How does a sundial work? It tells the time with the sun: a fixed rod or blade, called a gnomon, casts a shadow onto a dial, and the shadow points at the hour. Try the interactive sundial below.
How does a sundial work? The three parts
- Gnomon: the rod or blade that casts the shadow.
- Dial plate: the surface with the hour lines.
- Sun: the moving light source. The earth turns about 15 degrees every hour, so the shadow moves about 15 degrees too.
Why the gnomon is tilted
On a correct sundial the gnomon points at the celestial pole, so its angle equals your latitude. Move the latitude slider above and watch the hour lines shift. A sundial made for another latitude will tell the wrong time, so check that a garden sundial matches where you live.
Sundial time vs clock time
A sundial shows local solar time, which can differ from your watch by several minutes because of the equation of time, your position inside your time zone and daylight saving time. That is why even a perfect sundial is a beautiful decoration rather than a precise instrument. For exact time, a quartz watch still wins. In short, this is the answer to “how does a sundial work?”
Types of sundials you can buy
- Garden sundial: a horizontal brass or bronze dial on a pedestal.
- Wall sundial: a vertical dial fixed to a south-facing wall.
- Armillary sphere: a decorative globe of rings with a shadow-casting arrow.
- Pocket or ring sundial: a small portable dial that works like a compass.
Browse some garden sundials below and check the stated latitude, size and material before you buy.
Buying tips
Choose solid brass or bronze for outdoor use, because it weathers well and the hour lines stay readable. Check that the gnomon angle matches your latitude, or pick an adjustable model. Place the dial on a level base in full sun, with the noon line pointing true north. If you prefer a watch on your wrist, see our watch type quiz or how a solar watch works. Background: Wikipedia on sundials.
How does a sundial work? FAQ
Does a sundial work when it is cloudy?
No. Without direct sun there is no sharp shadow, so the dial cannot show the time.
Does a sundial work at night?
No, although some special moon dials exist.
Why are the hour lines unevenly spaced?
Because the shadow moves faster near sunrise and sunset than at midday on a flat dial.
