The ’shadow-maker’ of the sundial is called a gnomon. The style should be parallel to the Earth’s axis of rotation, and point to the celestial pole. On a standard garden sundial, this line is the top edge of the gnomon. The linear feature that casts the shadow from which the time can be read is often called a style
In addition to the Hour line, the face of the dial can contain further information , such as the horizon, the equator and the tropics. These lines are referred to as the dial furniture
The line on the dial plate perpendicularly beneath the style is called the substyle, that is below the style. On many wall dials, the substyle is not the same as the noon line. The angle, on the dial plate, the noon line makes to the substyle is called the substyle distance, an unusual use of the word distance to mean an angle. The angle the style makes perpendicularly with the dial plate is called the substyle height, an unusual use of the word height to mean an angle
In common designs such as the horizontal sundial, the sun casts a shadow from its style onto a flat surface marked with lines indicating the hours of the day. Hence, if such a sundial is to tell the correct time, the style must point towards true North and the style’s angle with horizontal must equal the sundial’s geographical latitude. A sundial is a device that measures time by the position of the Sun. However, many sundials do not fit this description, and operate on different principles. As the sun moves across the sky, the shadow-edge progressively aligns with different hour-lines on the plate. Such designs rely on the style being aligned with the axis of the Earth’s rotation
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 style. Some sundials indicate both the time and the date by the shadow of a particular point on the gnomon. That point is called the nodus. The nodus may be the tip of a gnomon with an arbitrary orientation
Sundials are not as common in the Southern hemisphere as in the North. To position a horizontal sundial correctly, one has to find true North or South. Also the hour numbers on a horizontal dial run anti-clockwise rather than clockwise. One proposition for this is that when Europeans arrived the mechanical clock was accurate enough for their purposes of time keeping and there was no need to erect sundials. The gnomon, set to the correct latitude, has to point to the true South in the Southern hemisphere as in the Northern Hemisphere it has to point to the true North. The same process can be used to do both. A sundial at a particular latitude in one hemisphere must be reversed for use at the reciprocal latitude in the other hemisphere. One half of the Earth’s globe is either north or south of the Equator. A vertical direct south sundial in the Northern Hemisphere becomes a vertical direct north sundial in the Southern Hemisphere ; an opposite
This correction — which may be as great as 15 minutes — is described by the equation of time. Third, the orbit of the Earth is not perfectly circular and its rotational axis not perfectly perpendicular to its orbit, which together produce small variations in the sundial time throughout the year. Sundials indicate the local solar time, unless otherwise corrected. This correction is usually made by numbering the hour-lines with two sets of numbers. For example, a sundial located west of Greenwich, England but within the same time-zone, shows an earlier time than the official time; it will show “noon” after the official noon has passed, since the sun passes overhead later, since the sundial is further in the west. A more sophisticated sundial design is required to incorporate this correction automatically; alternatively, a small plaque can be affixed to the sundial giving the offsets at various times of the year. To obtain the standard clock time, three types of corrections need to be made. Second, the practice of daylight saving time shifts the official time away from solar time by an hour or, in rare cases, by another amount. This correction is often made by rotating the hour-lines by an angle equaling the difference in longitudes. First, the solar time needs to be corrected for the longitude of the sundial relative to the longitude at which the official time zone is defined
For illustration, the Benoy Dial uses a cylindrical lens to create a sheet of light, which falls as a line on the dial surface. Benoy dials can be seen throughout Great Britain, such as. Light may also be used to replace the shadow-edge of a gnomon. Whereas the style usually casts a sheet of shadow, an equivalent sheet of light can be created by allowing the sun’s rays through a thin slit, reflecting them from a long, slim mirror , or focusing them through a cylindrical lens
