How Does a Sundial Work?
A sundial tells time from the shadow of the sun. The slanted edge that casts the shadow, called the gnomon or style, is tilted to point at the celestial pole, so it is parallel to Earth's axis. As Earth turns 15 degrees every hour, the sun appears to swing around that axis, and the gnomon's shadow sweeps across the dial. The hour lines are spaced so the shadow crosses one line per hour. On a flat horizontal dial like this one they are not evenly spaced: they bunch up around noon and spread out toward morning and evening, by an amount that depends on your latitude.
How to Read a Sundial
Read the hour where the shadow of the gnomon's slanted edge falls, just like the hour hand of a clock. The 3D dial above is laid out for your latitude, with the gnomon pointing to true north (or true south in the Southern Hemisphere) and tilted at your latitude angle. Watch the shadow line up with each Roman numeral as you press Play the day, and try changing the date to see how the shadow gets shorter in summer and longer in winter while still telling the same hour.
Why Sundial Time Differs From Clock Time
A sundial shows apparent solar time, where noon is the moment the sun is highest in the sky. Your clock shows standard time for your whole time zone. Three things make them disagree:
- The equation of time. Earth's orbit is an ellipse and its axis is tilted, so the sun runs up to about 16 minutes fast (early November) or 14 minutes slow (mid February) compared with a perfectly even clock. It matches exactly only four times a year.
- Your longitude. A time zone shares one clock time across about 15 degrees of longitude. For every degree you are east of your zone's central meridian, the sun reaches its highest point 4 minutes earlier, and 4 minutes later for every degree west.
- Daylight saving time. In summer, clocks are moved an hour ahead, so a sundial runs about an hour behind.
The box under the dial adds these up for your location, and shows today's solar noon, the clock time when the sun is highest and the shadow points straight along the noon line.
How to Set Up a Real Sundial
- Choose a dial made for a latitude close to yours, or adjust the gnomon so its angle equals your latitude.
- Place the dial on a flat, level surface in full sun.
- Point the gnomon to true north, not magnetic north. A compass can be off by several degrees, so a better method is to turn the dial at your local solar noon until the shadow sits exactly on the XII line.
- Add the corrections above to convert sundial time to clock time, or simply enjoy telling time the way people did for thousands of years.
A Short History of Sundials
Shadow clocks were used in ancient Egypt more than 3,000 years ago, and the Greeks and Romans built elaborate dials for public squares. The idea of tilting the gnomon parallel to Earth's axis, which makes every hour the same length through the year, spread in the Islamic world and Europe in the Middle Ages. Sundials were used to set mechanical clocks until the 1800s, when railways created the need for standard time zones. Explore more about the sun with sunrise times, golden hour and the sun and sky tools.
