Question

Define light year and explain how and why light years are used to measure distances in the universe

Answer 1

Distances ANYWHERE could be described in terms of inches, feet,
yards, miles, kilometers, nanometers, centimeters, millimeters, or
even AU (Astronomical Units).  But if you tried to use any of these
to describe the distance of anything away from the Earth's surface, 
you'd be faced with a ridiculous number.  It would be very hard to
remember this number or tell other people about it, and impractical
to write this number down in your notebook.

For example: The average distance between Earth and the moon is

38,440,145,820 centimeters,

and the distance to the nearest star outside the solar system
is approximately

1,671,433,491,000 kilometers.

Isn't that ridiculous ! ?

Obviously, we need much bigger units to describe these distances,
so that we can talk about them in more convenient numbers.  so the
light year was invented to fill this need.  It's simply the distance that
light travels through vacuum in one year.

Definition of 1 light year = exactly 9,460,730,472,580,800 meters