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Space Station Glossary D

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Dark Matter:

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.

Degree of Arc:

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.

Diffraction Grating:

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.

Digital Image:

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.

Click image to see more about Nasa Image Gallery