Astronomy dictionary - U

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.

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.