U

Umbra

The portion of a shadow cone in which none of the light from an extended light source (ignoring refraction) can be observed.

 

Universal Time (UT)

A generic reference to one of several time scales that approximate the mean diurnal motion of the Sun; loosely, mean solar time on the Greenwich meridian (previously referred to as Greenwich Mean Time). In current usage, UT refers either to a time scale called UT1 or to Coordinated Universal Time (UTC); in this volume, UT always refers to UT1. UT1 is formally defined by a mathematical expression that relates it to sidereal time. Thus, UT1 is observationally determined by the apparent diurnal motions of celestial bodies, and is affected by irregularities in the Earth’s rate of rotation. UTC is an atomic time scale but is maintained within 0s.9 of UT1 by the introduction of 1-second steps when necessary. (See leap second.)

 

UT1

See Universal Time (UT).

 

UTC

See Coordinated Universal Time (UTC).