Light Year: Definition & Overview

Instructor: Richard Cardenas

Richard Cardenas has taught Physics for 15 years. He has a Ph.D. in Physics with a focus on Biological Physics.

In this lesson, you will learn about the concept of a light year and that it is not a measure of time but of distance. Discover how to calculate a light year and explore uses of the light year in astronomical measurements.


You may have heard the term 'light year' used in science fiction or on educational channels about astronomy. But what exactly is a light year? The term light year refers to a measurement of distance, not time. It is the total distance traveled by light in one year. So when you hear the phrase 'Galaxy X is 10 light years away', then you can multiply the speed of light by 10 years to find out how far Galaxy X is from us. You can also think of a light year in terms of the time light takes to travel that distance. When we say Galaxy X is 10 light years away, that means that it will take light from Galaxy X ten years to reach the Earth.

Calculating a Light Year

In order to calculate the distance covered by light in a year, we need to know the speed of light. The speed of light is a constant and has a magnitude of approximately 300,000,000 meters/second (yes you read that right, 300 million meters per second). It is very fast; in fact, it is equivalent to about 671,000,000 miles per hour (671 million miles per hour). So we will now calculate the value of a light year in both meters and miles.

To calculate the value in meters, we need to know how many seconds there are in a year. To calculate the value in miles, we need to know how many hours there are in a year. The values are


First, let us calculate the value of a light year in meters:


Next, let us calculate the value of light years in miles:


What does it all mean? Light travels very far in a year whether you measure it in miles or meters. Let us put this in perspective. Let us convert the distance between each planet to the sun in light years.

Planet Distance in light years
Mercury 0.000006
Venus 0.000011
Earth 0.000016
Mars 0.000024
Jupiter 0.000082
Saturn 0.000150
Uranus 0.000303
Neptune 0.000475

It takes light from the Sun about 8 minutes to reach Earth. Contrast that to some galaxies and stars.

Celestial Object Distance in light years
Proxima Centauri (star) 4.22
Alpha Centauri (star) 4.37
Canis Major Dwarf (galaxy) 25,000
Andromeda (galaxy) 2,538,000
z8_GND_5296 (galaxy) 30,000,000,000

Proxima Centauri is the closest star to the Earth, while Canis Major Dwarf Galaxy is our closest neighboring galaxy. It takes 4.22 years for the light from Proxima Centauri to reach us and 25,000 years for light from Canis Major Dwarf Galaxy to reach us. In 2012, the most distant galaxy from us was found to be z8_GND_5296 galaxy, which was determined to be 30 billion light years from us.

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