## 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.

### Introduction

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.

### Core

- View the video Powers of ten.
- Complete the module Writing numbers in exponential notation.
- Discuss why astronomers need to use exponential notation when referring to distances in space.
- 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.
- Explain that not only are distances very great but so are the sizes of celestial objects.
- Conduct the Incredible two-inch universe activity to compare the sizes of objects in the universe.

### Conclusion

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)