Equatorial ring

An equatorial ring was an astronomical instrument used in the Hellenistic world to determine the exact moment of the spring and autumn equinoxes.

On other days of the year, the Sun passes to the north or south of the ring, and will illuminate the bottom half.

For latitudes away from the equator, the ring merely needs to be placed at the correct angle in the equatorial plane.

The main disadvantage with the equatorial ring is that it needed to be aligned very precisely or false measurements could occur.

Ptolemy mentions in the Almagest that one of the equatorial rings in use in Alexandria had shifted slightly, which meant that the instrument showed the equinox occurring twice on the same day.