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A centimetre (international spelling as used by the International Bureau of Weights and Measures; symbol cm) or centimeter (American spelling) is a unit of length in the metric system, equal to one hundredth of a metre, centi being the SI prefix for a factor of 1/100 . The centimetre was the base unit of length in the now deprecated centimetre–gram–second (CGS) system of units.
The light-hour is a unit of length useful in astronomy, telecommunications and relativistic physics. It is defined as the distance that light travels in free space in onehour, and is equal to exactly 1.0792528488×1012 metres.