Equatorial Ring

An equatorial ring is an ancient astronomical instrument used during the Hellenistic period to determine the precise moment of the spring and autumn equinoxes. Mounted vertically in the east-west plane and typically forty five to ninety centimeters across, the ring worked by catching the sun's shadow; at the equinox the sun rises due east and sets due west, and the shadow it casts falls exactly along the inside of the ring, an effect that could pin down the equinox to within about an hour. Such rings were installed outside temples in major cities such as Alexandria and Rhodes to help manage the calendar, though Ptolemy recorded cases where a poorly aligned ring produced a false reading.