L is for Lightyear

"Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space."
--Douglas Adams

Space is so big, we measure it in lightyears (ly). Don't let the name fool you. It's not a measurement of time, but distance. A lightyear is the distance light can travel (in a vacuum) in a year (365.25 Earth days).

How far is that? Exactly 9,460,730,472,580,800 meters (or you can round it up to 9.461 x 10¹⁵ meters).  Yep. That's really big. (Why measure it in meters instead of, say, kilometers? Because the meter is a basic unit of Système International. Makes calculation easier and more precise, I promise.)

If that number makes your head spin, sit down, rest awhile and let your brain chemistry reset itself.

Want an easier number to handle?  An astronomical unit (AU) is the distance from the Earth to the Sun. A lightyear is 63,241 AU long.

When it comes to measuring distances, astronomical units tend to be used for stuff within our Solar System, lightyears for the local neighbourhood of stars, and parsecs for everything else. After all, space is really, really big.

A parsec is about 3.26 lightyears. Why the strange number? Because of how we measure faraway stuff.

I'm sure you've played with parallax in high school science, where you hold out your thumb, look at a distant tree with one eye closed, then switch eyes, noting how your thumb appears to move compared to the tree. That's how we measure stuff in space, only we use, the month of June as one eye and the month of Decmeber as the other eye, a nearby star as our thumb and a faraway star as the tree.

Works like this:

A parsec (pc) is the distance that one AU subends an angle of one arcsecond. An arcsecond is 1/3600 of a degree from a circle. As you know, 360 degrees make a circle. (Parsec is an abbreviation of the "parallax of one arcsecond".) If you understood all that, cool. If not, just stick with 3.26 lightyears, as you probably will never need to calculate parallax.

So, how far away from us is stuff?

• Alpha Centauri (nearest known star): 4.6 lightyears (ly)
• Distance to the star Bellatrix (in Orion): 244.6 ly or 75 parsecs (pc)
• Distance from the Earth to the centre of the galaxy: about 26,000 ly or 8 kpc (kiloparsecs)
• Milky Way Galaxy is 100,000 ly or about 30 kpc wide.
• Distance to Andromeda Galaxy (nearest major galaxy): 778,000 parsecs (778 kpc).
• Farthest known galaxy, GN-z11: 13.3 billion lightyears (3,985,819 kpc) 

And that's about as far as we can see. For all we know, there is more stuff out there. We honestly have no idea how big the Universe is.

No hardcore stuff today, unless you want to convert kiloparsecs to meters. I think I've given you enough to keep your head spinning for a while.

_______________________
Even Her Grace likes to take a break now and then.