In contrast, other dials are self-aligning; for example, two dials that operate on different principles, such as a horizontal and analemmatic dial, may be mounted together on one plate, such that their times agree only when the plate is aligned properly. Sundials were an important aspect of the Greek and Egyptian civilizations up to the 16th century. In the other case, the shadow-casting object — the sundial’s gnomon — may be a thin rod, or any object with a sharp tip or a straight edge. First, some sundials use a spot of light, or a line of light, to indicate the time, where others use the edge or tip of a shadow. Third, sundials may use many types of surfaces to receive the spot or line of light, the shadow-tip or shadow-edge. The installation of many dials requires knowing the local latitude, the precise vertical direction , and the direction to true North. Fourth, sundials differ in their portability and their need of orientation. Second, sundials employ many types of gnomon. Sundials can be categorized in several ways. 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. Planes are the most common surface, but partial spheres, cylinders, cones and even more complicated shapes have been used for greater accuracy or intriguing aesthetics. In the former case, 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; a line of light may be formed by allowing the rays through a thin slit or focusing them through a cylindrical lens
If the gnomon is not aligned with the celestial poles, even its shadow will not rotate uniformly, and the hour lines must be corrected accordingly. If the shadow-casting gnomon is aligned with the celestial poles, its shadow will revolve at a constant rate, and this rotation will not change with the seasons. This cone and its conic section change with the seasons, as the Sun’s declination changes; hence, sundials that follow the motion of such light-spots or shadow-tips often have different hour-lines for different times of the year, as seen in shepherd’s dials, sundial rings, and vertical gnomons such as obelisks. The rays of light that graze the tip of a gnomon, or which pass through a small hole, or which reflect from a small mirror, trace out a cone that is aligned with the celestial poles. This model of the Sun’s motion helps to understand the principles of sundials. In other cases, the hour-lines are not spaced evenly, even though the shadow is rotating uniformly. Alternatively, sundials may change the angle and/or position of the gnomon relative to the hour lines, as in the analemmatic dial or the Lambert dial. The hour-lines will be spaced uniformly if the surface receiving the shadow is either perpendicular or circularly symmetric about the gnomon . This is perhaps the most commonly seen design and, in such cases, the same set of hour lines may be used throughout the year. This conic section is the intersection of the cone of light rays with the flat surface. The corresponding light-spot or shadow-tip, if it falls onto a flat surface, will trace out a conic section, such as a hyperbola, ellipse or a circle
Nodus-based sundials may use a small hole or mirror to isolate a single ray of light; the former are sometimes called aperture dials. The oldest example is perhaps the antiborean sundial , a spherical nodus-based sundial that faces true North; a ray of sunlight enters from the South through a small hole located at the sphere’s pole and falls on the hour and date lines inscribed within the sphere, which resemble lines of longitude and latitude, respectively, on a globe. Most of the sundials described below use shadow to indicate time, whether it be the shadow-edge of the style, or the shadow-point of the nodus. However, light may be used in equivalent ways
The sun casts a shadow from the gnomon or style to a surface called the dial face or dial plate
Most equiangular sundials have a fixed gnomon style aligned with the Earth’s rotational axis, as well as a shadow-receiving surface that is symmetrical about that axis; examples include the equatorial dial, the equatorial bow, the armillary sphere, the cylindrical dial and the conical dial. However, some other designs are equiangular, such as the Lambert dial. A dial is said to be equiangular if its hour-lines are straight and spaced equally
It is traditional for a sundial to have a motto. The motto is usually in the form of an epigram, and often represents the sense of humor of the dial maker
The path of the Sun on the celestial sphere is known as the ecliptic, which passes through the twelve constellations of the zodiac in the course of a year. Since the celestial axis is aligned with the axis about which the Earth rotates, its angle with the local horizontal equals the local geographical latitude. For navigational and sundial purposes, it is an excellent approximation to assume that the Sun revolves around a stationary Earth on the celestial sphere, which rotates every 23 hours and 56 minutes about its celestial axis, the line connecting the celestial poles. The principles of sundials can be understood most easily from an ancient model of the Sun’s motion. Science has established that the Earth rotates on its axis, and revolves in an elliptic orbit about the Sun; however, meticulous astronomical observations and physics experiments were required to establish this. Unlike the fixed stars, the Sun changes its position on the celestial sphere, being at positive declination in summer, at negative declination in winter, and having exactly zero declination at the equinoxes