A light year is the distance that light travels in one year through vacuum or empty space. Light moves though vacuum at just over 186,282 miles (299,792 km) per second, reaching an astounding 5.8 trillion miles (9.4 trillion km) in one year. With such great distances, relative terms like “miles” and “kilometers” become inefficient and meaningless. Instead, astronomers speak in terms of light years to describe great distances.
Before we can appreciate speaking of distances in terms of light years, it helps to grasp how far a single one spans. In terms of our own solar system, defined for this exercise by the orbit of the former planet Pluto, the solar system would have to be 800 times larger to be a single light year across. Put another way, the sun is about 93 million miles from earth, and one would have to log 31,620 round trips from the earth to the sun to travel the distance of a light year.
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In addition to the light year, scientists also speak in terms of light seconds and light minutes. One astronomical unit (AU) — the distance from the earth to the sun — is 8.3 light minutes. In other words, it takes 8.3 minutes for light from the sun to reach the earth. Stated conversely, the sun is 8.3 light minutes away from earth.
There is no star that lies precisely one light year away from our solar system. The closest neighboring star is Proxima Centauri at 4.2 light years distance. Next is double-star Alpha Centauri A and Alpha Centauri B, 4.3 light years away.
Knowing how fast light travels, one can begin to appreciate the great distances of the universe when speaking in terms of light years. For example, the light given off by Alpha Centauri today will take 4.2 years to reach earth, so when we observe that star, we are seeing it as it was 4.2 years ago. In essence, we are looking back in time when we look out into space.