### What is a light year?

Many people hear the phrase 'light year' and, perhaps understandably, assume it to be a unit of time. But it isn't*; it's a unit of length, or distance.

Put another way, the term 'light year' does not belong in the same box as words such as second, minute, hour, day, month, year and decade. Instead, it does belong in the box containing such words as inch, metre, mile, kilometre, parsec, foot, millimetre, astronomical unit, furlong and kilometre.

So what is a light year?
A light year is defined fairly simply as the distance that light travels in one year**. As the speed of light is around 300 million m/s*** and there are around 32 million seconds in a year we can work out how long a light year is in metres:
Distance (m) = speed (m/s) x time (s)
If we use the rough figures above, we get 300,000,000 m/s x 32,000,000 s = 9,600,000,000,000,000 m
That's almost ten quadrillion metres. Quite a long way.

Proper scientists (not me) have calculated the distance of a light year with a bit more accuracy, and state it to be 9.4605284x1015m****, which isn't far off what we calculated above, but this number is so big it's hard to deal with, so how does it compare to other units of distance that we can comprehend?

One light year is approximately the same distance as:

Some light year facts
• The Earth is about 0.0000016 light years from the Sun.
• Voyager 1 is about 0.0032 light years away from us.
• The Oort cloud is about 1.6 light years in diameter.
• The Sun's gravity dominates local space out to about 2 light years away.
• The nearest star to us (other than the Sun) is about 4 light years away.
• The centre of the milky way is about 26,000 light years away.
• The milky way measures about 100,000 light years in diameter.
• Our nearest galaxy, Andromeda, is about 2,500,000 light years away.
• The edge of the visible universe is 46,500,000,000 away.

* Nor is it a year in which there are 20% fewer calories.
** In a vacuum: this is important because light travels at different speeds depending on what it is travelling through. Hence, light can travel further in one year in a vacuum than it can through water, for example.
*** The units (m/s) are 'metres per second'.
**** If you'd like help deciphering large numbers and 'standard form', why not head over to Maths Questions and ask your questions over there?

1. What's the Oort cloud??????

2. 3. An Oort cloud sounds like it should be in Doctor Who - love it!