The Astronomy Portal


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

Astronomy (from Greek: ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, galaxies, and comets. Relevant phenomena include supernova explosions, gamma ray bursts, quasars, blazars, pulsars, and cosmic microwave background radiation. More generally, astronomy studies everything that originates beyond Earth's atmosphere. Cosmology is a branch of astronomy that studies the universe as a whole.

Astronomy is one of the oldest natural sciences. The early civilizations in recorded history made methodical observations of the night sky. These include the Babylonians, Greeks, Indians, Egyptians, Chinese, Maya, and many ancient indigenous peoples of the Americas. In the past, astronomy included disciplines as diverse as astrometry, celestial navigation, observational astronomy, and the making of calendars. Nowadays, professional astronomy is often said to be the same as astrophysics.

Professional astronomy is split into observational and theoretical branches. Observational astronomy is focused on acquiring data from observations of astronomical objects. This data is then analyzed using basic principles of physics. Theoretical astronomy is oriented toward the development of computer or analytical models to describe astronomical objects and phenomena. These two fields complement each other. Theoretical astronomy seeks to explain observational results and observations are used to confirm theoretical results.

Astronomy is one of the few sciences in which amateurs play an active role. This is especially true for the discovery and observation of transient events. Amateur astronomers have helped with many important discoveries, such as finding new comets. (Full article...)

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Photograph of the crescent of the planet Neptune (top) and its moon Triton (center), taken by Voyager 2 during its 1989 flyby

The definition of planet, since the word was coined by the ancient Greeks, has included within its scope a wide range of celestial bodies. Greek astronomers employed the term asteres planetai (ἀστέρες πλανῆται), "wandering stars", for star-like objects which apparently moved over the sky. Over the millennia, the term has included a variety of different objects, from the Sun and the Moon to satellites and asteroids.

In modern astronomy, there are two primary conceptions of a 'planet'. Disregarding the often inconsistent technical details, they are whether an astronomical body moves like a planet (that is, whether its orbit and relationship to other bodies are similar to those of the classical planets) or whether it looks like a planet (that is, whether it is round or has planetary geology). These may be characterized as the orbital definition and the geophysical definition. (Full article...)


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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.

Astronomy News

26 May 2021 – May 2021 lunar eclipse
A total lunar eclipse occurs, the second-shortest of the 21st century, with totality lasting for only 14 minutes. Nicknamed the Super Flower Blood Moon, the eclipse was visible over the Pacific Ocean and Oceania, with South and East Asia seeing the eclipse at moonrise, and the Americas seeing the eclipse at moonset. (Space.com)
17 February 2021 – Discoveries of exoplanets
Astronomers announce the discovery of HD 110082 b, a sub-Neptune exoplanet that is three times larger than Earth and which orbits a relatively young star. (Phys.org)

July anniversaries

Space-related Portals

Astronomical events

All times UT unless otherwise specified.

3 July, 11:41 Close approach of Venus and M44
4 July, 19:59 Mercury at greatest western elongation of 21.6 degrees from the Sun
5 July, 14:48 Moon at apogee
6 July, 02:59 Earth at aphelion
7 July, 04:39 Conjunction of the Moon and Mercury
10 July, 01:17 New moon
21 July, 10:24 Moon at perigee
24 July, 02:37 Full moon
28 July, 03:17 Peak of Delta Aquariids meteor shower



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