BC Gnomonics

The demonstrational armillary sphere is a three-dimensional model of the celestial sphere. As its name would suggest, it is a sphere made up of a number of bands and circles which represent key elements of celestial motion.

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The function of the armillary sphere is to model the position of the sun (and occasionally stars and planets) in the local sky. When the sphere is set for its latitude of use and oriented north, its features align with those of the true celestial sphere.

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While this type of armillary sphere cannot be used for observation or time-finding, it is very useful as an educational and predictive model. Its main function is to track the apparent movement of the sun across the sky for any given day of the year.

How to use

1. Using the latitude scale on the meridian circle, elevate the polar axis to match the latitude of use.

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2. Orient the sphere true north. This can be done using the embedded compass or external method. If magnetic declination is not known, it can be found here (the given value is the difference between magnetic and true north. If magnetic declination is 15ºW, rotate 15ºE of magnetic north to point true north).

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3. Identify the sun's current position in the ecliptic circle. Take note or otherwise mark this location.

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4. Rotate the free portion of the armillary sphere east-to-west to simulate the movement of the sun.

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Overview of Instrument