An armillary sphere is a scientific model which demonstrates the workings of the celestial sphere, a concept which was historically used by astronomers and other scientists to understand the heavens. Viewing the Earth as the center of the universe, the celestial sphere consists of the objects visible from Earth. Using an armillary sphere, someone can calculate the position of something in the sky relative to an observer on Earth. This model can also be used to visually show people how the celestial sphere works.
The armillary sphere is primarily viewed as a scientific curiosity today, although these devices can be used for some basic astronomical calculations. Numerous models can be seen on display in scientific museums which collect old instruments, and some people collect vintage spheres because they find them visually interesting. Like many antique scientific instruments, armillary spheres are often works of art in addition to working models, and they can vary widely in size from desktop versions to devices which fill large rooms.
The construction of the sphere consists of a series of nestled rings which rotate around a single axis. In the middle of the rings, a globe or ball representing the Earth is positioned. Some of the rings are fixed, indicating the position of features like the equator and the tropics, while others can be rotated. The rings are covered in a series of markings which people can use to align the rings when attempting to arrive at a measurement.
Relative to Earth, the objects in the celestial sphere appear to move. In fact, objects in the universe are all moving, but because they are moving together, these movements are usually not readily apparent to observers. The apparent movement of objects in space is due to the Earth's rotation and orbit around the sun, but early astronomers were not aware of this. The armillary sphere helped them understand and explain the movements of the heavens, and this device was regularly retooled to keep up with new discoveries and scientific information.
China seems to deserve the credit for invention of the armillary sphere, as numerous depictions and discussions of the instrument can be seen in Ancient Chinese art and writings. The Greeks also appear to have developed a version independently, and they are sometimes mistakenly referred to as the original inventors of the device. In any case, armillary spheres were used and manufactured well through the 19th century, and several modern companies make reproductions for people who collect scientific curiosities.