1.1 Eratosthenes Measures the Earth (6 pages)

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The two hemispherical bowls of the Jayaprakash Yantra:
the foreground shows about one fourth of one bowl;
the second bowl can be seen in the background.

Each bowl contains part of the hemispherical measuring surface in such a way that where
one bowl has part of the measuring surface, the other bowl has a space where an observer
can walk and vice versa. In this way all portions of the hemisperical surface are easily
accessible to the observer in a way that would not be possible using a single hemispherical
bowl.

A youngster examines the graduated markings in one bowl of the Jayaprakash Yantra.

In the Jayaprakash Yantra a ring suppended by wires at the center of the sphere served the
same purpose as the tip of the vertical pole or "gnomon" in Eratosthenes' device. The

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