Astronomy, 10 -ARI
Human beings have long been fascinated by the celestial sphere above, whose twinkling lights have inspired not only scientific theories but also many artistic endeavors. Humankind's fascination with the world beyond Earth has led to many landmark moments in history, as when space exploration took a giant step forward with the advent of technology that allowed humans to successfully travel to the Moon and to build spacecraft capable of exploring the rest of the solar system and beyond.
Astronomy Encyclopedia Articles By Title
10 Important Dates in Mars History
The question of whether life could have started on Mars has been asked for centuries—but there’s much more to Mars......
10 Important Dates in Pluto History
Always an oddity as a planet, Pluto was demoted to dwarf planet in 2006. That fateful day is, however, just one......
10 Places to Visit in the Solar System
Having a tough time deciding where to go on vacation? Do you want to go someplace with startling natural beauty......
10 Women Scientists Who Should Be More Famous
Not counting well-known women science Nobelists like Marie Curie or individuals such as Jane Goodall, Rosalind......
12 Questions About the History of Space Exploration Answered
The universe formed roughly 13.8 billion years ago. We humans have been exploring a small portion of the universe......
2001 Mars Odyssey
2001 Mars Odyssey, U.S. spacecraft that studied Mars from orbit and served as a communication relay for the Mars......
2024 Solar Eclipse FAQ
What is an eclipse? An eclipse occurs when one celestial body passes in front of another and obscures it. On Earth......
2024 YR4
2024 YR4, an asteroid expected to pass close to Earth on December 22, 2032. For a brief period after its discovery,......
21-centimetre radiation
21-centimetre radiation, electromagnetic radiation of radio wavelength emitted by cold, neutral, interstellar hydrogen......
5 Mysteries of Jupiter That Juno Might Solve
The Juno spacecraft arrives at Jupiter on July 4, 2016, after a journey of nearly five years and 2.7 billion km......
5 Things People See in the Moon
The Moon keeps one side facing Earth because its rotation period is the same as its orbital period. The Earth-facing......
5 Unforgettable Moments in the History of Spaceflight and Space Exploration
Humans have made great strides in spaceflight and space exploration in the relatively short amount of time since......
5 Weird Facts About Venus
The ancient Greeks knew the planet Venus by two different names: Phosphorus and Hesperus. Find out more about the......
51 Pegasi
51 Pegasi, fifth-magnitude star located 48 light-years away from Earth in the constellation Pegasus, the first......
51 Pegasi b
51 Pegasi b, the first extrasolar planet confirmed to orbit a sunlike star. The planet orbits a fifth-magnitude......
6 Questions About the Sun Answered
Earlier versions of these questions and answers first appeared in the second edition of The Handy Answer Book for......
61 Cygni
61 Cygni, first star whose distance from Earth was measured. German astronomer Friedrich Wilhelm Bessel obtained......
7 Accidents and Disasters in Spaceflight History
Closed quarters, vehicles faster than the speed of sound, zero gravity, and extremely volatile rockets. Do any......
7 Important Dates in Jupiter History
When ancient astronomers named the planet Jupiter for the Roman ruler of all the gods and heavens, they had no......
7 Important Dates in Mercury History
Until the last part of the 20th century, Mercury was one of the least-understood planets. Find out how these seven......
7 Questions for Astronaut Jessica Meir
Jessica Meir made history on October 18, 2019, when she and fellow astronaut Christina Koch made the first all-woman......
8 Questions About the Moon Answered
The Moon is Earth’s only natural satellite, and it’s been a source of human fascination since prehistoric times.......
9 Britannica Articles That Explain the Meaning of Life
The articles in this list don’t have all the answers. However, they serve as a useful primer on the basics—what......
9 Ghostly Planets
Humanity has sent probes to every planet, so we now have a decent idea of what’s in our neighborhood. Even before......
A Model of the Cosmos
Sometimes it’s hard to get a handle on the vastness of the universe. How far is an astronomical unit, anyhow? In......
Abbot, Charles Greeley
Charles Greeley Abbot was an American astrophysicist who, as director of the Smithsonian Astrophysical Observatory......
aberration, Constant of
Constant of aberration, in astronomy, the maximum amount of the apparent yearly aberrational displacement of a......
Abū al-Wafāʾ
Abū al-Wafāʾ was a distinguished Muslim astronomer and mathematician, who made important contributions to the development......
Acaba, Joseph
Joseph Acaba is an American astronaut who, over three spaceflights to the International Space Station (ISS), has......
Accretion disk
Accretion disk, a disklike flow of gas, plasma, dust, or particles around any astronomical object in which the......
