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Astronomy & Telescopes Glossary - E
Earthshine - Faint light reflected from the Earth onto the dark part of the moon.
Eclipse - The obscuring of one astronomical body by another as seen from the Earth. The most common eclipses are solar eclipes and lunar eclipses.
Ecliptic - Theplane defined by the Earth's orbit around the Sun. Most planets in our solar system appear close to the ecliptic plane. The Earth's axis is tilted 23.5° from the ecliptic, which causes our seasons.
ED glass - A high-quality optical glass found primarily in apochromatic telescopes. Many ED (extra-low dispersion) lenses have a high content of fluorite, which reduces aberration.
Elongation - The angle between a planet and the Sun as seen from the Earth.
Emission nebula - A nebula that emits light and glows
Encke Division - The Encke Division of Saturn splits the A Ring into two parts.
Equatorial mount - A type of telescope mount with an axis parallel to the axis of the earth. An Equatorial (or EQ) mount provides tracking of celestial objects for astrophotography when combined with a motorized drive.
Erfle eyepiece - A specific type of telescope eyepiece wich offers a wide field of view of 68 degrees or higher.
Eye Relief - The distance between the eyepiece and the eye where you can see the entire image through a telescope or binoculars. Eyeglass wearers need longer eye relief.
Exit Pupil - The diameter of light from the eyepiece that reaches the pupil of the eye.