A sundial is a device that measures time by the position of the Sun. In common designs such as the horizontal sundial, the sun casts a shadow from its style onto a surface marked with linesindicating the hours of the day. The style is the time-telling edge of the gnomon, often a thin rod or a sharp, straight edge. As the sun moves across the sky, the shadow-edge aligns with differenthour-lines. All sundials must be aligned with the axis of the Earth's rotation to tell the correct time. In most designs, the style must point towards true celestial north (not the north magnetic poleor south magnetic pole). That is, the style's horizontal angle must equal the sundial's geographical latitude.
It is common for inexpensive decorative sundials to have incorrect hour angles, and thesecannot be adjusted to tell correct time.[
By: Diego Correa 203
Diego Correa N.L.8 203
Carbia Satteducato Leonor
There are different types of sundials: Some sundials use aline of light to indicate the time. Others use the edge of a shadow. The spot of light may be formed by allowing the sun's rays through a small hole or reflecting them from a small circular mirror. Aline of light may be formed by allowing the rays through a thin slit or focusing them through a cylindrical lens.
When the sundial reads by shadows, the shadow-casting object — the sundial's gnomon —may be a thin rod, or any object with a sharp tip or a straight edge. Sundials employ many types of gnomon. The gnomon may be fixed or moved according to the season. It may be oriented vertically,horizontally, aligned with the Earth's axis, or oriented in an altogether different direction determined by mathematics.
Sundials also may use many types of surfaces to receive the light or shadow.Planes are the most common surface, but partial spheres, cylinders, cones and other shapes have been used for greater accuracy or beauty.
Sundials differ in their portability and their need for...