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The total or partial blocking of one celestial body by another

Emission line

A bright line in a spectrum caused by emission of light. Each chemical element emits and absorbs radiated energy at specific wavelengths. The collection of emission lines in a spectrum corresponds to the chemical elements contained in a celestial object.


Tables of data which list the positions of the Sun, Moon, planets and other Solar System objects

Escape velocity

The minimum velocity required for an object to escape the gravity of a massive object.


The process of allowing electromagnetic radiation to fall on light-sensitive materials such as the camera's CCD detector. An exposure is also the image created by the process. A long exposure time is needed in order to obtain an image of dim and distant celestial objects.

Extrasolar planet

A planet that orbits a star - other than the sun