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Space · Galaxies and the universe

The light-year

Not a time, a distance

Only spacecraft can see it

The light-year
The light of a distant star sets off and travels for years before it reaches our eyes.

The numbers

What it is
A distance, not a time
What it is for
Measuring distances between stars
Another unit
The parsec, equal to 3.26 light-years

How it happens, step by step

  1. Light travels

    Light does not arrive instantly: it moves, extremely fast but not infinitely fast, and it takes time.

  2. A year of travel

    The distance it covers in one whole year is taken as a unit of measure, and it is called a light-year.

  3. Made for the stars

    Between the stars miles turn into useless rows of zeros, while light-years stay numbers you can grasp.

  4. The light set off

    The light reaching your eyes tonight set off as many years ago as the star is light-years away.

  5. You see the past

    So you do not see the star as it is now, but as it was when that light set off on its trip.

To get an idea

A star a hundred light-years away is seen as it was a hundred years ago, not as it is now.

The Sun we see is the Sun of a little over eight minutes ago.

Fun facts

These are not in the book: there was no room on paper.

  1. The year used in the definition is the Julian year, three hundred sixty-five and a quarter days long.

  2. Looking at a distant star always means looking at its past, never at its present.

  3. Sunlight takes a little over eight minutes to travel all the way to the Earth.

  4. The farther a telescope looks, the further back in time it can see.

  5. A star we see shining tonight might already have gone out long ago.

  6. The official definition of this unit was set by the International Astronomical Union.

Test yourself

The first question is the one printed under the QR code, with the answer the book did not give. The other four only exist here.

Question 1 of 5★ is the one from your book

What does a light-year measure?

Question 2 of 5

Why is it used instead of miles?

Question 3 of 5

How do we see a star a hundred light-years away?

Question 4 of 5

How long does sunlight take to get here?

Question 5 of 5

What is a parsec?

Riddles

My name says year and everyone thinks of time, but I measure space: I am the distance the fastest thing there is covers in twelve months. What am I?

Thanks to me every telescope becomes a machine that looks back: the farther you reach, the older the scene you see. What am I?

Without me you would have to write rows of zeros to say how far away a star is, and those numbers would mean nothing to anyone. What am I?

Try it yourself

Activity 1

Look up the distance in light-years of a star you know and subtract that number from the current year: that is the year the light you see set off.

Activity 2

Time a distant flash of lightning and the thunder that follows: the light arrives at once, the sound later. It is the same idea, on a tiny scale.

The words

parsec
Another unit of distance in astronomy, about 3.26 light-years.

Sources: Wikidata (Q531), Wikipedia (Anno luce).