How long does light take?
Send light to us from a GPS satellite, the Moon, the Sun, Mars and the nearest star, and work out how long each journey takes. Find out why we see the Sun as it was eight minutes ago.
Core concept
Light travels at about 299 792 km every second, so the time it takes is distance ÷ speed: a fraction of a second from a satellite, 8 minutes from the Sun and more than 4 years from the nearest star
Topic: Our planet and sky
Light is the fastest thing there is: it travels about 299 792 km every second, fast enough to go round the Earth more than seven times in a second. On Earth it seems to arrive at once, but space is so big that even light takes time to cross it. The time is the distance divided by the speed: How far something travels in a certain time, like kilometres per hour. All key words. Light from the Moon, 384 400 km away, takes just over a second. Light from the Sun, about 150 million km away, takes about 8 minutes, so we always see the Sun as it was 8 minutes ago. Light from Proxima Centauri, the nearest star after the Sun, takes over 4 years. The distance light travels in a year is called a light-year, and astronomers use it to measure the distances to stars.
What you will learn
- Work out how long light takes to travel a distance.
- Explain why we see the Sun and stars as they were in the past.
- Describe what a light-year is.
Before you start
- Know that time = distance ÷ speed.
Materials
- A calculator
Equipment
- None
Procedure
- Record the light from the Sun.
- Change to the Moon and record.
- Change to Mars and record, then to Proxima Centauri and record.
How this simulation is simplified
- Light travels at 299 792 km/s; a year is 365.25 days (31 557 600 s).
- Distances: GPS satellite 20 200 km, Moon 384 400 km, Sun 149 600 000 km, Mars at its closest 54 600 000 km, Proxima Centauri 40 140 000 000 000 km.