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A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
U
Ultraviolet
Electromagnetic radiation at wavelengths shorter than the violet end of
visible light. The atmosphere of the Earth effectively blocks the
transmission of most ultraviolet light, which can be deadly to many
forms of life.
Umbra
The area of total darkness in the shadow caused by an eclipse.
Universal Time (UT)
Also known as Greenwich Mean Time, this is local time on the Greenwich
meridian. Universal time is used by astronomers as a standard measure of
time.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z