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Curiosity · The scale of the universe

The light year: measuring distance with time

A light year is not a time but a distance: the roughly 9.46 trillion km that light travels in a year. Find out why looking at the sky means looking into the past, and why we see the Sun as it was 8 minutes ago.

  • 15 min
  • 2 simulations
  • Mini quiz

Updated:

Preview

The question

When you hear that a star is "100 light years away", do you think of a length of time or a distance? And here's an intriguing question: is the Sun you see right now really the Sun as it is right now?

Try it yourself

No account needed: change the values and check your answer. Some simulations ask for a prediction before they start; it doesn't matter whether it's right or wrong.

Number line

The journey of light from the Moon

Drag the marker or move it with the arrow keys, then press Check.

Prediction

Predict first

How long do you think light reflected from the Moon takes to reach your eye?

It doesn’t matter whether you’re right; the simulation opens once you choose.

Planet observatory

The planet four light hours away

Tap the planets to check their distances from the Sun. Multiply the distance in AU by 8.3 minutes and convert the result to hours.

Prediction

Predict first

What does sunlight taking 4 hours to reach a planet tell you about that planet?

It doesn’t matter whether you’re right; the simulation opens once you choose.

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Embed the simulation on your site

Paste this code into the HTML of your blog or school site. Visitors try the simulation without an account; no progress is saved.

The embedded page sets no cookies and does no tracking. A small link back to Kuazara sits underneath.

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Frequently asked

5 questions

Is a light-year a unit of time?

No, it is a unit of distance: the distance light travels in a vacuum in one year, about 9.46 trillion km. Saying "you reach this star in 10 light-years" is like saying "you reach this city in 10 kilometres".

Where does 9.46 trillion km come from?

Light travels about 299,792 km per second in a vacuum. A year has about 365.25 × 24 × 60 × 60 = 31,557,600 seconds. Multiplying the two gives about 9.46 trillion km.

Is the Sun you see the Sun as it is right now?

No. Sunlight takes about 8 minutes 20 seconds to reach us, so you see the Sun as it was that long ago. If the Sun suddenly went out, we would only notice after that much time.

Have most of the stars we see in the sky already died?

That is very unlikely. Almost all the stars we can see with the naked eye are closer than a few thousand light-years, while stars live for millions or even billions of years. The chance that you are seeing a star that went out while its light was on the way is very low.

Why is looking far away the same as looking into the past?

The image you see is as old as the time the light took to reach you. You see Proxima Centauri as it was about 4 years ago, and the light from the Andromeda Galaxy entering your eye tonight set out about 2.5 million years ago.