Astronomy Dictionary

Browse through Sky at Night Magazine’s comprehensive astronomical dictionary, with over 300 entries.

Uu
Ultraviolet radiation

Electromagnetic radiation that has a shorter wavelength and higher energy than visible light.

Umbra

The part of the shadow cast by an eclipsing body that results from the Sun being totally eclipsed.

Umbra

The shadow cast by an eclipsing body during a total eclipse; also the central, dark region of a sunspot group.

Universal time (UT)

A timescale based on the rotation of the Earth on its axis. Greenwich Mean Time (GMT) was measured from the Greenwich Meridian at midday until 1928, when UT replaced it. Now, UT is measured from midnight at the Greenwich Meridian. The term GMT is still often used though.

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