Astro Glossary

  • Earth

    Our home planet, the third from the Sun in the solar system. The Earth orbits the Sun at an average distance of 93 million miles (149 million km/1 astronomical unit) 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 and oxygen with traces of argon, carbon dioxide, and other compounds. Liquid water covers 71 percent of the planet’s surface. Earth has one moon.

  • Earth's Atmosphere

    The layers of gas that are gravitationally bound above the surface of our planet.

  • Earth's Moon

    The only natural satellite of Earth.

  • Eclipses, Occultations and Transits

    An eclipse is the result of the total or partial masking of a celestial body by another along an observer’s 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. An occultation occurs when a celestial object is eclipsed by the Moon or another solar system body. A transit is either the act of one celestial body passing in front of another or the time at which a celestial object is highest in the sky. The time at which a celestial object crosses the meridian is called the transit time.

  • Ecliptic

    The Sun's path against the background of stars. It is defined by the plane of Earth's orbit around the Sun. It crosses the 12 constellations of the zodiac plus Ophiuchus, the serpent bearer. The other major planets of the solar system stay close to the ecliptic as they circle through the starry background, as does the Moon. 

  • Egyptian Astronomy and Folklore

  • 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.

  • Electromagnetic Spectrum

    The full range of electromagnetic radiation in order of wavelength from longest to shortest, or frequency from lowest to highest (radio waves, infrared, visible light, ultraviolet, X-rays, and gamma rays). 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.

  • Elliptical Galaxies

    Galaxies that are shaped like fat eggs or footballs. Ellipticals are among the largest galaxies, with some spanning more than one million light-years. They probably formed from the mergers of two or more smaller galaxies, such as spirals. Unlike spirals, where the stars all orbit the center of the galaxy in the same direction, the stars in ellipticals move in random orbits.

  • Eridanus, the River

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