The sundial works by a simple principle: a vertical rod (the gnomon) casts a shadow whose direction tells the hour. But shadows hold more secrets than just time...
Horizontal Dial: Lies flat on the ground. The most common garden variety. Easy to read but the hour lines are unevenly spaced.
Vertical Dial: Mounted on a south-facing wall. Common on churches and public buildings. The gnomon juts out from the wall.
Equatorial Dial: The dial plate tilts to match Earth's equator. Hour lines are evenly spaced. Elegant but only works half the year (sun's on the wrong side for the other half).
Armillary Sphere: A three-dimensional globe of rings. Beautiful and complex, often seen in formal gardens.
Portable Dial: Pocket or ring sundials carried by travelers. Some included compasses for orientation.
At solar noon, the shadow points due north (in the northern hemisphere). As the sun moves 15° per hour across the sky, the shadow sweeps across the dial face.
For a horizontal sundial to work correctly, the gnomon must be tilted to match your latitude. A gnomon at 45°N points 45° above horizontal, aiming at the celestial pole (near Polaris).
This alignment means the shadow rotates uniformly: 15° per hour, matching Earth's rotation.