### Theory:

Length can be defined as the difference between the two points.
For an example if you want to measure your body length, you should find the length between your head and your toe leg.

Similarly, we use this fundamental quantity to measure the distance also. Usually, we calculate the distance in centimetre or meter. But if the distance is huge we use Kilometer.

Have ever wondered how we calculate the distance at sea?

We use the term nautical miles to measure the distance at sea.

$$1$$ mile $$=$$ $$1.6$$ Kilometers.

$$1$$ nautical mile $$= 1.852$$ Kilometers at sea.

Now we know how we calculate the distance at sea, but what about space?

To measure the very large and enormous  distance we use the following units.

1. Light years.

2. Astronomical units

3. Parsec

1. Light years:
A light-year is a distance travelled by light in one year in a vacuum and it is equal to m or $9.46\phantom{\rule{0.147em}{0ex}}×{10}^{13}\phantom{\rule{0.147em}{0ex}}\mathit{km}$.
Know that light travels $3×{10}^{8}\phantom{\rule{0.147em}{0ex}}m$ per second.

$$1$$ Light year:

To calculate one light-year, multiply the total number of days in a year, hours in a day, minutes and seconds, and the product of this value should be multiplied by $3×{10}^{8}\phantom{\rule{0.147em}{0ex}}m$.

$$1$$ Light year $$= (365 × 24 ×60 × 60) ×( 3×10^8)$$

$$1$$ Light year $$=$$ 31536000 $$×( 3×10^8)$$

$$1$$ Light year $$=$$ 9460800000000000 $$= 9.46×10^15$$m

2. Astronomical units:
Astronomical units used to calculate the distance between the Sun and the Earth.

It is the mean distance of the center of the Sun from the center of the earth.

$1\phantom{\rule{0.147em}{0ex}}\mathit{AU}\phantom{\rule{0.147em}{0ex}}=1.496×{10}^{11}$
3. Parsec:
Parsec is the unit of distance, we can use this method to measure astronomical objects outside the solar system.
$1\phantom{\rule{0.147em}{0ex}}\mathit{Parsec}\phantom{\rule{0.147em}{0ex}}=3.28\phantom{\rule{0.147em}{0ex}}\mathit{Light}\phantom{\rule{0.147em}{0ex}}\mathit{years}$.