In this activity, learners will measure the length of their shadow from the Sun and compare it three to four months later. This process will help learners investigate Earth's rotation and discover the change in seasons by the elevation of the Sun. This resource includes an explanation of why learners will observe changes in their shadow's length as well as a description of Eratosthenes' first known measurement of the circumference of the earth. Learners can measure the circumference of the Earth using shadows in the related activity "The Shadow Knows II." This resource also includes winter and summer safety tips.