What is a Light Year?

R. Kayne

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.

Outer space.
Outer space.

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.

According to the conclusions of Albert Einstein, the speed of light is an absolute speed limit throughout the galaxy.
According to the conclusions of Albert Einstein, the speed of light is an absolute speed limit throughout the galaxy.

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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.

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Discussion Comments


@helene55, the other thing about science fiction is that it allows for so many other differences in the way people behave and think, as soon as you introduce a society where people can travel across galaxies in seconds and have light year-speed communications. I think the new societal ideas that television shows like Star Trek and Firefly, for example, have been able to introduce to us are what keep people watching, even when they don't understand the science or feel it is realistic.


When you think about the crazy number of miles in a light year, it shows, to me at least, why people are so fascinated by science fiction and fantasy novels, television shows, and movies. Even the suggestion that people could travel that far seems much more adventurous from an entertainment standpoint than anything that takes place on Earth.


how can we be sure that were looking back in time when we look at a star. if that's the case then it would be the same on earth just in smaller amounts. it's not really possible.

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