A unit of length is a standard measurement used to quantify the distance between two points. Length can be measured in various units, which can vary depending on the measurement system being used. In the International System of Units (SI), the primary unit of length is the meter (m). Other common units of length in the SI system include: - Kilometer (km), which is equal to 1,000 meters - Centimeter (cm), which is equal to 0.
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A unit of length is a specified reference magnitude used to express a length. In , the physical length is unchanged; only its numerical value and chosen unit change. This separates a physical dimension from a unit convention.
The SI base unit is the metre, symbol . It is defined by fixing the vacuum speed of light at exactly , with the second defined independently through the caesium frequency. Thus realizing a metre can connect length measurement to a measured time interval or a calibrated optical frequency.[ref]
The International System of Units (SI), ninth edition, SI Brochure. Section 2.3.1, definition of the metre
Unit conversion should preserve dimensions: converting to changes a length by a factor of in its numerical representation, while the corresponding area changes by . Astronomical units, light-years and parsecs are useful for different astronomical distance scales; the choice does not alter the distance being measured.