A sundial is a device that tells the time of day 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 lines indicating 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 different hour-lines. All sundials must be aligned with their styles parallel to the axis of the Earth's rotation to tell the correct time throughout the year. The style's angle from the horizontal will thus equal the sundial's geographical latitude. It is common for inexpensive mass-produced decorative sundials to have incorrect hour angles, which cannot be adjusted to tell correct time.
There are different types of sundials. Some
sundials use a shadow or the edge of a shadow while others use a line or spot of light to indicate the time.
The shadow-casting object, known as a gnomon, may be a thin rod, or other 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.[citation needed]
With
sundials using light to indicate time, a line of light may be formed by allowing the sun's rays through a thin slit or focusing them through a cylindrical lens. A spot of light may be formed by allowing the sun's rays to pass through a small hole or by reflecting them from a small circular mirror.
[citation needed]
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.[citation needed]
Sundials differ in their portability and their need for orientation. The installation of many dials requires knowing the local latitude, the precise vertical direction (e.g., by a level or plumb-bob), and the direction to true North. Portable dials are self-aligning; for example, it may have two dials that operate on different principles, such as a horizontal and analemmatic dial, mounted together on one plate. In these designs, their times agree only when the plate is aligned properly.
In medieval weaponry, the breastplate is the front portion of plate armour covering the torso. It has been a military mainstay since ancient times and was usually made of leather, bronze or iron in antiquity. By around 1000 AD, solid plates had fallen out of use in Europe and knights of the period were wearing chain mail in the form of a hauberk over a padded tunic.[1] Plates protecting the torso reappeared during the 13th century in the form of the cuirass or alternatively as plates directly attached to a knightly garment known as the surcoat.[1] Around 1300 this developed into the Coat of Plates which continued to be in use for about a century.[1] True breastplates reappear in Europe in 1340 first composed of wrought iron and later of steel.They were between 1 mm and 2.5 mm strong.[2] In order to prevent the wearer from bring cut by their own armour, the design featured outward turned edges that also increased stiffness.[2] In some cases, further strength was added by a ridge running down through the centre of the plate.[2] These early breastplates only covered the upper torso with the lower torso not being protected by plate until the development of the fauld around 1400.[2] Around 1450, the breastplate had expanded to cover the entire torso and could consist of one or two plates.Bullet-proof vests are the modern descendant of the breastplate.
