This module uses interactive simulations to introduce students to the horizon and celestial coordinate systems and the apparent rotation of objects in the sky (caused by the Earth's rotation). The simulations demonstrate the horizon plane, azimuth, and altitude; compare the advantages and disadvantages of the horizon and celestial coordinate systems, and show the paths of stars and other objects as they 'rotate' through the sky. The 'Rotating Star Explorer' lets users place stars, select the observer's position, and run an animation that shows the movements of the stars in both celestial and horizon views. The module includes a pretest, posttest, and a student guide.