Measurements are done normally with objects deemed measurable.
Things like the width of a road, water level, sound level, or even amount of electrical current, are common things people measure.
But how about length that are way beyond human's grasp or sight?
I am talking about measurement out to space!
To measure parameters related to outer space, we still require measurement and mathematics, but in another form; not with the ruler, though.
The distance between star and star, star and planet, etc, are fantastically large.
Therefore, to express these long interstellar distance in the range of normal kilometre, the measurement becomes awkward.
Different units are, thus, invented to cover this aspect of distance measurement. Indices become useful in this manner.
1) Astronomical Unit (AU)
This unit is defined as the distance between the Earth to the Sun. The distance is about 150 million kilometres. (149,597,870 kilometres)
2) Light Year
This unit is defined as the distance taken by light to travel in one year.
Note however that light travels at the speed of 3 x 108 metre per second. You can therefore, imagine how far is 1 light year! Only our imagination can go that far!
Distance of Solar System to nearest star is slightly more than 4 light years.
The width of our Milky Way in our galaxy is 100 thousand light years.
Remark: Light year is a measurement of distance and NOT the duration of time.
Long measurement, short measurement.....
Do you see the beauty of Maths here .....