How big is space?
Turn cosmic distances into something you can picture: read a light-year correctly, shrink the Sun to a football, and see why looking far away means looking into the past.
THE RULER
A light-year is a distance, not a time
Light travels at about 300,000 km every second. Nothing goes faster. A light-year is how far light gets in one year: roughly 9.5 trillion km. Kilometres become useless in space, so astronomers measure distance by how long light takes to cross it.
The Moon is about 1.3 light-seconds away. The Sun is 150 million km away, which is about 8 light-minutes. The nearest star after the Sun, Proxima Centauri, is 4.2 light-years away.
speed of light = 300,000 km per second
seconds in one year = 60 x 60 x 24 x 365 = 31,536,000
one light-year = 300,000 x 31,536,000 kmabout 9,460,000,000,000 km (9.5 trillion km)A passenger jet at 900 km/h would need more than a million years to cover one light-year.
Check yourself
Neda imagines sending a radio message (radio is a kind of light) to a planet around Proxima Centauri, 4.2 light-years away. If someone there answers immediately, when is the earliest she could hear the reply?
- In 4.2 years
- In about 8.4 years
- Almost at once, because radio needs no time in empty space
- In 4.2 light-years
Show the answer
In about 8.4 years
Right. The message needs 4.2 years to get there and the answer needs 4.2 years to come back. A light-year tells you the distance, and the distance tells you the delay.
Shrink the Sun to a football
Make the Sun a football, 22 cm across, and shrink everything else by the same amount. The Earth becomes a 2 mm bead, about a pinhead, circling 24 metres away. Jupiter is a large marble 120 metres off. Neptune is a small bead 700 metres away. Between them: nothing. Where the model breaks: it freezes everything in place, while the real planets are moving at tens of kilometres per second.
| In the model | Size | Distance from the football |
|---|---|---|
| Sun | football, 22 cm | 0 |
| Earth | 2 mm bead | 24 m |
| Jupiter | 2 cm marble | 120 m |
| Neptune | 8 mm bead | 700 m |
| Nearest star | another football | over 6,000 km |
Check yourself
In the football model, with the Sun in the middle of your city, the nearest other star would be a second football somewhere on the edge of the same city.
Show the answer
False
False. The whole solar system out to Neptune fits inside a circle you could walk across in 20 minutes, but the next football is more than 6,000 km away, roughly the distance from London to Chicago. Space between stars is far emptier than space between planets.
A TIME MACHINE
Looking far is looking back
Light takes time to arrive, so you never see anything as it is now. You see it as it was when the light left. For a friend across the room the delay is a few billionths of a second. For the sky it's minutes, years, or millions of years. A telescope is a way of seeing the past, and the farther it looks, the older the view.
The Sun you see is the Sun of 8 minutes ago. Proxima Centauri appears as it was 4.2 years ago. Our galaxy, the Milky Way, is about 100,000 light-years across, so light from its far side set out long before writing was invented.
Check yourself
Match each thing in the sky to how old the view is
Show the answer
- The Moon → About 1.3 seconds old
- The Sun → About 8 minutes old
- Proxima Centauri → About 4.2 years old
- The Andromeda galaxy → About 2.5 million years old
How we know: measuring a star with parallax
- Try it with your thumb
Hold your thumb at arm's length and look at it with one eye, then the other. It jumps against the background. Bring it closer and it jumps more. The size of the jump tells you the distance.
- Use the Earth's orbit as two eyes
Astronomers photograph a star, wait six months, and photograph it again. By then the Earth has moved to the other side of its orbit, 300 million km away.
- Measure the shift
A nearby star shifts slightly against the far more distant stars behind it. This shift is called parallax. Even for the nearest star it is far less than a thousandth of a degree.
- Solve the triangle
You know the base of the triangle (the width of Earth's orbit) and the angle. Simple geometry gives the distance. The Gaia space telescope has done this for more than a billion stars.
Check yourself
Kian compares two photos of the same patch of sky taken six months apart. Star A has shifted a little against the background. Star B, right next to it, has not moved at all that he can measure. What can he conclude?
- Star A is closer to us than star B
- Both are at the same distance, since they sit side by side in the photo
- Star B is closer, because it is steadier
- Star A is brighter than star B
Show the answer
Star A is closer to us than star B
Yes. A bigger parallax shift means a nearer object, just like your thumb jumps more than a distant tree. Star B is too far for its shift to show.
Three mistakes to leave behind
- "A light-year is a long time." It is a distance, about 9.5 trillion km.
- "The night sky is a snapshot of now." Every point of light is a different age. Some stars you see may have changed a great deal since the light left.
- "Space is crowded with stars." Stars are footballs thousands of kilometres apart. Space is almost entirely empty.
Check yourself
Imagine the Sun stopped shining this very second. Mina is standing outside at noon. When does her sky go dark?
- At the same instant, since the Sun is what lights the sky
- After about 1.3 seconds
- Only after a full year has passed
- About 8 minutes later
Show the answer
About 8 minutes later
Right. The light already on its way keeps arriving for about 8 minutes, because the Sun is 8 light-minutes away. Mina is always seeing the Sun's past.
Lesson recap
- A light-year is a distance: about 9.5 trillion km, how far light goes in a year at 300,000 km/s.
- With the Sun as a football, the Earth is a pinhead 24 m away and the nearest star is another football over 6,000 km away.
- Light takes time to arrive, so every view of the sky is a view of the past: 8 minutes for the Sun, 2.5 million years for Andromeda.
- We measure distances to nearby stars with parallax: the small shift a star shows when the Earth moves across its orbit.