A tellurium (Latin tellus, the earth), is a planetary machine to demonstrate the movements of the earth and the moon. These rotate on a lever arm around the sun. In addition, the earth revolves around itself (day and night) and the moon around the earth. With a tellurium the seasons and phases of the moon can be represented. The question of why we see only one side of the moon on Earth, through the bound rotation of the earth and the moon, can be answered well here.