Moons
Moons

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Moons

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

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