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Earth and Space 6-8 / Week 07 / Tuesday
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Week 07 · How Far: Distances

Tuesday

Light takes time
// Walking the solar system one step at a time
⏱ about 20 min

Tuesday: Light Takes Time

Rocket flicks the Sky Deck lamp on and off. "Light is instant," he says. "On, and it is everywhere."

"Is it?" Raven asks. "What do you notice when a storm is far away? Flash first, thunder later."

"Sound is slow," Rocket says. "Light is fast. But fast is not the same as instant."

Nova dims to a soft glow. "The sunlight on your hand right now left the Sun before you walked outside," she says.

Rocket stares at his hand. "How long before?" Nova tilts. "Long enough to pour a drink and sit down. Find the number."

Raven is already flipping pages. "Eight minutes," she reads. "The Sun we see is eight minutes old."

Sunlight is eight minutes old

Light is the fastest thing there is, but it still takes time to travel.

NASA says it takes about eight minutes for light from the Sun to reach our planet.

OpenStax Astronomy puts it this way. Light takes slightly more than 8 minutes to travel one astronomical unit.

So the latest news we receive from the Sun is always 8 minutes old. If you look at the Sun's glow at sunset, you see the Sun as it was eight minutes earlier.

Never look directly at the Sun to check. Watch the glow on the horizon or the shadows on the ground instead.

Farther means older light

The farther a planet is from the Sun, the longer its sunlight has traveled before it arrives.

Neptune is about 30 times farther from the Sun than Earth. Its sunlight has traveled far longer than eight minutes.

Beyond the planets, distances grow so large that kilometers stop being useful. Astronomers switch to light-years.

NASA defines a light-year as the distance light travels in one year, about 9.5 trillion kilometers.

The nearest star beyond the Sun, Proxima Centauri, is 4.24 light-years away. Its light left it more than four years ago.

FromToTravel time for light
The SunEarthabout 8 minutes
Proxima CentauriEarthabout 4.24 years
OLD LIGHT
  • Read the question.
  • Tap your answer.
About how long does sunlight take to reach Earth?
What is a light-year?
A planet farther from the Sun receives sunlight that has traveled how long?
LIGHT TRAVEL TIME, SHORTEST TO LONGEST
  • ?Sunlight reaching Earth
  • ?Sunlight reaching Neptune
  • ?Starlight from Proxima Centauri reaching Earth
WHY THIS EXERCISEOrdering by distance is ordering by travel time. The two always match.
Light takes about ____ minutes to cross one astronomical unit.
Proxima Centauri is 4.24 ____ away.
Try it
Ask someone to stand across the yard and clap. Watch the hands and listen. Did you hear the clap at the same instant you saw it?
Write what you noticed. Sound is slow enough to catch. Light is too fast to catch across a yard.
Safety first
Never look directly at the Sun, with or without glasses, binoculars, a camera or a phone.
Check the Sun's position only by the glow on the horizon or by where shadows fall.
Try this
Set a timer for eight minutes and do something ordinary, like reading. When it rings, think: sunlight just made the whole trip.
Write one sentence about how far light traveled while you read.
Draw the Sun on the left and Earth on the right. Draw a wavy line of light between them and write 8 minutes above it. Add a tiny Proxima Centauri far off the page edge with 4.24 years.

Clear thinking. Tomorrow is Sky Deck Lab: you walk the whole solar system out across the yard.

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