Triangular figure lying on the plane of the solstitial colure. Its vertex coincides with the center of the celestial sphere; the angles at its base coincide with the intersection between the colure and the two tropics. The base angles indicate the solstice points; the center of the base indicates the equinoctial points; the sides coincide with the inclination of the ecliptic (23°27'). On the base are projected the twelve divisions of the zodiac signs read from the ecliptic. Lines originating from these points converge at the vertex of the triangle. Parallel to the base are several lines whose reciprocal distances correspond to the tangents of the angles between 0° and 60°. Regiomontanus applied the trigon to the construction of a special sundial that bears his name.

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