Moons

This free course is available to start right now. Review the full course description and key learning outcomes and create an account and enrol if you want a free statement of participation.

Free course

# 4.1.4 Calculating the distance to the Moon

## Activity 1

Which is the correct equation for calculating the distance to the Moon? Note that ‘time’ refers to the time a laser pulse takes to get to the Moon and back to Earth.

a.

distance = (time/speed)/2

b.

distance = (speed × time)/2

c.

distance = (speed/time)/2

The correct answer is b.

b.

This is correct because speed = distance/time. Multiplying both sides by time gives: distance = speed × time. Then speed × time is divided by 2 because the time obtained by shining the laser is the time for the light to go to the Moon and back to Earth, whereas all you want to know is the time for the light to reach the Moon.

What is the speed of the laser light?

a.

The speed of light

b.

The speed of sound

c.

It is different depending on the laser

The correct answer is a.

a.

This is correct. Since a laser is simply a light source so laser light travels at the speed of light, independent of the frequency of the laser.

What is the distance to the Moon?

a.

~770 000 km

b.

~260 000 km

c.

~390 000 km

The correct answer is c.

c.

This is correct. The speed of light is approximately 3 × 10<sup>8</sup> m s<sup>−1</sup> and the time taken for the light to go to and from the Moon is about 2.6 seconds. Using the equation given above, distance = (speed × time)/2, you get distance to the Moon ~390 000 km.

MOONS_1

### Take your learning further

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has over 40 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses.

If you are new to university level study, find out more about the types of qualifications we offer, including our entry level Access courses and Certificates.

Not ready for University study then browse over 900 free courses on OpenLearn and sign up to our newsletter to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus