What is a light year?
Inspired by the likes of Star Wars and Star Trek, my little boy has become firmly fascinated with all things ‘space’ of late. Last weekend, shortly after destroying an elaborate Lego spaceship I’d spent most of Saturday afternoon building for him, he inquired “Dad, what’s a light year? Does it measure time?”
Even though a year is usually used as a unit of time, light years actually measure distance. Simply, one light year is the distance that light can travel in one year. Light can travel at a speed of 300,000 kilometres per second in the vacuum of space. Based on that, it can travel 9,460,730,472,581 kilometres in one year (365.25 Earth days).
Why do we use light years to measure distance?
We are all used to measuring distances in metres, kilometres, miles and the like. On Earth, it is only a few hundred or thousand kilometres to wherever we may need to travel. But in space, things are rather different.
For example, the nearest star beyond the Sun – Proxima Centauri – is some 40,000,000,000,000 km away. Using light years however, the star is at a distance of 4.24 light years which is much simpler to comprehend. In other words, light years make it easier for astronomers to communicate about the huge distances in space without having to deal with unfeasibly long numbers.
To give some other examples, the nearest spiral galaxy to Earth – Andromeda – is approximately 2.3 million light years away. Try writing that down in kilometres! Our own galaxy – The Milky Way – is estimated to be between 100 million and 150 million light years across. And the edge of the visible universe is some 14.5 billion light years away.
Other astral measurements
Light years aren’t the only measurement that astronomers use. A ‘parsec’ for example is 3.3 light years and this enables us to define distances in even smaller numbers.
Astronomers also use the Astronomical Unit or AU to describe distances, though this tends to be for our own galaxy. An AU is equal to approximately 150 million kilometres (149,597,871) or about the distance between the Earth and the Sun (which varies actually, due to the Earth’s orbit around the Sun not being perfectly circular).
So there we go, a bite-size answer explaining light years. Thanks for reading.
Sources: How It Works magazine, How Stuff Works, NASA, Universe Today