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Space Station Glossary D
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Dark Matter:
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Matter that is too soft to be detected by
telescopes. Astronomers deduce its survival
by measuring its gravitational influence.
Dark substance makes up most of the total
mass of the universe.
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Degree of Arc:
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One degree of arc is 1/360 of a complete
circle. The obvious sizes of objects as seen
from Earth could be measured in degrees of
arc. The angular width of the full moon or
the Sun as seen from Earth is one-half of
a degree.
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Diffraction Grating:
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A device that splits glow into its component
parts or also spectrum. A diffraction grating
frequently consists of a mirror with thousands
of intimately spaced parallel lines that spread
out the light into similar bands of colors,
or distinct fine lines or bars.
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Digital Image:
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A visible picture that is recorded by an
electronic detector and subdivided into small
image elements (pixels). Each element is assigned
a figure that corresponds to the intensity
recorded at its physical location on the detector.
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