Achernar
Achernar, brightest star in the constellation Eridanus and the ninth brightest star in the sky. Achernar (Arabic......
Achondrite
Achondrite, any stony meteorite containing no chondrules (small, roughly spherical objects that formed in the solar......
Active galactic nucleus
Active galactic nucleus (AGN), small region at the centre of a galaxy that emits a prodigious amount of energy......
Adams, John Couch
John Couch Adams was a British mathematician and astronomer, one of two people who independently discovered the......
Adams, Walter
Walter Adams was an American astronomer who is best known for his spectroscopic studies. Using the spectroscope,......
Adhara
Adhara, the 22nd brightest star in the sky and one of the 57 stars of celestial navigation. The Bayer designation......
AG catalog
AG catalog, compilation of the positions of all stars brighter than the ninth magnitude, compiled by the Astronomische......
AIM
AIM, U.S. satellite designed to study noctilucent clouds. AIM was launched on April 25, 2007, by a Pegasus XL rocket......
Airy, Sir George Biddell
Sir George Biddell Airy was an English scientist who was astronomer royal from 1835 to 1881. Airy graduated from......
Aitken, Robert Grant
Robert Grant Aitken was an American astronomer who specialized in the study of double stars, of which he discovered......
Akari
Akari, Japanese satellite observatory that carried a 67-cm (26-inch) near- to far-infrared telescope. On February......
Akatsuki
Akatsuki, space probe that investigated Venus in Japan’s first mission to the planet. An H-IIA rocket launched......
Akiyama Toyohiro
Akiyama Toyohiro was a Japanese journalist and television reporter, the first Japanese citizen and the first journalist......
Al Saud, Sultan ibn Salman
Sultan ibn Salman Al Saud is an astronaut who was the first Saudi Arabian citizen, the first Arab, the first Muslim,......
al-Ḥanafī, ʿAlam al-Dīn
ʿAlam al-Dīn al-Ḥanafī was an Egyptian mathematician, astronomer, and engineer. He wrote a treatise on Euclid’s......
Alcor
Alcor, star with apparent magnitude of 4.01. Alcor makes a visual double with the brighter star Mizar in the middle......
Aldebaran
Aldebaran, reddish giant star in the constellation Taurus. Aldebaran is one of the 15 brightest stars, with an......
Aldrin, Buzz
Buzz Aldrin is an American astronaut who was the second person to set foot on the Moon. A graduate of the U.S.......
Alfonsine Tables
Alfonsine Tables, the first set of astronomical tables prepared in Christian Europe. They enabled calculation of......
Alfvén, Hannes
Hannes Alfvén was an astrophysicist and winner, with Louis Néel of France, of the Nobel Prize for Physics in 1970......
Algol
Algol, prototype of a class of variable stars called eclipsing binaries, the second brightest star in the northern......
ALH84001
ALH84001, meteorite determined to have come from Mars and the subject of a contentious scientific claim that it......
alien
alien, hypothetical intelligent extraterrestrial being. See extraterrestrial intelligence. See also extraterrestrial......
Allende meteorite
Allende meteorite, meteorite that fell as a shower of stones (see meteorite shower) after breaking up in the atmosphere......
Almagest
Almagest, astronomical manual written about 150 ce by Ptolemy (Claudius Ptolemaeus of Alexandria). It served as......
Almucantar
Almucantar, in astronomy, any circle of the celestial sphere parallel to the horizon; when two objects are on the......
Alpha Centauri
Alpha Centauri, triple star system, the faintest component of which, Proxima Centauri, is the closest star to the......
Alpha Crucis
Alpha Crucis, brightest star in the southern constellation Crux (the Southern Cross) and the 13th brightest star......
Altair
Altair, the brighest star in the northern constellation Aquila and the 12th brightest star in the sky. With the......
Altitude and azimuthazimuth
Altitude and azimuthazimuth, in astronomy, gunnery, navigation, and other fields, two coordinates describing the......
Amalthea
Amalthea, small, potato-shaped moon of the planet Jupiter and the only Jovian satellite other than the four discovered......
Amaterasu
Amaterasu, (Japanese: “Great Divinity Illuminating Heaven”), the celestial sun goddess from whom the Japanese imperial......
Ambartsumian, Viktor
Viktor Ambartsumian was a Soviet astronomer and astrophysicist best known for his theories concerning the origin......
Amici, Giovanni Battista
Giovanni Battista Amici was an astronomer and optician who made important improvements in the mirrors of reflecting......
Anaxagoras
Anaxagoras was a Greek philosopher of nature remembered for his cosmology and for his discovery of the true cause......
