Skip to content

Light from the Sun

Space and time · level 3, lesson 4 of 6

A question to start

If the Sun suddenly went out, how long would it be before we noticed?

The idea

The Sun is about 149.6 million km away. Light takes

\[149{,}600{,}000 \div 300{,}000 \approx 499 \text{ seconds},\]

which is about 8 minutes and 20 seconds. So the Sun you see is the Sun as it was 8 minutes ago. If it went out, the sky would stay bright for 8 more minutes.

Mars is farther from the Sun, so sunlight takes longer to get there:

Sunlight takes Mars Earth
On average about 13 minutes about 8 minutes

When you look at a star, the light is even older. The nearest star beyond the Sun is so far away that its light takes more than four years to reach us.

Earth8 min 20 sMarsabout 13 min

Worked examples

1. Seconds to minutes. 499 seconds: 8 minutes (480 seconds) and 19 seconds left over. About 8 minutes 20 seconds.

2. To Mars. \(228{,}000{,}000 \div 300{,}000 = 760\) seconds: 12 minutes 40 seconds, about 13 minutes.

3. The difference. \(760 - 499 = 261\) seconds: sunlight takes about 4 minutes longer to reach Mars than Earth.

Try it

  1. About how long does sunlight take to reach Earth?
  2. If you look at the Sun's light now, when did it leave the Sun?
  3. Does sunlight take longer to reach Earth or Mars?
  4. Light from Jupiter, 778 million km from the Sun: about how many minutes from the Sun?
Answers
  1. About 8 minutes 20 seconds.
  2. About 8 minutes ago.
  3. Mars.
  4. \(778{,}000{,}000 \div 300{,}000 \approx 2{,}593\) seconds: about 43 minutes.

Common mistake

"We see the Sun as it is now." We see it as it was about 8 minutes ago. Looking far into space is looking back in time.

Where it leads