# What is a Light Year?

Of all the things we know about, the speed of the light is fastest! Think about anything you think travels fast and light travels faster than that. Can you guess how fast?

Light can travel 300,000 Kilometers in one second!

Can you imagine how fast that is? It is so fast that light can travel about seven times around Earth in one second!!

That’s about light. What is a light year then?

Light Year is the distance that light can travel in one year. Considering that light can travel about 300,000 kilometers (km) each second, in one year, it can travel 9,500,000,000,000 kilometers. Phew! It would be easier to say that one light year is about 10 trillion km.

Aren’t so many zeroes astonishing? Distances in space are so vast that if we measure them in kilometers or miles the result will be mind boggling huge numbers!

Take an example.  The distance of our galaxy to the next nearest large galaxy – the Andromeda is 21,000,000,000,000,000,000 km. What? Can you even interpret this number? It is 21 quintillion km. This is a number so large that it becomes hard to read and hard to understand.

In the universe, the kilometer is just too small to be useful. Thus, astronomers use other units of distance. They use light year to measure distances in space the same way we use Kilometers to measure distances on Earth.

Now using measure of light year we can comfortably say that the nearest spiral galaxy – Andromeda is 2.3 million light-years away or nearest star to us is about 4.3 light-years away. Easier to understand, right?

Did you know?
Almost 300 years ago, a Danish scientist named Olaus Roemer, was the first man to estimate the distance light travels in a year – and using that he figure out the approximate speed of light!

Useful Resource:

We suggest parents to refer to excerpts provided by American Museum of Natural History to further develop insights on how the speed of light was measured first by Olaus Roemer. The excerpt is based on original publication Cosmic Horizons: Astronomy At The Cutting Edge, edited by Steven Soter and Neil deGrasse Tyson.

1. Dan says:

Amazing. Very Useful

2. kc says:

oh cool thanks

3. grace chivonivoni says:

Thanks a lot, this really helped me.

4. Arusha says:

Whooooooooooaaaaaaaaaaaaaaaaa!That sure is a lot of zeroes.this tells us how vast space is!
Let’s start small:there’s earth or rather the solar system
The sun is the biggest- WE think!(but it’s not -there are several stars millions of light years away)
Then there’s the Galaxy-Milky way-in which our home-Earth is
Ther are several trillion stars in ONE Galaxy
Imagine how many stars would be there assuming that there are several galaxies!Sounds hard

• kinooze says:

Great comment. It is certainly hard to fathom the vastness of space. Think how many years it took us to prepare for a space probe to our nearby planet Mars. Can you imagine how less we have covered – given the distance to Mars is just over 3 light minutes!