Answer key: Light Travel Time Worksheet
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Goal: Use the speed of light to describe distances in the solar system and beyond, and explain why seeing far away means seeing the past.
Completed results table
| Setting | Light travel | In minutes | In years |
|---|---|---|---|
| Sun to Earth (1 AU) | 499 s | 8.32 min | 1.58e-5 years |
| Earth to Moon (0.00257 AU) | 1.28 s | 0.02 min | 4.06e-8 years |
| Sun to Neptune (30.18 AU) | 15,059.96 s | 251 min | 4.77e-4 years |
| Sun to Jupiter (5.2 AU) | 2,594.82 s | 43.25 min | 8.22e-5 years |
| Proxima Centauri (4.24 light-years) | 1.34e+8 s | 2,230,070.4 min | 4.24 years |
Answers
How long does sunlight take to reach Earth? Give your answer in seconds and in minutes and seconds.
About 499 seconds, which is 8.32 minutes or 8 minutes 19 seconds.
Predict how many minutes sunlight takes to reach Jupiter (5.2 AU), then check with the lab.
About 43 minutes: 5.2 × 8.32 ≈ 43.2 minutes (2,595 seconds).
If the Sun suddenly went dark, how long would it take people on Earth to notice? Explain.
About 8 minutes 19 seconds. The light already on its way would keep arriving until the last of it reached us.
Radio signals travel at the speed of light. About how long does a message take to reach a spacecraft near Neptune? Why can’t engineers steer it live?
About 4.2 hours (15,060 seconds, or 251 minutes). A command and its reply take more than 8 hours round trip, so spacecraft follow instructions planned in advance.
The Moon is about 1.28 light-seconds away. Is a light-second a unit of time or of distance? What about a light-year?
Both are distances: how far light travels in one second (about 300,000 km) or in one year (about 9.46 trillion km). The Moon’s average distance is 1.28 light-seconds.
Light from Proxima Centauri takes 4.24 years to reach us. When you look at it, are you seeing it as it is today? Explain.
No. You see it as it was 4.24 years ago, when that light left the star. The farther away an object is, the further back in time you are seeing it.