Tellurians are models of the sun, moon and earth, or just the moon and earth. They are not very common; this example at Kenyon College is the only one I have ever seen. 

   However, the 1860 E. S. Ritchie catalogue suggests that this is really a gyroscope. The usual spinning wheel, mounted in gymbals, occupies the place occupied by the ivory ball. As the counterbalanced system is rotated around the axis passing through the base, the wheel, representing the earth, always spins in the same orientation. 

   The present model, when used with an external light source to simulate the sun, can be used to illustrate lunar eclipses. 

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