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Light from the Moon

Space and time · level 3, lesson 3 of 6

A question to start

The Apollo astronauts left mirrors on the Moon. Scientists on Earth still fire lasers at them today. Why?

The idea

The Moon is about 384,400 km away. Light takes

\[384{,}400 \div 300{,}000 \approx 1.3 \text{ seconds}\]

to get there. So the Moon you see is the Moon as it was 1.3 seconds ago.

The mirrors left by Apollo 11, 14 and 15 bounce laser light straight back to where it came from. Scientists time how long the light takes to go there and back: about 2.6 seconds. From that time, they work out the distance to the Moon very precisely. That is how we know the Moon is drifting away from Earth, by about 3.8 cm a year.

The mirrors need no power, so they still work, more than 50 years later.

1.3 seconds there1.3 seconds backEarthMoon

Worked examples

1. One way. \(384{,}400 \div 300{,}000 \approx 1.28\) seconds.

2. There and back. \(2 \times 1.28 \approx 2.6\) seconds.

3. From time to distance. Light took 2.56 seconds there and back. One way: 1.28 seconds. \(1.28 \times 300{,}000 = 384{,}000\) km.

Try it

  1. About how long does light take to reach us from the Moon?
  2. Why do scientists bounce lasers off mirrors on the Moon?
  3. A laser pulse takes 2.6 seconds to come back. How far away is the Moon?
  4. Why do the mirrors still work after 50 years?
Answers
  1. About 1.3 seconds.
  2. To measure the distance to the Moon very precisely.
  3. One way is 1.3 seconds: \(1.3 \times 300{,}000 = 390{,}000\) km, close to 384,400 km.
  4. They need no power; they just reflect light.

Common mistake

Using the round-trip time as the one-way time. The laser goes there and back. Halve the time before working out the distance.

Where it leads