Sundial
A device that tells time using the Sun's shadow or light.
A sundial is a device that uses the Sun’s position in the sky to show the time of day when sunlight is direct. In its simplest form, it has a flat plate, called the dial, and a gnomon that casts a shadow onto it. As the Sun moves, the shadow shifts across hour-lines marked on the dial. The style—the time-telling edge of the gnomon—casts a shadow that indicates the hour, while the gnomon itself may be a rod, wire, or decorative metal piece. For year-round accuracy, the style must be parallel to Earth’s rotational axis, and its angle from horizontal equals the local latitude.
The term “sundial” can also apply to any device using the Sun’s altitude or azimuth to tell time. Sundials are often valued as decorative items, metaphors, or subjects of mathematical study. Even a simple stick in sand or a nail in a board can track time by marking shadow edges at intervals. However, many cheap, mass-produced sundials have incorrectly aligned gnomons, shadow lengths, or hour-lines, making them impossible to adjust for accurate time.
Different types of sundials exist. Some use a shadow or its edge; others use a line or spot of light. The gnomon may be a thin rod or an object with a sharp tip or straight edge, and it can be fixed or adjusted seasonally. It may be vertical, horizontal, aligned with Earth’s axis, or oriented differently based on mathematics. A line of light can come from a slit or cylindrical lens, while a spot of light can come from a small hole, window, oculus, or mirror—ranging from a pinhole to the Pantheon’s oculus.
Sundials use various surfaces to receive light or shadow. Planes are most common, but partial spheres, cylinders, cones, and other shapes are used for accuracy or beauty. Sundials differ in portability and orientation needs. Installing many requires knowing local latitude, the vertical direction (via a level or plumb-bob), and true north. Portable dials are self-aligning; for example, two dials on one plate—like a horizontal and analemmatic type—agree only when the plate is correctly oriented.
Sundials show only local solar time. To get national clock time, three corrections are needed. First, Earth’s elliptical orbit and tilted axis cause solar time to vary from clock time by up to 16 minutes, 33 seconds throughout the year—this is the equation of time. A sophisticated sundial can incorporate this with a curved style or hour lines; simpl
- type
- Timekeeping device
- earliest_known
- Shadow clocks from ancient Egyptian and Babylonian astronomy (1500 BC or BCE)
- key_component
- Gnomon (shadow-casting object) and dial face
- accuracy_requirement
- Style must be parallel to Earth's axis; angle from horizontal equals local latitude
- corrections_needed
- Equation of time, longitude offset, daylight saving time adjustment
Lore & Background
The earliest sundials known from the archaeological record are shadow clocks from ancient Egyptian astronomy and Babylonian astronomy, dating to around 1500 BC or BCE. By 240 BC, Eratosthenes had estimated the circumference of the world using an obelisk and a water well, and a few centuries later, Ptolemy charted the latitude of cities using the angle of the Sun. The people of Kush created sundials through geometry. The Roman writer Vitruvius listed dials and shadow clocks known at that time in his De architectura. The Tower of the Winds in Athens included both a sundial and a water clock for telling time.
Reader's Guide
Sundials function by casting a shadow or throwing light onto a surface known as a dial face. The shadow-casting object, the gnomon, may be a rod, wire, or elaborately decorated metal casting. For accuracy throughout the year, the style (the time-telling edge of the gnomon) must be parallel to the Earth's axis of rotation, and its angle from horizontal equals the sundial's geographical latitude. Sundials indicate local solar time only; to obtain national clock time, three corrections are required: the equation of time (up to 16 minutes, 33 seconds), longitude relative to the official time zone, and daylight saving time if applicable. Inexpensive mass-produced decorative sundials often have incorrectly aligned gnomons, shadow lengths, and hour-lines that cannot be adjusted to tell correct time. Sundials have been in common use since the 16th century, and treatises on their manufacture were published by Giovanni Padovani in 1570 and Giuseppe Biancani around 1620.
Did You Know?
- The style of a sundial must be parallel to the Earth's axis of rotation for the sundial to be accurate throughout the year.
- The equation of time correction can be as great as 16 minutes, 33 seconds.
- A canonical sundial indicated the canonical hours of liturgical acts and was used from the 7th to the 14th centuries by religious orders.
- The Tower of the Winds in Athens included both a sundial and a water clock.
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