This is an activity about oscillation. Learners will observe, time, and graph the data of the side to side motion of the mirror used in the soda bottle magnetometer activity to determine the mirror's oscillation amplitude. This activity requires prior construction and experience in use of a soda bottle magnetometer, which is the eighth activity in the Exploring the Earth's Magnetic Field: An IMAGE Satellite Guide to the Magnetosphere educators guide. This is the tenth activity in the guide.