You can use a bar magnet and a compass to get a rough idea of the strength of Earth's magnetic field.

What happened?

The force on a compass due to the Earth's magnetic field is relatively weak. Think of it as a gentle magnetic breeze.
Magnetic Wind
However the field near the household magnet is dense like a tornado. Strong near the magnet and very weak as you move away.
Magnetic Wind

Challenge 1: How strong is the Earth's magnetic field?

Find (or make) a compass & 1 household magnet.
Bar Magnet
Compass
Can you determine the exact distance at which the household magnet force equals that due to the Earth's magnetic field?
What does this distance tell us?