Question

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Explain the enormous distances between objects in space. How do we use light in space?

Answer 1

The universe is so large that to think of its size in terms of miles or kilometers, the numbers would be so large as to seem meaningless.
The Sun is about 92 million miles away (92,000,000 mi). The distance to the nearest star is over 23 trillion miles (23,462,784,000,000 mi). As you can see, working with numbers this large for even the shortest distances in our universe would get very cumbersome.
For distances within our galaxy (the Milky Way) sometimes astronomers use Astronomical Units. One Astronomical Unit (1 au) is approximately the average distance from the Earth to the Sun. This unit is especially handy to use when astronomers need to triangulate an object within our solar system, say the distance to Pluto. The distance from where we view it on one side of the sun and where we view it on the other side (six months later) is 2 au. That becomes the base for the triangle. Beyond the solar system though, triangulation becomes difficult in terms of au as it results in too small of angles. Enter the light year...
Astronomers use Light Years to measure or state distances, from our nearest star (Alpha Centauri), to the ends of the universe - really. Light travels 186,000 miles per second, or a little more than 5.8 trillion miles per year. In terms of light years the nearest star is just 4 light years away. The diameter of the Milky Way galaxy is approximately 100,000 light years. And the distance to the known edge of the universe is approximately 13.7 billion light years away (13,700,000,000 ly).
How is it that the "Time" it takes light to travel a distance is a meaningful measurement? If you told somebody that it takes you 1 hour by car to drive from your town to the next town, they might know what that feels like. If you said it was 60 miles away, they would probably think about it in terms of how long it would take them to travel that distance. If "we" always moved at the same speed, we could use time to measure distances. Light does travel at the same speed, relative to us earthlings, so we CAN use light, and light years, as a meaningful and relatively accurate measurement.