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Light-minutes and light-years

Space and time · level 5, lesson 5 of 6

A question to start

A light-year sounds like a length of time. It is not. What is it?

The idea

Space distances get so big that kilometres stop being handy. So astronomers often give a distance as how long light takes to cross it.

  • A light-second is about 300,000 km.
  • A light-minute is \(60 \times 300{,}000 = 18\) million km.
  • A light-year is the distance light goes in a year: about 9.46 trillion km.

The Moon is about 1.3 light-seconds away. The nearest star beyond the Sun, Proxima Centauri, is just over 4 light-years away.

Distance from the Sun Mars Earth
In light-minutes about 12.7 about 8.3
In au about 1.52 1
the Moon1.3 light-sthe Sun8.3 light-minProximaover 4 light-yr× 380× over 250,000

Worked examples

1. A light-year. A year is about 31.6 million seconds. \(31{,}600{,}000 \times 300{,}000 \approx 9.5\) trillion km.

2. Mars in light-minutes. 228 million km \(\div\) 18 million km \(\approx 12.7\) light-minutes.

3. Proxima in km. It is just over 4 light-years away. \(4 \times 9.46\) trillion \(\approx 38\) trillion km, so a little more than that.

Try it

  1. Is a light-year a time or a distance?
  2. About how many km is a light-minute?
  3. Jupiter is about 778 million km from the Sun. How many light-minutes is that?
  4. Why do astronomers use light-years for stars?
Answers
  1. A distance: how far light goes in a year.
  2. About 18 million km.
  3. \(778 \div 18 \approx 43\) light-minutes.
  4. Star distances in km are huge, hard-to-read numbers.

Common mistake

"A light-year is a year." It is a distance, about 9.46 trillion km.

Where it leads