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a device used for the visual demonstration of the earth’s annual motion about the sun and its diurnal rotation about its axis. In the tellurian a smaller sphere, representing the earth, moves about a larger sphere or some light source, for example, a lightbulb equipped with a reflector, representing the sun. In addition, the earth sphere rotates about an axis passing through its center and maintaining a constant inclination, like the earth’s axis. The tellurian may sometimes include a still smaller sphere, representing the moon in its revolution about the earth. The simplest tellurians are equipped with sliding parallelograms, which can be moved by hand, so that the relative positions of the axes may be held constant.