AstroGlossary - E
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Earth
The third planet from the Sun in the Solar System, and the planet on which people live. The Earth orbits the Sun at an average distance of 1.0 A.U. and has a mass of 6x10^24 kg. It is made of solid, rocky materials such as iron, nickel, and silicon, and it has an atmosphere composed mainly of nitrogen, oxygen, argon, and carbon dioxide. Liquid water oceans cover 75% of the planet's surface. Earth has one moon. Earth's Moon
The only natural satellite of Earth. eccentricity
A measure of the shape of the orbit of a body or ellipse that compares the lengths of the semimajor axis (longest length) and semiminor axis (shortest length). The eccentricity of a circle is zero, while a highly elliptical orbital path of a comet might be 0.9. eclipse
Total or partial masking of a celestial body by another along the line of sight. Solar eclipses result from the Moon blocking the Sun relative to the Earth; thus Earth, Moon and Sun all lie on a line. Lunar eclipses work the same way in a different order: Moon, Earth and Sun all on a line. In this case the Earth's shadow hides the Moon from view. eclipsing binary
A system of two stars that periodically eclipse one another from our point of view on Earth. Astronomers cleverly use these observations to calculate mass, size, and distance. One noted eclipsing binary is called Algol, with a period of 2.87 days and a shift in magnitude of 1.2. See also: ecliptic
The Sun's path in the sky. Planets and the Moon closely follow this path, since their orbital planes are nearly aligned with Earth's orbital plane. The ecliptic is tilted 23.5 degrees from the celestial equator. See also: equinoxeffective temperature
The temperature of a star's outermost layer, the photosphere. Temperatures vary throughout a star, from millions of degrees in the core to thousands of degrees in the outer stellar atmosphere. Astronomers measure the temperature of the photosphere, and combine that with other information to calculate the core temperature. Einstein, Albert
A Swiss mathematician and physicist who lived from 1879 to 1955. Einstein was born in Germany to Jewish parents, and after moving to Switzerland, he took a job at a patent office. When he began publishing scientific papers, he quickly rose to fame and was recognized as a leading thinker of the time. He held several professorial positions in Switzerland, Germany, and the United States during his lifetime. In 1922, Einstein won a Nobel Prize for work he had done with the Photoelectric Effect. Over the course of his life, Einstein made major advances in physics, such as developing the Theory of Special Relativity and the Theory of General Relativity. He was arguably the most important physicist of the 20th century. See also: Relativity, Specialelectromagnetic radiation
Energy that travels through space at the speed of light. The total range of electromagnetic wavelengths and frequencies is called the electromagnetic spectrum. Our eyes are sensitive to a sliver of the electromagnetic spectrum we call visible light, from red to blue. Beyond the blue end, as wavelength decreases and frequency increases, lie ultraviolet light, X-rays, and gamma-rays. In the other direction past red, as wavelength increases and frequency decreases, lies infrared, microwave, and radio regions of the electromagnetic spectrum. Radio waves are the longest, extending on the order of meters, while X-rays and gamma-rays are the shortest, on the order of atomic size. Light wavelengths are measured in microns, millionths of meters. For all forms of electromagnetic radiation, the speed of light must remain constant, regardless of wavelength or frequency. See also: spectrumelectromagnetic spectrum
The full range of electromagnetic radiation in order of wavelength from longest to shortest, or frequency from lowest to highest. In terms of visible light, a light spectrum runs from red to blue. Astronomers scan as much of the electromagnetic spectrum as possible to learn about the nature of the universe and how it came to be. See also: spectrumellipse
A closed shape resulting from the intersection of a circular cone and a plane, resembling an oval. In space, orbiting bodies follow elliptical paths. See also: elongation
The angle between a planet and the Sun in the sky. Because Mercury and Venus are inside the Earth's orbit, their elongation angles are never more than 23 degrees (for Mercury) or 46 degrees (for Venus), which is why they appear as "morning" or "evening" stars, never as "midnight stars." All other planets, though, with orbits outside the Earth's, can appear at any elongation angle. When a planet reaches the greatest angular separation east of west of the Sun in the sky, it is said to have reached greatest elongation. The maximum elongation for Venus and Mercury is 46 and 23 degrees, respectively. The maximum elongation of planets outside the orbit of the Earth is 180 degrees, when the planet is said to be at opposition. At these points in the sky, planets are visible for the longest period of time. See also: oppositionequatorial coordinates
A system of coordinates that uses right ascension and declination to locate objects in the night sky. A telescope that uses equatorial coordinates is said to have an equatorial mount. See also: right ascensionequinox
The time of year when the Sun appears in the sky at the intersection of the ecliptic and the celestial equator. Two such points exist: The vernal equinox (approx. March 21) marks the start of spring. The autumnal equinox (approx. Sept. 20) marks the start of autumn. At each of these points, the Sun rises due east and sets due west. See also: escape velocity
The minimum speed necessary to escape the gravitational pull of a celestial body. A rocket launched from Kennedy Space Center must accelerate to about 17,500 mph to enter orbit, or about 25,000 mph (11.2 km/second) to escape Earth's gravitational pull and travel to another planet. For a black hole, the escape velocity is greater than the speed of light. Since lightspeed is the ultimate cosmic speed limit, nothing can achieve the speed needed to escape from a black hole. event horizon
A black hole's point of no return. Any light or matter crossing within this boundary is doomed by the hole's gravity. Beyond this point, the escape velocity is greater than the speed of light, the ultimate speed limit. In essence, although it is not a physical boundary, the event horizon marks the black hole's "surface." extrasolar planet
A planet that orbits around a star other than the Sun.
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