A Light Year (.com)

Check out alightyear.com; it's an interactive representation of just how long a light year is - and that's long in DISTANCE, not in time! Scroll down past the numbers and you'll start to see the planets, with their distances in light minutes - notice that the Voyager Spacecraft, which is currently 3 times farther away than Pluto, is still within one light DAY of Earth - 19 billion kilometers (you can see the real-time distance of both Voyagers here), but light travels farther than that in a single day. Then continue scrolling, and scrolling, and scrolling to get a picture of just how far a light year really is.

Cool graphic from Cosmos with Neil Degrasse Tyson

Even though a light year sounds like a time period, what it really represents is how far light travels in one Earth year. I say "Earth year" because a 'year' is really just how long it takes the Earth to go one full time around the sun - aliens would have very different lengths in mind based on their home planet's orbit!

Also check out this video; NASA is about to land a spacecraft ON A MOVING COMET!