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.

Comments:

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.