How Fast is the Speed of Light and How Far is a Light Year?

How Fast is the Speed of Light and How Far is a Light Year?

Light speed

Light speed and light years are terms that are commonly used in astronomy, but what are they and what do they mean?


Light speed is how fast light travels. Just like a plane’s speed is measured in miles or kilometres per hour, light has a top speed too. The speed of light is 186,282 miles per second. If you want to compare it to the speed of a plane then the speed of light is 671 million miles per hour or 1,080 million kilometres per hour. That is fast enough to travel around the world seven and a half times every second. Nothing in the known universe can travel faster than the speed of light and so it is called the universe’s speed limit.


But What is Light? 


Light is made of particles called photons and so, the speed of light is how fast the photons travel. But light is a wave not a thing I hear you say. Well, when you get down to the level of very tiny things like particles they behave as both a wave and a particle. That’s the wonderful weirdness of quantum mechanics.


What is a Light Year?


A light year is a measurement of distance not speed. This is where confusion sometimes arises. Because things in the universe are so far away from each other you need a unit of measurement to be much larger than a mile or kilometre. So, a light year is how far a photon of light will travel in an earth year. This is 5.88 trillion miles or 9.46 trillion kilometres. That is a pretty big unit of measurement.


The nearest star to us is around 4.3 light years away. This means it would take 4.3 years to reach it if you were travelling at the speed of light. Pretty depressing for the future of the human race if we are to ever achieve interstellar travel. However, because time and the speed of light are intertwined as Einstein described, as we approach the speed of light time slows down. This would mean that if you were on a spacecraft travelling at 99.9% the speed of light towards the nearest star you would only age 3 or 4 weeks on the journey. The problem is by the time you return to planet Earth everybody else would have aged around 9 years. So maybe there is hope for interstellar travel within a human lifespan after all. 

Related Posts:  Astronomy

Back to Learn Astronomy

Or now go to:

Star Magnitude - Just What Can You See in the Night Sky?

 Privacy policy and cookies | Disclaimer | Contact Us | Credits | Resources | Site Map © 2012-2014