### 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.
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.

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.

For measuring larger distances, such as the distance between two cities or countries, a larger unit of length called $$kilometre$$ ($$km$$) is used.

Have you ever wondered how sailors calculate the distance at sea?

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.

$\mathit{Speed}\phantom{\rule{0.147em}{0ex}}\mathit{of}\phantom{\rule{0.147em}{0ex}}\mathit{light}=3×{10}^{5}\phantom{\rule{0.147em}{0ex}}\mathit{km}/\mathit{sec}$

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).

$\mathit{Time}=365×24×60×60=3.154×{10}^{7}\mathit{seconds}$

$\begin{array}{l}\mathit{Light}\phantom{\rule{0.147em}{0ex}}\mathit{year}=\mathit{Speed}\phantom{\rule{0.147em}{0ex}}\mathit{of}\phantom{\rule{0.147em}{0ex}}\mathit{light}×\mathit{Time}\\ \\ =\left(3×{10}^{5}\right)×\left(3.154×{10}^{7}\right)\end{array}$

$1\phantom{\rule{0.147em}{0ex}}\mathit{light}\phantom{\rule{0.147em}{0ex}}\mathit{year}=9.4607305×{10}^{12}\phantom{\rule{0.147em}{0ex}}\mathit{kilometres}$

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