In this lesson, students will study how stars may appear to be equidistant from the Earth, use a model to demonstrate the arrangement of stars, and study the importance of the parallax effect in determining distance. When early astronomers observed the sky, they imagined the vast array of stars embedded in a giant "crystalline sphere" that encompassed the Earth and the Sun. They believed that the planets resided on a smaller transparent sphere concentric to the one on which the stars moved. Only over time did astronomers realize that, as the Earth moved around the Sun, some stars shifted in position relative to the background of others. They realized that this shifting occurred because of a parallax effect when the Earth was on opposite sides of the Sun.