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Geoff Hitchcox,

You wrote: “When making a Horizontal Sundial, the Gnomon needs to be at an angle to the flat surface equal to the latitude of the installation.”

You’ll also have a true, but more general, statement by changing a few words in that sentence to read:

When making any sundial, the gnomon needs to be elevated above horizontal at an angle equal to the latitude of the installation.

You wrote: However the 'reference surface' on an Armillary Sundial is the equatorial armilla (Latin for ring) which is aligned parallel to the Earth's equator and thus the axis of rotation, and therefore local latitude. The gnomon must therefore be at an angle of the co-latitude, as shown in the diagram attached from Chapter 17 in the book "Sundials" by Albert Waugh (1973).”

You misinterpreted Waugh’s words and image regarding the gnomon angle in Figure 17.3.  I’ve attached a screen shot of something I did recently to clarify a few things regarding angles for myself. At left is a diagram of a basic armillary for a latitude of 41 degrees. I’ve added letters A, B, and G to it, as used in Waugh’s figure. Angle A-G is the latitude and A-B is the colatitude, as stated in the book. The image and words at the right side of the attachment may help clarify angles in an equatorial sundial with a time ring / scale as used in the Mystic sphere.

Chuck V.

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