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ELI5 Yahaaa

A distance, not a time

What Is a Light Year?

It sounds like a duration. It is actually how far light gets in one year.

Sounds like A length of timeActually is A distance: 9.5trillion km
Same trick as calling a journey 'two hours away'.

  1. Step 1 of 5

    Kilometres run out of usefulness.

    To the nearest star40 trillion kmTo the Sun150 million kmAround the Earth40,000 km

    The nearest star is 40,000,000,000,000 km away. Writing space distances in kilometres means counting zeroes, and nobody can feel a number like that.

  2. Step 2 of 5

    So astronomers measure with light.

    Light 7 laps of theplanet per second

    Light is the fastest thing in the universe — 300,000 km every second. Fast enough to circle Earth seven times while you say the word.

  3. Step 3 of 5

    One light year is one year of that.

    1 300,000 km persecond2 Keep going forone year3 9,500,000,000,000km

    Keep that speed up for a full year and you cover about 9.5 trillion kilometres. That distance is one light year.

  4. Step 4 of 5

    It makes the sky measurable.

    Furthest visible13 billion lyAcross our galaxy100,000 lyNearest star4.2 ly

    The nearest star is 4.2 light years. Our galaxy is 100,000 across. The furthest thing we see is 13 billion light years out. Now the numbers fit in your head.

  5. Step 5 of 5

    Looking far away is looking into the past.

    1 Light leavesthe star2 Travels foryears3 You see thepast

    Light from a star 4 light years away left 4 years ago. You never see space as it is now — every glance is a look backwards in time.

    Some stars you can see tonight have already died. Their last light is still on its way.

The short version

A light year is a distance — the 9.5 trillion km light covers in a year — and it makes every glance a look back in time.

Try it yourself

Say 'that shop is ten minutes away'. You just measured distance with time, exactly like a light year.

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