Have you ever fancied hitchhiking the galaxy? There's more than can be seen in one lifetime. You'd need a pocketful of rejuvenating pills - or a cartload, more like. It's a big place. Well, quite big. Well, not so big really...

There are lots of ways of portraying the size of the universe, some visual and some involving lots of zeros. We'll go with the zeros in this Entry, but will try to cut the number down to a single eyeful.

Start with Something Small

Looking at the size of things, we have to start somewhere so we'll take something that most of us have some idea of what it is - an atom of hydrogen. Just for a vague idea of the size of one, it's about a tenth of a nanometre across, which means you'd get ten thousand million of them in a metre - ten million in one millimetre of your ruler.

10,000,000 to a millimetre - that's only a couple short of an eyeful of zeros.

If we were to look at things smaller than the constituents of a hydrogen atom (one electron, one proton), that could be a problem for many of us – we don't really know anything much about it except maybe a few names.

They are elementary particles that lack internal structure – they aren't made up of anything else. They are things belonging to Quantum Field Theory, like fermions, quarks, leptons - and one that some of us may have heard of: the Higgs boson.

We'll eschew those things and simply say that such things are small. Small, that is, on similar sorts of scales that the things we'll be looking at are big, with our hydrogen atom sort of somewhere in the middle-ish.

Roughly Speaking

One electron orbiting one proton is the simplest atom – hydrogen.

• You would need 1,000 electrons (if you could lay them side by side) to measure the diameter of a proton.

• You would need 100,000 protons to measure across a hydrogen atom.

• As we've said, you can fit about 10,000,000 hydrogen atoms to a millimetre on your ruler.

Let's Pause Here for a Moment

Let's look at something living, something important - to us, anyway:

• The head of a human sperm? It's maybe 150 to the millimetre. Head width, half that length (and the tail is about 8 times as long as the head).

• The human egg, when ready for fertilisation, is on the large size for a cell, 7 or 8 to the millimetre, ie 20 or so sperm head lengths.

Now Back To the Business In Hand

• Across a ping-pong ball is about 40 millimetres (300 human eggs, or thereabouts).

Very Roughly

• 45 ping-pong balls would measure a tallish human.

• It would take seven million (7,000,000) of those humans, laid out end-to-end, to span the Earth's diameter (about 22,000,000 to go around the Earth at the equator).

Got that? Good

• From here to the Sun? 150,000 Earths.

• Across our solar system, nearly one and a half million Earths.

• Our nearest star is up towards 4,500 solar systems away.

• Across our galaxy (the Milky Way) is over 100,000,000 solar systems.

• Our galaxy to the nearest other one? Edge to edge, about a galaxy.

• Then there are galactic clusters - across ours is worth something like 50 galaxies.
  Gravity keeps clusters together. That is, there is enough matter, just close enough together for gravity to keep it more or less that way.

Now Things Start to get Really Big

• After clusters, there are collections of clusters, called superclusters, which gravity does not keep together.

• The distance between our supercluster and the next nearest might be measured in a few tens of clusters, as could supercluster diameters.

• Then there are sheets, some tens of superclusters across - and there are voids, too, about which there's not much else to say.

• The furthest visible reaches of our universe at present (in 2011) are at some 6 Sheets away. – that's 13,000,000,000 light years away, which means that light has taken that many years to get to us. We are, therefore, looking back in time that many years - the light we see now left there that long ago (accounting for more than 150,000,000 of our lifetimes).

• The age of the universe is thought to be some 750,000,000 years older than that (another 9,000,000 or so lifetimes), so presumably(?) there is quite a lot of stuff further out that we can't see, yet.

And

Each new development in astronomy enables us to see further outwards (and backwards in time).

Some scientists say we will eventually see all the way back to the Big Bang. Others say not.

Finally

There, how's that? An eyeful of nine zeros at most. Nifty, eh?

So, dear reader, there we are. If you've read, learned and inwardly digested that little lot, it may give you some idea of how important you are in the grand scheme of things.