Sundial

:This article pertains to the astronomical instrument. For the psychedelic rock band, see Sun Dial.

Design & principles of operation

Terminology

The 'shadow-maker' of the sundial is called a gnomon.

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The sun casts a shadow from the gnomon to a surface called the dial face or dial plate (often shortened to face).

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Most sundials indicate time on the dial face by

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the shadow of a line in space

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called the stylehttp://www.sundialsoc.org.uk/glossary/alpha.htm#S.

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On a standard garden sundial, this line is the top edge of the gnomon.

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The style should be parallel the Earth's axis of rotation.

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In common speech, sometimes style refers to the entire gnomon.

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Some sundials indicate both the time and the date by the shadow of a particular point on the gnomon.

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That point is called the nodus.

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The nodus may be the tip of a gnomon with an arbitrary (usually horizontal or vertical) orientationhttp://www.sundialsoc.org.uk/glossary/alpha.htm#N.

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A few sundials have both a style and a nodus, with the nodus in the form of a small sphere or a notch on a polar-pointing gnomon, or simply the tip of the gnomon.

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In general, the best material for a face is a very light color to give a high contrast with the shadow. The numerals should be dark, visible on the unshaded portion of the face. The gnomon should be sturdy, preferably metal, because gnomons are usually thin, and can break easily. The traditional luxury materials are a white marble face, with markings inlaid in black marble. Traditional styles are thick bronze to prevent corrosion.

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It is traditional for a sundial to have a motto.

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Equatorial or Equinoctialhttp://www.sundials.co.uk/tbequ.htm sundial

The simplest sundial is a disk mounted on a bar. The bar must be parallel to the Earth's axis of rotation. The disk forms a plane parallel to the plane of the Earth's equator. The disk is marked so that one edge of the shadow of the bar shows the time as the Earth rotates. Usually noon will be at the bottom of the disk, 6AM on the western edge, and 6PM on the eastern edge. In the winter, the north side of the disk will be shaded, and hard to read. In the summer, the south side will be shaded.

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In the above design, the bar is the style.

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The disk in the above design is called the face.

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In the summer, the north end of the bar is the nodus, but in the winter, the south end of the bar is the nodus.

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A series of cocentric circles can be drawn on the face which plot the path of the shadow of the nodus on specific days, thus the dial can be used as a calendar as well as clock. The style shows the time and the nodus the date. One disadvantage of this design is that with a solid face, near the equinox, when sun is just on the celestial equator, the dial is hard to read.

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Garden sundial

The classic garden sundial uses the same principle, except the lines of the disk are projected, using trigonometry, onto a face that is parallel to the ground. The advantage of the garden sundial is that it keeps time all year, and its face is never completely shaded in the daytime (as vertical sundials are). For use in a public area, this sundial can be made visible by placing it in a square, or making the face of frosted glass, elevated high in the air, and visible from underneath. The top edge of the gnomon is parallel with the axis of the Earth's rotation. The shadow will cross time markings on the face.The markings of each edge are aligned with the edge of the gnomon that produces the shadow. The angle of the face markings from the root of the gnomon (the substyle) are calculated from the formula face-angle = arctan(sin(latitude)*tan(hour-angle)).

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The angle of the style (gnomon)= 90 - latitude. (See Logo programming language for a sample program to draw a garden sundial)

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Vertical sundials

Although they are rare in modern life, sundials on vertical south-facing walls (north-facing in the southern hemisphere) are a traditional ancient convenience. They are easy to see from large distances and inexpensive to arrange. One sturdy method is to paint the sundial on the wall, and construct the gnomon as a tripod of metal bars. Fancy sundials used to have faces of inlaid stone.

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A problem is that vertical sundials only keep time for the part of the year in which the sun illuminates the wall. They are very similar to garden sundials. The formula for a south-facing sundial face is face-angle = arctan(cos(latitude)*tan(hour-angle)). The angle of the style (gnomon)= latitude.

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It used to be traditional to place four sundials on the roof or sides of a tower to provide the time. In this way, the time was available to all for the entire year. In principle, sundials can be placed on any surface, at any angle, given the correct trigonometric projection of the face. For example, sundials on roofs are harder to calculate but quite practical.

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