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All About Astronomy
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Astronomy Dictionary
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Click on an underlined word for more information on that subject.

E



EAGLE NEBULA

The Eagle Nebula (M16) consists of enormous columns of cool interstellar hydrogen gas and dust that are about 7,000 light-years from Earth (in the constellation Serpens). This star-forming cloud is illuminated by ultraviolet light that is emitted from newborn stars. The columns are called EGG's (Evaporating Gaseous Globules) because the ultraviolet light boils off some of the hydrogen gas (H2), shaping the nebula in a process called photoevaporation. The biggest column is about 1 light-year tall.


EARTH

The Earth is the third planet from the sun.


EARTH GRAZER

An Earth grazer is a meteoroid (or other space debris) that enters the Earth's atmosphere and disintegrates, traveling nearly parallel to Earth's surface. The meteor burns up slowly, putting on a beautiful display.


EARTHSHINE

Earthshine is the faint light that is reflected from the Earth onto the dark part of the moon.
Planet Orbital Eccentricity
Mercury 0.206
Venus 0.007
Earth 0.017
Mars 0.093
Jupiter 0.048
Saturn 0.056
Uranus 0.047
Neptune 0.009
Pluto 0.248

ECCENTRICITY
Eccentricity is a measure of how an orbit deviates from circular. A perfectly circular orbit has an eccentricity of zero; an eccentricity between 0 and 1 represents an elliptical orbit. A parabolic orbit has an eccentricity equal to 1; a hyperbolic orbit has an eccentricity greater than 1. Neptune, Venus, and Earth are the planets with the least eccentric orbits in our solar system. Pluto and Mercury are the planets with the most eccentric orbits in our solar system.


ECLIPSE

An eclipse happens when the moon blocks the Sun or the Earth's shadow falls on the moon.

Some Eclipsing Binaries Magnitude Range Period
(days)
Delta Librae 4.9-5.9 2.3
Algol
(Beta Persei)
2.1-3.4 2.9

ECLIPSING BINARY

An eclipsing binary is two close stars that appear to be a single star varying in brightness. The variation in brightness is due to the stars periodically obscuring or enhancing one another.


ECLIPTIC

The ecliptic is the plane defined by the Earth's orbit around the Sun; conversely, in the course of a year, the sun traces a path in the sky along the ecliptic. Most of the planets in our solar system appear close to the ecliptic plane from Earth. The Earth's axis is tilted at a 23.5° from the ecliptic (which causes the seasons).

EDDINGTON, ARTHUR

Arthur Eddington (1882-1945) was an English astronomer who first described the internal structure of a star.


EGG NEBULA

The Egg nebula (CRL2688) is a planetary nebula about 3,000 light-years from Earth. This huge cloud of dust and gas is expanding outwards at very high speeds. It was formed by a red giant star a few hundred years ago (this red giant was once a Sun-like star). Dust obscures our view of the star within.
EINSTEIN, ALBERT
Albert Einstein (1879-1955) was a German/American physicist. He revolutionized our conception of the universe with his Theories of Special and General Relativity.

Special relativity supplanted Newtonian mechanics, yielding different results for very fast-moving objects. The Theory of Special Relativity is based on the idea that speed has an upper bound; nothing can pass the speed of light. The theory also states that time and distance measurements are not absolute but are instead relative to the observer's frame of reference. Space and time are viewed as aspects of a single phenomenon, called space-time. Energy and momentum are similarly linked. As a result, mass can be converted into huge amounts of energy, and vice versa, according to the formula E=mc2.

General Relativity expands the theory of special relativity to include acceleration and gravity, both of which are explained via the curvature of space-time. His theories explained the perturbations in the orbits of Mercury.

Einstein won the Nobel Prize in 1921 for explaining the photoelectric effect.


EINSTEIN-ROSEN BRIDGE

A Einstein-Rosen Bridge (named for Albert Einstein and Nathan Rosen, and commonly known as a wormhole in space) is a mathematical solution to Einstein's theory of General Relativity. A Lorentzian wormhole would theoretically provide a shortcut through widely-separated parts of space-time, through a black hole and out of a white hole (moving faster than the speed of light). Many physicists believe that wormholes have no physical reality because wormholes require "exotic matter," matter which is repelled by gravity (rather than attracted by it).


EJECTA

Ejecta is rocks, dust, and other material thrown from a volcano, impact crater, or exploding star.

