Armillary Dial

armillary dial based in Swedish design
Figure 24: Armillary sundial

The above figure shows an armillary sundial based on a traditional Swedish design. This armillary sundial is made of iron with some brass, and it is about 22 inches tall.

General Appearance

The armillary dial consists of an assembly of rings representing the principal circles of the celestial sphere. In Figure 24, only the meridian, celestial equator and equinoctial colure , which is the circle passing through the NCP and the equinoxes, are found on the dial. More complicated armillary dials have several more rings to model the tropics, arctic circles or horizon. This multitude of rings may block the shadow of the gnomon on the time scale, or at least cause confusion when reading the dial. Its name originated from the Latin word, armilla, which means a ring.

The armillary dial is an extension of the equatorial dial, with two rings representing the celestial equator and the meridian. A third ring is usually added to represent the horizon. The hour lines are marked on the inside of the equatorial ring. In Figure 25, the circle ABCDEFG is the meridian, DHGI the celestial equator, CHFI the horizon. The gnomon is the thin long rod which runs through holes pierced in the meridian ring.

general appearance of armillary sundial
Figure 25

More Pictures of Armillary Dials

complicated armillary sundial 1
Figure 26

complicated armillary sundial 2
Figure 27

complicated armillary sundial 3
Figure 28

How it works

The shadow of the gnomon, cast among the hour lines on the equatorial ring, shows the time. At noon, the shadow of the meridian ring will fall across the gnomon and also across the 12-o'clock line. At the time of the equinoxes, the Sun will be on the celestial equator, and hence in the plane of the equatorial ring of the armillary dial. The shadow of the upper part of the ring (D in Figure 25) will fall on the lower part of the ring, and the hour lines will be shaded.

Measuring the Sun's declination

At the times of the equinoxes, the shadow of the equatorial band falls at the center of the gnomon, half way between E and B in Figure 25. In the summer, when the Sun is high, the shadow falls further down the gnomon closer to B, while in winter it falls closer to E. We can calibrate the gnomon rod to show the Sun's declination by the position of the shadow on the gnomon.