Brief description

A sundial is a device that tells the time of day when there is sunlight by the apparent position of the Sun in the sky.

Use / Function

• Timekeeping: Indicating the time of day.
• Navigation: Determining latitude and direction (North/South).
• Decoration: Garden ornament.

Operating principle

As the sun moves across the sky, it casts a shadow from a gnomon (a thin rod or sharp edge) onto a flat plate (the dial). The position of the shadow indicates the time.

• Gnomon Alignment: Must be parallel to the Earth’s axis of rotation (pointing to the celestial pole).

How to create it

1. Base: Create a flat surface (horizontal, vertical, or inclined).
2. Gnomon: Install a rod at an angle equal to the local latitude.
3. Alignment: Orient the gnomon to point True North (in Northern Hemisphere) or True South (in Southern Hemisphere).
4. Marking: Mark the shadow position at each hour using a reliable clock or by geometry.

• Technical level: Intermediate (requires astronomical knowledge).

Materials needed

• Essential: Stone or metal for durability outdoors.
• Tools: Compass, level, protractor.

Variants and improvements

• Horizontal dial: The most common garden type.
• Vertical dial: Mounted on walls.
• Equatorial dial: The dial plate is parallel to the equator.
• Analemmatic dial: The gnomon is vertical and moved depending on the date.

Limits and risks

• Night/Cloud: Only works when the sun is shining.
• Equation of Time: Solar time varies from clock time throughout the year (up to ~16 mins).
• Latitude: A fixed dial only works accurately at the latitude it was designed for.

Related Inventions

• Timepieces
• Water Clock
• Compass