How fast is fast?
186,282miles per second
7.5trips around Earth in one second
1.3seconds from the Moon
8 min 20 sfrom the Sun
Nothing with mass can reach this speed. It's the universe's speed limit.
Light-years: distance, not time
| Object | Distance | Its light left... |
|---|---|---|
| The Moon | 1.3 light-seconds | A moment ago |
| The Sun | 8.3 light-minutes | Before you finished your coffee |
| Proxima Centauri (nearest star) | 4.2 light-years | About four years ago |
| Center of our galaxy | About 26,000 light-years | Before farming began |
| Andromeda Galaxy | About 2.5 million light-years | Before modern humans existed |
One light-year is about 5.9 trillion miles, the distance light travels in a year.
Looking back in time
Every view is old news
- You see the Sun as it was 8 minutes ago
- You see stars as they were years ago
- Some stars you see may no longer exist
Why it matters
- Telescopes are time machines
- The farthest light shows the young universe
- Radio to Mars takes 3 to 22 minutes each way
Remember
Look up at night and you're seeing the past.
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Answer key
- How fast does light travel?About 186,000 miles per second186,282 miles per second.
- How old is sunlight when it reaches you?About 8 minutes8 minutes 20 seconds.
- A light-year measures:DistanceIt's how far light travels in a year, about 5.9 trillion miles.
- What is the nearest star after the Sun?Proxima CentauriAbout 4.2 light-years away.
- Why is a telescope like a time machine?Distant light shows objects as they were long agoThe farther away, the older the light.