Where Am I?

I get the odd question asking where certain things are: Europa, the Moon, the Voyager probes, and even some things that don't even exist. But do we have a good handle on where we are? Regular readers with a keen memory may recall this post which addresses the same question with a cool video. I'm taking it a bit further here, though.

Imagine you have a penpal. Maybe you do have a penpal; imagine this penpal is an alien, somewhere out in space, somewhere out in the vast expanse of the universe. How does he, she or it address your letters? For each of us, the Earth-based part of our address is going to be different, so I'll leave that bit off. Beyond that, how would your address look on your green and betentacled penfriend's correspondence?

He'd have to send it to the right planet. But where is Earth? You may have heard our blue-green planet referred to as the 'third rock from the Sun'; we're the third closest large body from our Sun. Mentioning our star's name is also useful because that's where solar systems get their name from: 'Sol' was a Roman Sun God, and we live in Sol's system- 'the' solar system. That would be the next line on our address, and we'd probably like to add "inner" to it, just to help the postman.

But where's the solar system?
Our solar system resides in the galaxy that we call the Milky Way, but just writing that is a bit like telling my postman that my house is somewhere in Europe. In fact, it's even less helpful than that when you think that the Milky Way measures around a hundred thousand light years across, and our solar system is somewhere in the region of a thousandth of a light year across. Add to that the fact that our Sun is only one of well over 100 billion stars, then we could at least do with a postcode.

Luckily, the Milky Way is a spiral galaxy, and we can narrow down the location of stars by talking about which of the spiral arms they're in. We're located on the inner rim of the Orion arm, about half way out from the centre of the galaxy.

But where's the Milky Way?
The massive Milky Way and Andromeda galaxies orbit each other in amongst a group of 50 smaller galaxies, all bound by gravity, known as the Local Galactic Group.

But where's the Local Galactic Group?
The LGG, resides in a volume of space 110 million light years across along with at least a hundred other galactic groups and clusters that make up the Virgo Supercluster. Superclusters are so large that they are not gravitationally bound. Consequently, this is the smallest scale that we can start to see effects due to the expansion of the universe.

But where's the Virgo Supercluster?
This is where things start to get sketchy. The Virgo Supercluster is part of a local group of superclusters known as... the local superclusters. Beyond this, there are vast, mind-bogglingly enormous structures made up of superclusters that we call 'walls' and 'filaments' that enclose even more mind-bogglingly vast voids. These are what make up what is known as the 'Observable Universe' (mainly because it's the bits of the universe that we are able to observe.

So in brief, here's that address:

  • [Insert your Earth-based address here],
  • The Inner Solar System,
  • The Orion Arm (Inner Rim),
  • The Milky Way Galaxy (nr. Andromeda),
  • The Local Galactic Group,
  • The Virgo Supercluster,
  • The Local Superclusters,
  • The Universe.
Here is an excellent set of maps that you can send your extraterrestrial correspondee so that they can find their way here (GPS is only useful for the last few thousand kilometres):

By Azcolvin429 (Own work), via Wikimedia Commons
You can click on the image to go to the original and read all the info on it, but be warned that, as befits the scales upon which we are working, the image is BIG.


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