ELARA

Elara is Jupiter's twelfth moon. Elara is 50 miles (80 km) in diameter and orbits 7,250,000 miles (11,737,000 km) from Jupiter. Elara has a mass of 8 x 1017kg. It orbits Jupiter in 259.6528 (Earth) days. Very little is known about Elara. Elara was discovered by C. Perrine in 1905.
Wave Wavelength
Gamma rays less that 0.1 Angstroms
X-Rays 0.1 to 200 Angstroms
Ultraviolet 200 - 4,000 Angstroms
Visible light 4,000 - 7,000 Angstroms
Near Infrared 7000 Angstroms to 10 microns
Mid-Infrared 10 microns to 60 microns
Far Infrared 60 microns to 300 microns
Sub millimeter 300 microns to 1 millimeter
Millimeter radio 1 millimeter to 1 centimeter
Microwave radio 1 millimeter to several centimeters

ELECTROMAGNETIC RADIATION
Electromagnetic radiation is energy in wave form.

ELECTROMAGNETIC SPECTRUM

The electromagnetic spectrum is full range of electromagnetic radiation, including: gamma rays, X-rays, UV rays, visible light, infrared, microwaves, and radio waves (in order of decreasing energy and increasing wavelength).
Atomic structure

ELECTRON

Electrons are tiny, negatively-charged atomic particles. In an atom, they orbit around the nucleus. Electrical currents are composed of electrons removed from atoms. The electron was discovered by J. J. Thompson in 1897.

ELEMENT

An element is a chemically pure substance composed of atoms of a single type. The lightest element is hydrogen, which makes up almost 80% of the universe. It is followed by helium, which makes up a bit under 20% of the universe. Other common elements are oxygen, nitrogen, silicon, iron, and carbon. Hydrogen (deuterium) helium and some lithium were created by nucleosynthesis just after the Big Bang. The next heaviest elements (like carbon, nirogen, and oxygen) are formed inside stars via fusion. Most stars fuse hydrogen, forming helium. The heavier elements (like iron, copper and gold) are created as massive stars undergo supernova. Every atom around us and in our bodies was made in the big bang or in a star! A few man-made elements, like seaborgium, have been created in laboratories.


ELLIPSE

An ellipse looks like a flattened circle. It consists of all the points in a plane that satisfy the following: a+b=(twice the length of the semi-major axis), where a is the distance from one focus to the point on the ellipse, and b is the distance from the other focus to the same point on the ellipse.


ELLIPTICAL GALAXY

An elliptical galaxy is a galaxy that has the shape of an ellipse. It is also called an "E" or "E-type" galaxy. M87 and M32 are elliptical galaxies.


ELLIPTICAL ORBIT

An elliptical orbit is an orbit that traces out an ellipse as the orbiter rotates around another body (which is located at one focus of the ellipse).


ELONGATION

Elongation is the angle between a planet (or moon or other object) and the Sun as seen from the Earth.

EMISSION NEBULA

An emission nebula is a nebula that glows; it emits light energy. The reddish light is produced when electrons and protons combine, forming hydrogen atoms. Emission nebulae are formed when energetic ultraviolet light from a very hot star excites a cloud of hydrogen gas; the UV radiation ionizes the hydrogen (it strips electrons from the hydrogen atoms). The free electrons combine with protons, forming hydrogen and red light. The Horsehead Nebula is in front of an emission nebula which illuminates the outline of the "horse head."


EMISSION SPECTRUM

An emission spectrum (also called a bright-line spectrum) is a discontinuous spectrum (discrete bright bands) that is emitted by atoms or molecules. The emission spectrum is characteristic of the chemicals which are emitting the photons. It looks like a series of bright lines on a black background (this is called spectroscopy).


ENCELADUS

Enceladus is one of the 18 moons of Saturn. It orbits at a mean distance of about 158,000 miles and revolves around Saturn in 33 hours. It has a diameter of about 400 miles. Enceladus reflects almost all of the sunlight that hits it and is very cold ( -201° C = -330° F). This icy moon was discovered by Wm. Herschel in 1789.


ENCKE DIVISION

The Encke Division splits the A Ring, the outermost of the major rings of Saturn. This gap is 200 miles (325 km) wide and is 83,000 miles (133,570 km) from the center of Saturn. It was named for Johann Encke, who discovered it in 1837.


