Equatorial Ring

Model of Ptolemy's ring used to measure the length of the year.

What it shows:

This is a model of the ring and method used by Claudius Ptolemy (2nd century A.D., Alexandrian astronomer) to determine the length of a year.


How it works:

The ring is mounted horizontally and tilted to be in the plane of the equator, i.e. the angle the sun makes with the horizon during the equinox (47.5° for Cambridge, MA). A vertical bank of five 200 watt light bulbs, mounted on the side wall of the lecture hall, represent various positions of the sun - the middle bulb corresponding to the sun at the equinox. The lights are individually controlled by a rotary switch positioned near the ring. All the lecture hall lights should be turned off for maximum contrast.


Ptolemy's bronze ring was set up in the middle of Gymnasium Square in Alexandria. He was able to catch the time of the equinox within a few hours which greatly reduced the uncertainty in reckoning the length of the year, especially if this measurement is repeated and averaged over many years.