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What Are Armillary Sundials?

As the sun journeys across the sky each day, shadows mark its movements on the earth. Sundials date back to prehistoric time and now dot gardens and walls around the world, enabling observers to track local solar time. Many different styles of sundials exist, including the armillary dial, a type of the equatorial dial. The word "armillary" derives from the Latin "armilla," meaning bracelet or ring.

History
- In 225 B.C., Eratosthenes first invented the armillary sphere, of which the armillary sundial takes after. First created with the earth in the center, the Greeks developed and used the armillary sphere as a teaching tool, while larger and more precise versions worked well as observational instruments, as used by Ptolemy. With Copernicus' heliocentric model of the universe, the use of armillary spheres denoting both Ptolemaic and Copernican models became commonplace teaching tools.
Armillary Sphere
- Like the modern day globe, which shows the countries and the oceans of the world, the armillary sphere maps out the heavens. Also called a celestial globe, armillary spheres were used by ancient astronomers to track constellations and their brightest stars. Rings wind around the hollowed out body of the sphere to create its look. While armillary spheres make beautiful sundials, the rings at times actually hinder the process of timekeeping.
Armillary Rings
- Rings symbolizing the principle circles of the sky and corresponding with the Earth's meridian and parallel lines band around the armillary sphere. Some spheres contain dozens of these bands, but most only have three or four. In the middle of the rod passing through the center of the sphere's skeleton sits the Earth or sun. The wide horizontal band in line with the equator denotes the zodiac, which measures positions of the sun, moon and most planets. The line that runs through this band represents the sun's path through the sky, also called the ecliptic.
Equatorial Dial
- The armillary dial exists as an extension of the equatorial dial, set apart by the bands that surround the sphere. The dial plate of an equatorial sundial sits parallel to the equator. The gnomon or rod, which casts the shadow for the sun, lies parallel to the Earth's axis and sets perpendicular to the center of the dial plate. The gnomon also points towards the direction of the celestial North Pole. Located on both the bottom and the top of the dial plate, to tell time during the summer and the winter, the hour markers sit 15 degrees apart. Thus, an armillary dial sets up the same way.

Astronomy