ENERGY

Energy is the capacity for doing work. Energy can change from one form (heat, chemical, nuclear, potential energy) into another but is always conserved. In nuclear reactions (fission or fusion), mass can be converted into energy.
Eon Time
Phanerozoic Eon 540 million years ago through today
Proterozoic Eon 2.5 billion years ago to 540 million years ago
Archaeozoic Eon 3.9 to 2.5 billion years ago
Hadean Eon 4.6 to 3.9 billion years ago

EON

Two or more geological Eras form an Eon, which is the largest division of geological time, lasting hundreds of millions of years.

EPHEMERIS

An ephemeris is a table listing the spatial coordinates of celestial bodies and spacecraft as a function of time.


EPICYCLE

Epicycles are circular orbits within orbits that were used to (incorrectly) describe the orbits of objects in the Ptolemaic system (about A.D. 150). In Ptolemy's model of the solar system, an orbiting planet (or moon) moved in a series of circular orbits (epicycles) and the center of these epicycles orbited in another circular orbit (called the deferent) that was a circle offset from the Earth. Many layers of epicycles were needed to approximate real (elliptical) orbits with their retrograde motion. Copernicus also used epicycles in his heliocentric (sun-centered) solar system model (in the mid 1500's). Epicycles were used to predict orbits until Kepler discovered the elliptical nature of orbits (early in the 1600's).


EPIMETHEUS

Epimetheus is one of the 18 moons of Saturn. This moon is covered by grooves, valleys, and craters over 30 km in diameter. Epimetheus orbits at a mean distance of about 151,422 km and revolves around Saturn in about 4 hours. Epimetheus has a diameter of about 200 km and an irregular shape. Epimetheus shares an orbit with Janus (they are only separated by about 50 kilometers (31 miles)). Once every four years they approach each other, exchange some momentum and switch orbits. This icy moon was discovered from work done by astronomers R. Walker in 1966 and J. Fountain and S. Larson in 1978.

EPOCH

An epoch is a division of a geologic period; it is the smallest division of geologic time, lasting several million years.


EQUATOR

The equator is an imaginary circle around the earth (or other planet or moon), halfway between the north and south poles.


EQUATORIAL TELESCOPE MOUNT

An equatorial telescope mount is a complex device that is aligned parallel to Earth's axis and is pivoted at a right angle in order to follow the apparent motion of celestial objects (caused by the Earth's spinning). Equatorial mounts can be fitted with an electric motor (a clock drive), that will automatically turn the telescope, compensating for the spinning of the Earth (this can also be done by hand).

EQUINOX

Equinoxes are days in which day and night are of equal duration. The two yearly equinoxes occur when the Sun crosses the celestial equator. The autumnal equinox occurs in late September; the vernal equinox occurs in late March.

ERA

Two or more geological periods comprise an Era, which is hundreds of millions of years in duration.

ERATOSTHENES

Eratosthenes (276-194 BC) was a Greek scholar who was the first person to determine the circumference of the Earth. He compared the midsummer's noon shadow in deep wells in Syene (now Aswan on the Nile in Egypt) and Alexandria. He properly assumed that the Sun's rays are virtually parallel (since the Sun is so far away ). Knowing the distance between the two locations, he calculated the circumference of the Earth to be 250,000 stadia. Exactly how long a stadia is is unknown, so his accuracy is uncertain, but he was very close. He also accurately measured the tilt of the Earth's axis and the distance to the sun and moon, and devised a method for determining the prime numbers up to a given number (the Sieve of Eratosthenes). Eratosthenes made numerous contributions to the sciences and arts in many fields, including geography, mathematics, astronomy, chronography (calendars), music, and literature. Eratosthenes was a brilliant all-around scholar; although not the top expert on any topic, he was well-versed in all subjects, and therefore nicknamed "Beta" (which is the second letter of the Greek alphabet).


ERIDANUS

[Abbreviation: Eri] Eridanus (the River) is a southern constellation that is may have been named for the river Nile, the river Euphrates, or for the river of tears wept by the mythical Heliades. Eridanus is located near the constellations Fornax and Lepus. The brightest star in Eridanus, alpha Eri, is Achernar (meaning "end of the river"); it is the 9th brightest star in the sky (magnitude 0.46). The second-brightest, beta Eri, is Cursa (meaning "chair/footstool of Orion" ). The third-brightest, gamma Eri, is Zaurak (meaning "boat" ). The fourth-brightest, delta Eri, is Rana (meaning "frog" ). The fifth-brightest, zeta Eri, is Zibal (part of the "Ostrich's Nest"). Eridanus was one of the original 48 constellations first noted by Ptolemy


