Other Units to Measure Distances in Space
Astronomical Unit (AU):
1 AU is defined as the distance from the _______________ to the ______________ and is
equal to 1.5 x 108 km.
1. How would you convert from km to AU?
2. Which is more convenient for expressing distances in our solar system, km or AU?
Explain your choice.
What is a Light Year?
Definition: ____________________________________________________________________
Why is the light year a more convenient way of expressing distances in space?
How Far Is a Light Year?
1. How long would it take for a pulse of sunlight to reach Earth?
Speed of light: 3 x 105 km/s
2. How far does light travel in a year?
3. If Proxima Centauri is 4.01x1013 km from Earth, what is its distance from Earth in light
years?
4. Polaris is a star 400 ly from Earth. Calculate the distance from Earth to Polaris in
kilometers.
Starlight is Really Ancient History:
It takes 8.3 min for light from our Sun to reach Earth. In other words, the event that
created that light, an explosion, for example, happened 8.3 minutes ago.
How old is light from Polaris? _______________________________________
Summary
In this exercise, you have learned different ways to measure distances in space: kilometres,
astronomical units, and light years.
Distance
Value (in km)
kilometre
1 km
astronomical unit
light year
Used to describe distances
between...
How Far Away are the Stars?
If a space shuttle travelled at 28 000 km/h, how long would it take to reach:
a) Proxima Centauri (the next nearest star to our Sun which is 4.01 x 1013 km away)
b) Andromeda galaxy (2 600 000 ly away)