This article describes and historically surveys the method of equal altitudes, also known as the Indian Circle, used for determining the meridian and cardinal directions by observing a gnomon's shadow. It covers the practical steps, potential errors, mathematical analysis of shadow curves (conic sections), and its widespread use in ancient and medieval Eastern (India, China) and Western (Roman, early medieval Europe) cultures for architecture, town planning, and sacred rituals.
Historical Dials,
How Sundials Work,
Mathematics of Dialling,
Sundial Design & Layout