A sundial uses the position of a shadow to indicate the passage of time. The shadow-casting part is called the gnomon. As Earth rotates, the Sun’s apparent position changes and the shadow points in different directions across the dial.

What time does a sundial show?

A traditional sundial indicates local apparent solar time, which need not match the civil time on your watch. Longitude, the equation of time over the year and daylight saving time can all contribute to the difference.

Why does alignment matter?

On a conventional horizontal sundial, the gnomon’s edge is parallel to Earth’s rotation axis. In the northern hemisphere it points towards true north, with its angle above the horizontal corresponding to the local latitude. Other types require different arrangements, so follow the instructions for the individual instrument.

A compass helps with orientation, but magnetic north differs from true north, and nearby metal or magnets can affect it. The dial’s design, levelling and location also influence the reading.

Trying a sundial

• Choose a sunny spot with a clear shadow.
• Set up the instrument according to its own instructions.
• Read the shadow position and compare it with your watch.
• Do not immediately treat a difference as a fault: corrections between solar and civil time must also be considered.

Marking a stick’s shadow each hour is an interesting experiment, but it does not create an accurate year-round sundial. The dial and gnomon need an appropriate design.

A sundial as a gift

The Urania Propitia sundial compass was listed in stock when this article was edited. Check its current description for intended use and availability. Explore our nautical home decor ideas or contact us with a question.

Források / Sources / Quellen: Royal Museums Greenwich: Sundials; U.S. Naval Observatory: Equation of Time; EAAE: How does a sundial work?.