päikesekell

A päikesekell, or sundial, is an ancient astronomical instrument used to tell time by the apparent position of the Sun in the sky. It works by casting a shadow from a gnomon onto a marked surface. As the Sun moves across the sky, the shadow moves, indicating the passage of time. The most common type of sundial is the horizontal sundial, where the dial plate is parallel to the horizon and the gnomon is angled to point towards the celestial pole. Vertical sundials, often found on the walls of buildings, have a vertical dial plate and a gnomon that may be perpendicular to the wall or angled.

The markings on a sundial are typically hour lines, which are calibrated to correspond to the local apparent solar time. Due to variations in the Earth's orbit and axial tilt, sundials do not show mean solar time directly, which is the time kept by most modern clocks. The difference between apparent solar time and mean solar time is known as the equation of time, and it can be accounted for using specific charts or by adjusting the sundial's design. Sundials were a primary means of timekeeping for centuries before the invention of mechanical clocks and continue to be used today as decorative garden ornaments and educational tools. Their design and placement require careful consideration of latitude and longitude to ensure accuracy.