Length is a measurement of distance or dimension.Length is a measure of one dimension. In most systems of measurement, length is a fundamental unit, from which other units are derived.
In engineering and physical sciences the word "length" is synonymous with distance.
Hundreds of years ago, people measured the length of objects against human body parts.The Egyptians are the earliest civilization that discovered a standard unit for length.The Egyptian cubit unit was based on the length of the arm. It was developed about 3000 BC.
The Greeks used the width of 16 fingers to find one foot. The Romans adopted the foot from the Greeks and divided it into 12 sections which was called unicae, which came to be known as an inch.
The two main systems to measure length are the metric and the English system.
In the SI, the basic unit of length is the metre and is now defined in terms of the speed of light. In1983 CGPM defined the meter:
The meter is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.
Commonly used units are the centimetre and the kilometre, that are derived from the metre.
In the Imperial system of units, commonly used units of length are the inch, the yard, the foot, and the mile.
A common unit of distance used primarily at sea and in aviation is nautical mile.The nautical mile is defined to be the average distance on the Earth's surface represented by one minute of latitude; it makes good sense at sea, where there are no mile markers but latitude can be measured.For many years the British set the nautical mile at 6080 feet (1853.18 meters) and until 1954 the U.S. nautical mile was equal to 6080.20 feet (1853.24 meters). In 1929 an international conference in Monaco defined the nautical mile to be exactly 1852 meters or 6076.115 49 feet,a distance known as the international nautical mile.
In chemestry are used units that denote microscopically small distance: micron and the ångström.
An ångström or aangstroem or angstrom (named after the Swedish physicist Anders Jonas Ångström ) is a non-SI unit of length equal to 0.1 nanometre (nm), and is internationally recognized. It is used in expressing the sizes of atoms and lengths of chemical bonds.
In astronomy are used units denote distances in the vastness of space, and are much longer than those used on Earth and include the light year, astronomical unit, and the parsec.
A light-year is the distance light travels in vacuum in one Julian year.A light-year is equal to:9,460,730,472,580.8;