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D - E - F

Select the letter that (probably) leads off the word in question. Please suggest new words or corrections.



  • dark matter: Unknown and theoretical substance in space whose present is detected by its gravitational effect on celestial objects.

  • declination: The coordinate system in the sky that corresponds to latitude on Earth. It is measured in degrees north or south of the Celestial Equator.

  • Deimos: One of two small moons orbiting Mars discovered in 1877 by American astronomer Asaph Hall.

  • Doppler effect: The process by which light or sound is altered in perceived frequency or wavelength by the motion of its source with respect to the observer. This is a key component in determining redshift.



  • Earth: The third planet from the Sun, and the planet that you are living on.

  • eclipse: The condition in which the Sun or the Moon is blocked from our view, either partially or fully. Also see "annular eclipse" and "total eclipse". In a "solar eclipse" the Moon passes in front of the Sun, and in a "lunar eclipse" the Earth's shadow passes between the Sun and the Moon, blocking the Moon from Earth's view.

  • ecliptic: The apparent path of the Sun across the sky as it travels across the constellations from Earth's perspective.

  • elliptical galaxy: An ellipsoidally shaped galaxy that does not contain much interstellar matter. They appear as small "wads" or orbs in a telescope.

  • elongation: The angular distance between two celestial bodies as seen from Earth.

  • emission nebula: nebular gas and dust that glows as a result of atoms being excited by high energy radiation from hot, massive, young stars.

  • Encke Division: A gap or "division" located in Saturn's A ring first named after J. Encke (possibly the discoverer).

  • ephemeris: A table giving the coordinates of a celestial body at a number of specific times during a given period.

  • epoch: An instant in time that is arbitrarily selected as a point of reference. Commonly used as the reference point in time when creating stellar maps. Ex. Epoch 2000.0

  • Europa: A moon of Jupiter's discovered by Galileo in 1610. There is strong evidence that underneath its surface there may exist liquid water.

  • eyepiece: The part of a visual telescope that is used to provide different magnifications of the image the viewer is seeing. Usually measured in millimeters. The lower the number of millimeters the higher the magnification.

  • eye relief: The distance between the lens of an eyepiece and the point behind the eyepiece where all the light rays of the exit pupil come to a focus and the image is formed. It is the distance from the eyepiece at which the eye can be held and correctly see the image produced by the eyepiece.



  • focuser: The part on a visual telescope that allows the user to focus an image into the eyepiece.

  • Foucault test: An optical test used to analyze the surface of mirror surfaces. It utilizes a point light source bounced off of an optical surface and back to a knife-edge (razorblade). The knife-edge splits the reflected light producing an accurate image of the mirror surface's smoothness, error, etc.

  • Full Moon: A phase of the Moon in which its entire face is lighted from Earth's perspective. It is 180 degrees opposite from the Sun in its orbit of Earth at this time.

  • FU Orionis: Young Sun-like stars, undergoing bursts of growth as gas falls onto them from their surrounding accretion disks. Over a year to a decade, they brighten by a hundred fold, then stay bright for a century or so before fading back into obscurity.


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