PDF chapter test TRY NOW

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 year 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.

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

**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).

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

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

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

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