Sundial

As Earth rotates, the Sun appears to move across the sky, changing the direction and length of shadows. A sundial turns that motion into a time indication. In a common fixed dial, a projecting pointer called a gnomon casts a shadow on a plate marked with hour lines. Other designs use a shaft of sunlight passing through an aperture. Many traditional hour lines are oriented so that the gnomon is parallel to Earth’s rotational axis; the gnomon’s angle above the horizon therefore depends on the observer’s latitude. The design must also account for how the dial is aligned relative to the cardinal directions. A dial made for one latitude or orientation may give incorrect readings if moved without adjustment.

Sundials range from large horizontal or vertical installations to portable instruments. A pocket dial may include an adjustable gnomon and a compass to help orient it; some historical examples carry scales or calendar features in addition to hour lines. Royal Museums Greenwich notes that the earliest known sundial, found in Egypt, dates to at least 1500 BCE. Early timekeeping also used unequal, or temporary, hours that varied in length with the season. The same museum describes the later adoption of equal-length hours, the flourishing of varied designs during the Renaissance, and the gradual replacement of sundials by clocks. Sundials remained useful for practical timekeeping and for setting clocks in some contexts into the nineteenth century.

A sundial measures apparent solar time: time based on the actual Sun’s position at the observer’s location. This is not identical to mean solar time, which represents the average motion of a hypothetical mean Sun and provides a more uniform basis for clocks. The difference, called the equation of time, changes throughout the year because Earth’s orbit is not perfectly circular and its rotational axis is tilted relative to its orbital plane. The U.S. Naval Observatory defines the equation of time as apparent solar time minus mean solar time; its magnitude can approach roughly 16 minutes. Longitude adds another correction because local solar noon occurs at different civil-clock times at different locations within a time zone. Daylight-saving rules and the time-zone meridian also affect conversion to a modern clock reading.

To use a fixed sundial, place it level and orient it according to its design, then read the hour line indicated by the gnomon’s shadow. The dial works only when sunlight is available and its accuracy depends on construction, alignment and reading precision. Its reading is local solar time, not automatically the time shown by a wristwatch. For a clock-time comparison, apply the date-specific equation-of-time correction and the longitude and civil-time adjustments appropriate to the location. Portable sundials may include a compass, a plumb line or a latitude adjustment to make setup easier, but they still require careful positioning.