An Inexpensive Armillary Sphere for Introductory Astronomy

Three-dimensional thinking is one of the most difficult skills introductory astronomy students struggle with. For instance, how does one’s latitude on Earth and the time of year affect the path of the Sun and stars across the sky? Why is the Sun high overhead at noon in June (in the northern midlatitudes) but not so in December? How does the time the Moon rises depend on its phase? Typically, astronomy instructors use diagrams, animations, and a lot of hand gestures to try to convey these concepts. Here I present a simple 3D model of the celestial sphere (see Fig. 1) that can be used in conjunction with 2D representations. This is not a new invention—the traditional name for it is “armillary sphere”—but this version is easy to build, easy to use, inexpensive, and large enough to gather a group around. The key to these desirable features is the use...