To measure the distance to the moon, astronomers reflect short pulses of light o
ID: 1428293 • Letter: T
Question
To measure the distance to the moon, astronomers reflect short pulses of light off of retroreflectors placed on the moon by Apollo astronauts. The round-trip time for these pulses indicate the Moon’s current distance.
a. The Moon’s distance varies from 363000km to 405000km over its elliptical orbit. What is the round-trip time for laser pulse to travel these distances?
b. The results of the experiment demonstrate that the Moon is moving away from Earth at 38mm per year. How much longer does it take a light to pulse to make a round trip each year?
Explanation / Answer
a)
speed of pulse,v = 3*10^8 m/s
when distance = 363000km
round trip distance, d = 2*363000 km = 2*363000000 m = 7.26*10^8 m
t = d/v
= (7.26*10^8)/(3*10^8)
= 2.42 s <----Answer
when distance = 405000km
round trip distance, d = 2*363000 km = 2*405000000 m = 8.1*10^8 m
t = d/v
= (8.1*10^8)/(3*10^8)
= 2.7 s <----Answer
b)
extra distance to be travelled = 38*2mm = 76 mm = 0.076 m
extra time = d/v
= (0.076)/(3*10^8)
= 2.53*10^-10 s <----Answer
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