The universe is big, and we have an address.

As you find your feet placed solidly on the ground in front of your house door, you might live with confident knowledge that your home is where your mail is sent to. Street name, city, country – it’s straight forward to find your way home. But what if you were lost on Mars and needed to give directions to the space pilot? Or if you were to order a package from a galaxy far far away?

Though it might be a while before you find yourself in that situation it shouldn’t stop us from asking what our address in the universe is. How do we keep track of locations when stars, planets and everything in between are moving in random directions and at different speeds? And how do we even define a proper map when the cosmological principle says that the universe is the same in all directions?

Let’s see if we can find out in today’s post about our place in space.

The Map Of Earth

Here on Earth we find each other by using a simple address. While street and city names are made up by whoever controls that, then an address is essentially a set of coordinates. You can even look up your exact coordinates on Google Maps which, for mail delivery purpose, would indeed be more accurate than using our current street and city names as these can cause confusion across countries that use different letters. (Poor mailman, though, having to use coordinates instead of street names).

The coordinate system is laid out on the surface of Earth, so it does not resemble the usual (x,y)-system we have seen so many times at school. Instead, it’s a spherical surface but nevertheless it’s still a reference system where each point on the surface has a unique coordinate. We use latitude and longitude instead of x and y to assign a location, but the principle is the same. This means two places can’t have the same coordinates, so if you know coordinates of a given location then you and everybody else will agree on how to find it. Simply because we all agree on which reference system to use.

This leads us to the next question: Does the Milky Way have a reference system as well?

Milky Way Coordinates

The Milky Way, much like Earth, also has a system of reference. Without it astronomers would be lost in their attempt to discuss galactic locations.

Artist’s depiction of the Milky Way Galaxy, showing the galactic longitude. Credit: NASA/JPL-Caltech/ESO/R. Hurt.

Our Milky Way has a coordinate system that references the Sun at the center. It uses spherical coordinates which is to a sphere what (x,y) is to a plane surface. This is a neat way to assign a specific location to a celestial body.

But of course we are a multi-reference-system species so we also have a coordinate system that references Earth at the center. It’s possible to convert coordinates between these systems, but only if you don’t scare easily by long equations.

The Really Large Scale

If we look beyond the 100,000 light years that make up our Milky Way, then it is compelling to think that these scales are far too big for anyone to even attempt at making a coordinate system. Not so much because galaxies are far away, but because everything moves so randomly in all directions at different speeds. It becomes close to impossible to define anything as a reference that will stay a reference even when everything moves.

The Milky Way is a part of The Local Group which contains galaxies within a span as big as 10 million light years. That’s 100 times bigger than our entire galaxy! Yet it’s not too big to squeeze into a coordinate system.

The Supergalactic Coordinate System is the name of this gigantic reference that includes The Local Group and even a bit beyond. That’s what I call a reference system!

So – back to our question. What is our cosmic address?

My best guess is:

Name, Street, City, Country, Earth, Solar System, Milky Way, Local Group

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