Horizon circle and window: light lines crossing at its middle
East and west horizon lines drawn through the window's angles intersect at its center
The text describes how the eastern horizon, drawn through the two window angles o f, reaches point d, while the western horizon, drawn through the other two angles r s, ends at c, so that the two lines intersect exactly in the middle of the window's space and thickness. It offers a practical proof: place two rods at g h and you will see the line from the middle of the real shadow straighten to the center m and to the horizon n f. A large diagram shows a semicircular arc lettered CERCHIO DE L'ORIZONTE TRAMONTANO, marked PONENTE (west) and LEVANTE (east), above a window opening with hatched shadow regions and crossing lines that meet at the central point M.
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East and west horizon lines cross at the window's midpoint
The horizon of the east, drawn so as to touch the two window angles o f, arrives at d; the horizon of the west, touching the other two angles r s, ends at c; and this intersection falls exactly in the middle of the window's space and thickness.
A demonstration with two rods
You will confirm this reasoning better by setting two rods at the place g h: there you will see the line drawn from the middle of the real shadow straighten toward the center m and toward the horizon n f.
Horizon circle, window opening, and crossing shadow lines
A large diagram fills the leaf: a semicircular arc lettered CERCHIO DE L'ORIZONTE TRAMONTANO, with PONENTE (west) and LEVANTE (east) at its rim and points F and N marked, sits above a rectangular window opening. Within it, crossing straight lines meet at a central point M, flanked by hatched shadow areas descending to D, H, G, C at the base.