Equatorial ring
An equatorial ring was an astronomical instrument used in the
equinoxes. Equatorial rings were placed before the temples in Alexandria, in Rhodes, and perhaps in other places, for calendar
purposes.
The easiest way to understand the use of an equatorial ring is to imagine a ring placed vertically in the east-west plane at the Earth's
horizontal
.
The equatorial ring was about one to two
cloudy
, an interpolation could be made between two days' measurements.
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. False readings can also be produced by atmospheric refraction of the Sun when it is close to the horizon.
Equatorial rings can also be found on
sundials
.
References
- Anton Pannekoek, (1989), A History of Astronomy, page 124. Courier Dover Publications
- James Evans, (1998), The History and Practice of Ancient Astronomy, pages 206-7. Oxford University Press.