AstroGlossary - D
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dark energy
A term used to describe of our lack of understanding of how the universe works on the largest scales. It may be a "repulsive" force that is causing the universe to expand faster as it ages, a discrepancy in the laws of gravity, or some other phenomenon. dark matter
Matter that produces no detectable energy, but that reveals its presence through its gravitational pull on the visible matter around it. Dark matter comprises more than 90 percent of all the matter in the universe. There are many possible forms for dark matter, but most current theories suggest a subatomic particle created in the Big Bang. declination
One of two measurements of the position of an object in the sky. Declination measures in degrees from 0 to 90 the position of a celestial object north or south of the celestial equator, just like latitude measures positions on the Earth north or south of the equator. The other coordinate is right ascension. degree
A unit of angular measure equal to 60 arc minutes or 3600 arc seconds. One degree is equal to the diameter of two full Moons. density
The amount of mass per unit of volume. diffraction
The bending of light around an opaque physical edge that demonstrates the wave nature of light. dome
In astronomy, the building that houses a telescope. The roof of a telescope dome has a dome shape, and there is a shutter that opens and closes, allowing the telescope to view the sky. Doppler effect
A measurable shift in the wavelength of a traveling wave caused by the relative motion of the source and observer. On the straight-aways of the Indianapolis 500, the sound of cars whizzing by an observer changes pitch from high to low. As the car approaches at more than 200 m.p.h., its motion compresses the sound wavelength (high pitch, or short wavelengths); the opposite occurs after the car rushes by (low pitch, or long wavelengths). Light behaves in somewhat the same way. Stars with relative motion toward an astronomer appear blue, i.e. the entire spectrum of star light shifts toward the blue end of the spectrum or is blue-shifted. Stars speeding away from the astronomer appear red, i.e. the entire spectrum is red-shifted. This is how astronomers know that the universe is expanding -- most everything is red-shifted.
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