Ask your child to tell you their address. When they reach the country, ask, "But where is our country?"
Keep asking "And where is that?" until they say "Earth" or "space". You have just started writing a cosmic address, and it is much longer than a street, town, and country. In this article, we will explore our place in the Universe, one step at a time, and see how far we can go.
🧒 Say in one sentence
You live on Earth, beside the Sun, inside a galaxy called the Milky Way - and that is only part of your address.
Game 1: Could the letter find you?
Take an envelope or a piece of paper and write your child's name on it. Ask, "Could the letter find you with only this?" Add the house, street, town and country one at a time, asking the same question after every line.
Do not finish the address yet. Keep the envelope beside you as you read. Whenever you reach a new level, add it and ask again. By the end, even an alien postal worker will know where to deliver the letter.
The whole address in one picture
Seven useful stops from you to everything we can observe. Each step is far bigger than the one before it, and the last jump is the one nobody can really picture.
This is the simplest address. Astronomers can add extra details such as the Local Bubble, the Orion Spur and the cosmic web. But these seven give us a clear route, so let us climb them one step at a time.
1. You
Every measurement has to start somewhere, so it may as well start with your child. Let's say it's about 1 metre, standing up.
Keep this number in mind. Everything that follows can be compared with things you know, at least in terms of scale. You are a tiny speck in the Universe, but you are also the starting point for measuring it.
2. Earth
Our planet is 12,742 km across. If you could drive without stopping at motorway speed, going all the way round the equator would take about three weeks.
That feels enormous, and for a walking animal it is. But Earth is a rocky ball of ordinary size, and it is not the biggest thing in its own neighbourhood. It is not even close.
Could the letter find you now? Add Earth, but keep going: one planet is still not much of an address.
3. The Solar System
One star, eight planets, a few dwarf planets, a great many moons, and a lot of ice and rubble. The planets occupy a disc about 9 billion km across, measured across Neptune's orbit - but the Sun's family does not stop there.
Beyond Neptune lie the Kuiper Belt and, much farther away, the Oort Cloud, a possible swarm of icy comet-like bodies stretching perhaps halfway to the next star. That means the Solar System has no single, tidy edge. For our address ladder, planetary Solar System means the Sun and the region containing its eight planets.
Almost all of it is empty. This is the fact that surprises people most, and the reason a solar-system model on a poster is so misleading: to fit on the page, the planets have to be drawn hundreds of times too close together. If you shrank the Sun to the size of a football, Earth would be a peppercorn about 25 metres away, and Neptune a grain of rice three quarters of a kilometre down the road.
The distances along the ruler are proportional. The football, peppercorn and grain of rice are enlarged above it so you can see them; at their true displayed scale, the planets would be almost invisible.
Light, which crosses your bedroom in less time than you can measure, needs a little over 8 minutes to reach us from the Sun, and about 4½ hours in total to reach Neptune's orbit.
Could the letter find you now? Add the Solar System. We have found the right star, but there are still billions of other stars around it.
4. Our star neighbourhood
Step outside the Solar System and the first thing you meet is lots and lots of emptiness.
The nearest star to the Sun is Proxima Centauri, and it is 4.2 light years away. In the football-sized-Sun model from a moment ago, where Neptune was three quarters of a kilometre off, Proxima Centauri would be about 6,500 kilometres away. Stars are not crowded. They are specks separated by oceans.
Within about 30 light years of us there are a few hundred stars. That is our neighbourhood. Yet even the nearest star is so far away that a spacecraft traveling at our current top speeds would take over 70,000 years to reach it.
5. The Milky Way
Now zoom out until those few hundred stars become a blur.
The Milky Way is our galaxy: a slowly turning spiral of a few hundred billion stars, held together by gravity, about 100,000 light years across. Light itself, the fastest thing there is, would need a hundred thousand years to cross it.
Left: seen from above, with our Solar System on the Orion Spur. Right: the same galaxy edge on. Its thickness is exaggerated so the disc remains visible; the real Milky Way is flatter. We are neither at the centre nor at the rim.
Two things about our address here are worth thinking about.
First, we are not in the middle. The Sun sits about 26,000 light years out from the centre, roughly halfway to the edge, on a smallish arm called the Orion Spur Orion Arm. It is an ordinary suburb.
Second, the whole thing is turning, and we are going round with it at about 220 kilometres every second. One full lap takes the Sun roughly 230 million years. The last time our Solar System was in this spot on its orbit, the first dinosaurs were just appearing.
The Milky Way is 100,000 light years across. Roughly how long does light take to cross the whole galaxy?
Make your prediction, then tap an answer to check!
6. The Local Group
Galaxies can be grouped together. Ours belongs to a group of roughly 80 known galaxies called the Local Group, spread across about 10 million light years. The count can change as astronomers discover faint dwarf galaxies or reconsider which ones belong.
Most of them are small. The two giant spiral galaxies are our Milky Way and Andromeda, 2.5 million light years away. On a clear dark night, far from town, Andromeda is faintly visible to the naked eye as a smudge of light. It is the most distant thing most people will ever see without a telescope.
Andromeda is moving towards us at about 110 kilometres per second. Astronomers once thought the two galaxies were certain to merge, but newer calculations make the future much less certain - something like a cosmic coin toss over the next 10 billion years. If they do meet, individual stars are so widely separated that direct star crashes would be very unlikely.
7. The observable Universe
Here is where the ladder stops. And it's one of the most confusing things about the Universe: how space and time are linked.
