Teaching sequence

Lesson objective

In this lesson students learn about the units used to measure distances in space and compare distances between celestial objects, and their sizes.


Explain to students that distances in space are so large they cannot be measured in common distance units. Instead, they are measured in light years. Define a light year as the distance light travels in one year. Calculate this distance.
Teacher note: To find more information on the light year, see Background information.


  1. View the video Powers of ten.
  2. Complete the module Writing numbers in exponential notation
  3. Discuss why astronomers need to use exponential notation when referring to distances in space.
  4. Explain to students that in astronomy distances are related to time. What astronomers see actually happened in the past because the distances are so great it takes light a long time to reach us.
  5. Explain that not only are distances very great but so are the sizes of celestial objects.
  6. Conduct the Incredible two-inch universe activity to compare the sizes of objects in the universe.


Use the learning object Explore: our place in space to review both the size and distances involved in the study of celestial objects and the universe.

Lesson Resources


Student activities

Digital resources

Powers of ten (1977), YouTube (9:00 min) 

Writing numbers in exponential notation, Science Learning Centre, University of Michigan 

The incredible two-inch universe, Chandra X-ray Observatory, NASA 

Explore: our place in space, Harvard Smithsonian Centre for Astrophysics 

Useful links

An atlas of the universe, atlasoftheuniverse.com. A learning object comparing scales of distance 

The scale of the universe 2, htwins.net. A learning object comparing scales of size 

The universe’s most distant object, NASA. Video (0:45 min)