The Celestial Sphere and Analemma JavaScript Model simulates the Two Sphere Universe theory of the Ancient Greeks. This theory supposes the stars to be fixed on the surface of a Celestial Sphere, with the spherical Earth at the center of this sphere. The simulation shows the motion of Sun and stars in this model, as well as the horizon plane for an observer on the spherical Earth. Two views are shown: one from outside the Celestial Sphere, and the other from the point of view of an observer on Earth looking up. Most of the characteristics of the simulation can be adjusted, including the tilt of the ecliptic relative to the equator, the latitude of the observer, the time of day and the time of year.

The Celestial Sphere and Analemma JavaScript Model was developed using the Easy Java/JavaScript Simulations (EJS) modeling tool. Although EJS is a Java program, EJS v5 and above can be used to create stand-alone JavaScript programs that run in almost any browser.