The humans on earth rigorously jotted down every aspect of light they could get their hands on. The cesium atom gave an accurate measure of time. They made use of the electron transition between the energy levels to define a second.
nd. astro.psu.edu. Web. 02 Jul. 2017.
And as the seconds turned to minutes, they noticed something peculiar.
Why is it that time passed by on earth when the photon hardly seemed to move at all?
The motion of every object in space left them with no other choice but to calculate their motion relative to the moving objects. Their calculations very much depended on the fact that they considered themselves to be stationary and were valid only in their frame of reference, from which light seemed to travel all the way from the sun for eight long minutes. From the point of view of light, however, it was absorbed as soon as it was emitted. The hundreds of years that it spent bumping around atoms in the sun weren’t accounted for, as well. They realized that the change in reference frame brought along a change in the numbers as well. The energy and speed of the photon so huge that the distance appeared to be point size. As soon as they realized all motion is relative, they named the distance traveled by light in a year relative to them as a light year.
McClure, Bruce. How far is a light year? 11 Jul. 2016. earthsky.org. Web. 02 Jul. 2017.
These humans, till today, make use of it to calculate distances across cosmos.
Following is the link for those looking to delve deeper into the world of the Sun!