Theory:

Length is the distance between one end and the other desired end. In general, it is the measurement of distance. For example, to find the height of a man, the distance between his head and toe should be measured.
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Units of length
The SI unit of length is \(metre\) (\(m\)). This unit is used to measure the length of cloth material, the height of a building, or the lamp post.
 
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For measuring very small lengths like the tip of a pencil or the length of a pen, the units of a \(millimetre\) (\(mm\)) and \(centimetre\) (\(cm\)) are used, respectively.
 
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For measuring larger distances, such as the distance between two cities or countries, a larger unit of length called \(kilometre\) (\(km\)) is used.
 
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Have you ever wondered how sailors calculate the distance at sea?
 
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Sailors use the term called 'nautical miles' to measure the distance at sea.
 
\(1\) \(mile\) \(=\) \(1.6\) \(kilometres\) (in land)
 
\(1\) \(nautical\ mile\) \(=\) \(1.852\) \(kilometres\) (at sea)
 
Now we know to calculate the distance at sea, but what about measuring huge distances between celestial bodies in space?
 
A term called 'light year' is used to measure the distance between celestial bodies.
A light -ear is defined as the total distance travelled by light in one year.
The speed of light is about \(300,000\) \(km\ per\ second\). So, the distance covered by the light travelling at a speed of \(300,000\) \(km\ per\ second\) in a year gives one light year.
 
Speedoflight=3×105km/sec
 
Calculation of one light year
To calculate one light-year, multiply the total number of days in a year, hours, minutes and seconds in a day. The product of this value should be multiplied by the speed of light (in seconds).
 
Time=365×24×60×60=3.154×107seconds
 
Lightyear=Speedoflight×Time=3×105×3.154×107
 
1lightyear=9.4607305×1012kilometres
 
Astronomical units and Parsec are the other units of distance used in space.
Reference:
Free image from, https://freesvg.org/height-measurement
https://cdn.pixabay.com/photo/2012/02/29/15/43/empire-state-building-19109_1280.jpg
https://cdn.pixabay.com/photo/2018/11/07/23/08/sailing-3801448_1280.jpg