Credit: NASA / ESA / Hubble Heritage Team (STScI/AURA)

Messier 82, also known as NGC 3034, Cigar Galaxy, and M82, is the prototype starburst galaxy about 12 million light-years away in the constellation Ursa Major. The starburst galaxy is five times more luminous than the whole Milky Way, and one hundred times more luminous than our galaxy's center. This mosaic image, taken by the Hubble Telescope, is the sharpest wide-angle view ever obtained of Messier 82.

April anniversaries

  • 3 April 2014 – NASA announces that the Cassini orbiter has found evidence of an underground body of water on Enceladus, a moon of Saturn
  • 12 April 1961 – Cosmonaut Yuri Gagarin becomes the first human to enter outer space when he is launched into orbital flight in Vostok 1
  • 19 April 1971 – The first space station, Salyut 1, is launched into orbit
  • 24 April 1990 – The Hubble Space Telescope, a powerful research tool and public relations boon for astronomy, is launched into orbit

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These books may be in various stages of development. See also the related Science and Mathematics bookshelves.

  • Astronomy
  • GAT: A Glossary of Astronomical Terms
  • Introduction to Astrophysics
  • General relativity
  • Observing the Sky from 30°S
  • Observing the Sky from 40°N


  • Solar System

Astronomical events

All times UT unless otherwise specified.

1 April, 17:54 Mercury at inferior conjunction
8 April, 05:31 Moon at apogee
16 April, 01:57 New moon
18 April Uranus at conjunction
20 April, 14:33 Moon at perigee
22 April, 20:00 Lyrids peak
29 April, 18:24 Mercury at greatest western elongation
30 April, 00:58 Full moon


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