Anaximander
Anaximander was a Greek philosopher who was the first to develop a cosmology, or systematic philosophical view......
Anaximenes Of Miletus
Anaximenes Of Miletus was a Greek philosopher of nature and one of three thinkers of Miletus traditionally considered......
Anders, William A.
William A. Anders was a U.S. astronaut who participated in the Apollo 8 flight (December 21–27, 1968), during which......
Andromeda
Andromeda, in astronomy, constellation of the northern sky at about one hour right ascension and 40° north declination.......
Andromeda Galaxy
Andromeda Galaxy, (catalog numbers NGC 224 and M31), great spiral galaxy in the constellation Andromeda, the nearest......
Andronicus of Cyrrhus
Andronicus of Cyrrhus was a Greek astronomer best known as the architect of the horologium at Athens called the......
Angel, Roger
Roger Angel is a British-born American astronomer whose lightweight mirror designs enabled the construction of......
Annadurai, Mylswamy
Mylswamy Annadurai is an Indian aerospace engineer who held a number of posts with the Indian Space Research Organisation......
Anomaly
Anomaly, in astronomy, originally the nonuniform (anomalous) apparent motions of the planets. In present usage,......
Antarctic meteorite
Antarctic meteorite, any of a large group of meteorites that have been collected in Antarctica, first by Japanese......
Antares
Antares, red, semiregular variable star, with apparent visual magnitude about 1.1, the brightest star in the zodiacal......
Anthropic principle
Anthropic principle, in cosmology, any consideration of the structure of the universe, the values of the constants......
Antikythera mechanism
Antikythera mechanism, ancient Greek mechanical device used to calculate and display information about astronomical......
Antlia
Antlia, constellation in the southern sky at about 10 hours right ascension and 30° south in declination. Its brightest......
Aouelloul Crater
Aouelloul Crater, large crater located 28 mi (45 km) southwest of Chinguetti, Mauritania, and thought to stem from......
Aphelion
Aphelion, in astronomy, the point in the orbit of a planet, comet, or other body most distant from the Sun. When......
Aphrodite Terra
Aphrodite Terra, the largest of three continent-sized highland areas (terrae) on the planet Venus. Aphrodite extends......
Apollo
Apollo, project conducted by the U.S. National Aeronautics and Space Administration (NASA) in the 1960s and ’70s......
Apollo 11
Apollo 11, U.S. spaceflight during which commander Neil Armstrong and lunar module pilot Edwin (“Buzz”) Aldrin,......
Apollo 13
Apollo 13, U.S. spaceflight, launched on April 11, 1970, that suffered an oxygen tank explosion en route to the......
Apollo 17
Apollo 17, U.S. crewed spaceflight to the Moon, launched on December 7, 1972, and successfully concluded on December......
Apollonius of Perga
Apollonius of Perga was a mathematician, known by his contemporaries as “the Great Geometer,” whose treatise Conics......
Apophis
Apophis, asteroid classified as a Near-Earth Object (NEO) and a Potentially Hazardous Asteroid (PHA). It is approximately......
Apus
Apus, constellation in the southern sky at about 16 hours right ascension and 80° south in declination. Its brightest......
Aquarius
Aquarius, in astronomy, zodiacal constellation lying in the southern sky between Capricorn and Pisces, between......
Aquarius/SAC-D
Aquarius/SAC-D, joint U.S.-Argentine space mission to map the salinity of Earth’s oceans. Aquarius/Satélite de......
Aquila
Aquila, constellation in the northern sky, at about 20 hours right ascension and on the celestial equator in declination.......
Ara
Ara, constellation in the southern sky, at about 17 hours right ascension and 55° south in declination. Ara has......
Arcturus
Arcturus, the fourth brightest star in the night sky, and the brightest star in the northern constellation Boötes,......
Area 51
Area 51, secret U.S. Air Force military installation located at Groom Lake in southern Nevada. It is administered......
Arecibo Observatory
Arecibo Observatory, astronomical observatory located 16 km (10 miles) south of the town of Arecibo in Puerto Rico.......
Ares
Ares, family of two launch vehicles, Ares I and Ares V, for the proposed Constellation program, the crewed U.S.......
Argelander, Friedrich Wilhelm August
Friedrich Wilhelm August Argelander was a German astronomer who established the study of variable stars as an independent......
Ariane
Ariane, family of launch vehicles developed as a means of independent access to space for the European Space Agency......
Ariel
Ariel, the first international cooperative Earth satellite, launched April 26, 1962, as a joint project of agencies......