The Universe is about 13.8 billion years old. Light travels fast, but not infinitely fast, so light from anything too far away simply has not had time to reach us yet. There is a boundary, and it is not the edge of the Universe. It is the edge of how much of it we can possibly see.
That bubble around us is called the observable Universe. Current estimates suggest between 200 billion and 2 trillion galaxies, most of them too faint for us to see individually.
🔬 Why 93 billion and not 27.6 billion?
If the Universe is 13.8 billion years old, the bubble should be 13.8 billion light years in every direction, so 27.6 billion across. Yet astronomers say it's 93 billion. The reason is that space itself has been stretching the whole time that light has been travelling. The light took 13.8 billion years to get here, but the galaxy that emitted it has been carried much further away in the meantime. Both numbers are correct: one measures travel time, the other measures where things are now.
And the most mind-bending part: everyone is at the centre of their own observable Universe. A creature in a galaxy far away has its own bubble and its own limit on what it can see. The Universe itself has no center, but everybody is in the middle of their own view.
Game 2: Build a Universe that nests
This version lets younger explorers feel what inside means. Use scraps of paper cut into different-sized shapes, or collect nesting bowls, boxes or food containers. Label them:
You · House · Town · Country · Earth · Solar System · Milky Way · Local Group · Observable Universe
📋 Make a cosmic nesting stack
- 1
Start with you
Draw yourself on the smallest piece of paper, or put a tiny figure in the smallest container. - 2
Build outwards
Place each card on a slightly larger card, or each container inside the next one. Say every name as you add it. - 3
Scramble the address
Mix up the cards and race to rebuild the Universe from smallest to largest. Ask, "What holds Earth? What holds the Milky Way?" whenever your child gets stuck. - 4
Reverse the journey
Take the stack apart from largest to smallest. You are travelling home from the observable Universe.
For a group of children
Give each child one address card. Without showing them the answer, ask everyone to stand in order from you to the observable Universe. Then reverse the line and travel home.
Reading the address backwards
Try it out loud with your explorer, largest to smallest, and watch the moment it lands:
The observable Universe, the Local Group, the Milky Way, the Orion Arm, the Solar System, Earth, your country, your town, your street, your house, you.
Eleven lines. Every one of them is a real place. The first three are things nobody had names for a few hundred years ago, and the whole idea that our galaxy is not the entire Universe is younger than most people's great-grandparents: it was settled in the 1920s.
That is the part we would want a child to take away. The address is enormous, yes. But it is also newly discovered, and there is nothing at all to say we have written the last line of it.
Key takeaways
- Your full address has more steps above your country: Earth, the Solar System, the Milky Way, the Local Group, the observable Universe.
- Earth is 12,742 km across; the Solar System is mostly empty space, which posters always hide.
- A light year is the distance light travels in one year. It also tells us how long the light has been travelling, so distant objects show us the past.
- The nearest star, Proxima Centauri, is 4.2 light years away. Stars are specks separated by enormous gaps.
- The Milky Way is about 100,000 light years across, and we sit about 26,000 light years from the centre, not in the middle.
- The Local Group holds roughly 80 known galaxies; Andromeda is the other giant spiral, 2.5 million light years away and visible to the naked eye.
- The observable Universe is 93 billion light years across, and it is a limit on what we can see, not the edge of everything.
- Because space has stretched while light travelled, the bubble is wider than the Universe's age in light years would suggest.
✂️ Free printable
Your cosmic address
2 pages · 167 KB
Use page one for the Could the letter find you? game: it is a fill-in address card from your name all the way out to the observable Universe. Page two is the ladder to colour in, with the size of every rung, ready for a classroom wall.
Frequently Asked Questions
What is our full address in the Universe?
Working outwards: your house, your street, your town, your country, Earth, the Solar System, the Orion Arm, the Milky Way, the Local Group, and the observable Universe. This is a useful simple address; astronomers can add other lines depending on how much detail they want.
What is a light year?
The distance light travels in one year: about 9.5 trillion kilometres. It is a measure of distance, not time, despite the name. It is useful because it also tells you how old the view is. Something 100 light years away appears to you as it was 100 years ago.
Where is Earth in the Milky Way?
About 26,000 light years from the centre, roughly halfway out to the rim, on a minor arm called the Orion Arm or Orion Spur. We are neither central nor on the edge, which is one reason the night sky shows a band of stars in one direction: that is us looking along the disc of our own galaxy.
How big is the observable Universe?
About 93 billion light years across. Current estimates suggest it contains between 200 billion and 2 trillion galaxies. The word "observable" is doing real work: it is the region whose light has had time to reach us in 13.8 billion years. What lies beyond it may well go on for ever. We simply cannot see it.
Is the Universe bigger than the observable Universe?
Almost certainly yes, and possibly infinitely so. The observable part is a bubble centred on us and limited by the speed of light and the age of the Universe. It says something about our view, not about where the Universe stops.
Are we at the centre of the Universe?
We are at the centre of our observable Universe, and so is everybody else at the centre of theirs. That sounds contradictory until you picture each observer with their own bubble of visible space. There is no known centre to the Universe as a whole.
How do I explain these distances to a young child?
Shrink everything. Make the Sun a football, and Earth becomes a peppercorn 25 metres away. Better still, take a roll of toilet paper into the hallway and pace out the planets one square at a time. Children reliably guess that the planets are close together, and the moment they discover otherwise is worth more than any number.
If you are interested in more science facts, check out our tools page and other articles. And till next time, enjoy the view of the night sky.
