A stick in sand is already a crude sundial
Sundials tell daytime from the Sun's apparent path when light is direct. In the narrow sense a gnomon casts a shadow on marked hours; broadly any device using solar altitude or azimuth counts. Stick-in-sand demos show the principle instantly.
A sundial reads the time of day from where the Sun appears in the sky, working only when direct sunlight falls on it. The classic form is a flat dial plate and a gnomon whose shadow sweeps across hour lines as the Sun moves. Usually only one edge of the gnomon, called the style, marks the hour, and it is normally set parallel to Earth's axis; some dials add a point, the nodus, whose shadow also shows the date.
Designs vary widely. Instead of a shadow, some dials use a line of light through a thin slit or a spot of light through a small hole or from a tiny mirror, and the receiving surface may be a plane, part of a sphere, a cylinder, a cone, or a helix. Many fixed dials must be installed knowing the local latitude, true vertical, and the direction of true north, while portable ones can align themselves. Cheap garden dials often have gnomons and hour lines set wrongly and cannot be corrected.
When the gnomon points at the celestial pole, its shadow turns at a steady rate all year, so one set of hour lines serves every season, which is why this is the commonest arrangement. If it points elsewhere, or if the dial follows the tip of a shadow or a spot of light, the hour lines must be spaced unevenly, and since that light traces a cone that changes with the Sun's declination, such dials often need separate lines for different times of year.
A dial shows local solar time, so three corrections give clock time. The equation of time allows for Earth's tilted axis and non-circular orbit and can reach 16 minutes 33 seconds; longitude matters too, since a dial west of its zone's meridian lags, showing perhaps 11:45 at official noon; and daylight saving usually adds an hour. The oldest known dials are Egyptian and Babylonian shadow clocks from about 1500 BC, and by 240 BC Eratosthenes had used an obelisk and a well to estimate Earth's circumference.
Source: Sundial