EROS

Eros, Asteroid #433, is a Near-Earth Asteroid (NEA). This elongated asteroid is 21 by 8 by 8 miles (33 by 13 by 13 kilometers). The force of gravity on Eros is 1000 times weaker than the gravity on Earth; it has no atmosphere. The density of Eros is 2.4 grams per cubic centimeter, roughly the same as the density of Earth's crust. The surface of Eros is littered with dust, rocks, boulders and craters. Eros varies from about 1.14 AU to 1.78AU from the Sun (it orbits the Sun in a slightly more elliptical orbit than the Earth and is a bit farther from the Sun than the Earth).
Planet or Moon Escape Velocity
meters/sec miles/hour
Mercury 4,300 9675
Venus 10,400 23,400
Earth 11,200 24,840
The Moon 2,361 5,300
Mars 5,000 11,250
Jupiter 59,500 133,875
Saturn 35,600 79,637
Uranus 21,300 47,925
Neptune 23,300 52,122
Pluto 1,100 2,461

ESCAPE VELOCITY
The escape velocity is how fast an object has to be moving away from a planetary object in order to escape its gravitational field.


ESKIMO NEBULA

The Eskimo Nebula (NGC 2392) is a plantary nebula in Gemini. It is a dying sun-like star, whose outer layers have begun to drift off into space. It was first sighted by William Herschel in 1787. The picture above is from the Hubble Space Telescope


ETA AQUARID METEOR SHOWER

The Eta Aquarids are a meteor shower that occur each year from April 21-May 12, with a maximum on May 5-6. This meteor shower occurs each year as the Earth passes through the orbit of Halley's comet, and icy debris from the comet burns up as it enters the Earth's atmosphere. The meteors in this shower seem to emanate from the constellation Aquarius, but they do not come from those stars.

EUDOXUS OF CNIDUS

Eudoxus of Cnidus (408-355 B.C.) was a Greek scholar (perhaps a student of Plato) who theorized that the Earth was at the center of the universe and that the other celestial objects (stars and planets) were set into geometric spheres around the Earth. His major contribution was inventing the modern notion of real numbers.


EUROPA

Europa is a large, dense, icy moon of Jupiter. Its surface is covered with long, crisscrossing trackways (but few craters) and frozen sulphuric acid. Its diameter is less than 2,000 miles (3,138 km), smaller than the Earth's moon. It takes Europa 3.55 days to orbit Jupiter. Its mean distance from Jupiter is about 420,000 miles (670,900 km). Its mass is 4.80x1022 kg. It was discovered by Galileo and S. Marius (independently) in 1610.


EVAPORATION

Evaporation is the process in which a liquid is transformed into a gaseous form.

EVENT HORIZON

The event horizon is the radius from a black hole inside of which it is impossible to escape (a "point of no return" called the Schwarzschild radius). It is also the radius at which a mass must be compressed down to in order to turn it into a black hole.


EVOLUTIONARY TRACK

An Evolutionary Track is the change in location of a star on the Hertzsprung - Russell (H-R) Diagram. As a star ages and evolves, you can trace out its history on the H-R diagram.

EVOLVED STAR

An evolved star is an old star that is near the end of its existence. Its nuclear fuel is mostly gone. The star loses mass from its surface, producing a stellar wind.

EXOBIOLOGY

Exobiology is the scientific field which studies the hypothetical biology, biochemistry, and life forms on worlds other than Earth. This field was pioneered by Carl Sagan.
Star Wobble

EXOPLANET

An exoplanet (short for extrasolar planet) is a planet that orbits a star other than the Sun. These exoplanets are detected by observing their star's "wobble" which the exoplanet's gravitational attraction causes.


EXOSPHERE

The exosphere is the outermost layer of the Earth's atmosphere. The exosphere goes from about 400 miles (640 km) high to about 800 miles (1,280 km). The lower boundary of the exosphere is called the critical level of escape, where atmospheric pressure is very low (the gas atoms are very widely spaced) and the temperature is very low.

EXPANDING UNIVERSE

The expanding universe is a model of the universe in which galaxies are receding from one another at a speed proportional to their separation - it is based on the observed Doppler redshift of distant galaxies. Hubble's constant describes the rate of the cosmic expansion.

EXTRAGALACTIC

Extragalactic means outside of, or beyond, our galaxy (the Milky Way Galaxy).


EYEPIECE

The eyepiece is the part of a telescope that you look into. It is a lens that magnifies the image formed by the main optical system. The eyepiece lens is close to the observer's eye and in better telescopes, is removable (to easily change the power of the telescope).
Astronomy Dictionary
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

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