On earth, we measure distance in kilometers or miles but out there is the space, where our earth is located, there will be so many zeroes behind a number, as the distance will be large.
To give you an idea lets talk about our star- the sun and the nearest star to our sun, i.e. Proxima Centuri. The distance between the two is 4.22 light year.
1 light= 9.46x 10^12 km or approximately 9.45 trillion kms or 5.9 trillion miles.
So, 4.22×9.46×10^12 km, so many zeroes, right?
Hence, we measure distance in light year. 1 light year is the distance light travels in an earth year of 365.25 days.
Let us understand a little better.
The distance between earth and sun is 149.6 million km or 0.0001496 trillion km
now, how much does light travel in 1 earth minute
9.5×10^12/365.25*24*60 ( 24 hours in day and 60 minutes in an hour = 18.06 million kms.
Now, time for light from sun to reach earth
149.6×10^6/18.06×10^6 and that’s approximately 8.5 minutes.
So, basically, we see the sun that’s 8.5 minutes old.