SUNDIAL

Abstract: A sundial having no gnomon comprises a plurality of planar surfaces mounted on a support, said surfaces constituting a sequence, the plane of each surface being tangential to a virtual helix, extending around an axis adapted to be parallel to that of the earth when the sundial is positioned at a predetermined latitude, solar time being indicated by the presence or absence of shadows cast on each face by the sunwards edge of that face.

Description: SUNDIAL This invention relates to a sundial, more specifically, it relates to a sundial of the type having no gnomon. Sundials are a well known means of time indication. They are usually based on an apparatus arranged to mark the shadow of the sun and thus make it possible to read the time from the shadow's position. The method commonly used employs the rotation of the earth to change the orientation of the sundial to the sun, thus advancing the indicated time throughout the day. A line or edge (the gnomon), running parallel to the earth's axis, is commonly used to cast a shadow onto a surface, on which time increments are indicated. The surface may, for instance, be a horizontal surface supported on a plinth, or a vertical surface on a wall of a building Less common but known are sundials in the form of a helix, where the axis of the helix runs parallel with the earth's axis. Time can be indicated by portions of shadows on the surface of the helix, the portions of the shadows changing as the earth rotates. For instance, according to GB-A-2212630, a sundial may comprise a single- turn helix wound around a gnomon which is a rod or wire extending axially of the helix. The shadow of the gnomon on the helix indicates solar time. It is not even necessary to provide a gnomon for a helical sundial. For instance, WO90/13854 comprises a strip-shaped body with surfaces substantially in the form of helicoidal surfaces, at least one of which is graduated. In such a sundial, the shadow of the edge of the strip itself provides the means of estimating time. A published study of sundials of that type, however, (Moir: British Sundial Society Bulletin 95-1 February 1995 pages 49 and 50) indicates that the action of the edge as a gnomon does not apply throughout the year. Between the autumnal and vernal equinoxes, the leading edge (eastern edge) does not act as a gnomon, and a sharp boundary for the shadow is not provided. The author proposes an alternative form showing two helices, lying on the same axis, the first directed upwardly from a central support and having a right hand twist, while the second, lying on the same axis but directed downwardly from the support has a left hand twist. It is also known that a flat surface, orientated to run parallel to the earth's axis, will fall from sun to shadow (or vice versa) at approximately the same time each day, and can be used as a time indicator. By basing the principle of the sundial on the orientation of a series of surfaces rather than the orientation of a shadow thrown upon a scale, a novel, inexpensive garden sundial can be produced by a simple casting method. The present invention provides a sundial comprising a plurality of planar surfaces mounted on a support, said surfaces constituting a sequence, the plane of each surface being tangential to a virtual helix, extending around an axis adapted to be parallel to that of the earth when the sundial is positioned at a predetermined latitude, solar time being indicated by the presence or absence of shadows cast on each face by the sunwards edge of that face. The word "virtual" as used herein means that the helix need not necessarily have a concrete existence. In general, it will be a mathematical abstraction, but each face will be tangential to the surface of a helix around the axis. It is to be understood that mathematically there is more than one form of helix, as will be apparent from a standard text on geometry, such as the article "Geometry" in the Encyclopaedia Britannica. The simple helix is defined as a curve cutting the generators of a right circular cylinder at a constant angle. Each point on this form of helix will be equidistant from the axis. A cylindroconical helix, or conical loxodrome, is defined as a curve cutting the generators of a cone of revolution at a constant angle. A loxodrome, otherwise known as a rhumb line or spherical helix is a curve cutting the meridians of a sphere at a constant angle. According to the invention, the helix may be of any of these types. According to various embodiments of the invention: • The axis of said virtual helix may extend upwardly from a point outside the base of the support. • The axis of said virtual helix may extend upwardly from a point inside the base of the support. • Each surface may constitute an end face of one of a series of flattened polyhedra. • Each surface may constitute an end face of a niche cut into the surface of a three-dimensional object. • A plurality of virtual helices may be provided, having the same or different axes, whereby faces of the different helices are displayed at different times. The invention will be further illustrated with reference to the accompanying Drawings in which: Figures 1 to 6 show various embodiments of a sundial constructed according to the principles of the present invention; Figure 7 illustrates diagrammatically the geometric principle of the operation of the sundial; and Figure 8a, b, c shows how embodiments of a sundial of the type illustrated in Figure 2, as they would be orientated at various latitudes. Referring now to the accompanying Drawings: Figure 1 shows an example of a four-sided dial at 15 minute increments of time. The shadow indicates a time of nearly 1430 (2.30 pm. The time of 1415 has passed the relevant face, as it is now in light. The fact corresponding to 1430 is just coming into light, indicating a time nearing that point. The axis A-A is parallel to the earth's axis and the centre of rotation for the faces. Angle °L indicates latitude (north or south) at which the sundial is positioned, with H-H as horizontal. Figure 2 shows a four-sided dial providing hourly increments. The time indicated is between twelve o'clock (midday) and 1300 (1 pm). The pattern is repeated on the opposite side but will run from shade to light rather than from light to shade as indicated. The last three/nine increment is repeated, to clarify time at the changeover point from one face to the next. Hours are marked on the faces. Time can be indicated as being between two points (the last face that is in sun and first face that is in shadow) or at one point, if a face is just turning in or out of shade. Figure 3 shows the dial sides incorporated on a cylindrical shape with gaps between each side. The time scale is indicated next to the face. Each face can be seen to be offset by 15° of arc, from vertically adjacent faces ( ≡* one hour) about the axis of the cylinder. Latitude can be adjusted by the positioning of the base on the support arm. Figure 4 shows a single set of faces cut into the surface of a cone. The faces are separated from each other and inclined at the angle corresponding to the latitude. Figures 5a and 5b show two faces at fifteen minute increments, set into a solid block. The time indicated is 0900 to 0915 h. The sundial has °A at 45° so is set to work at a latitude of 45° north. The sundial would be orientated to be on the north-south N-S line indicated (in the northern hemisphere). Figure 5a shows how the shadow will face when the sun is shining. Figure 5b shows no shadow, but is otherwise identical to Figure 5a. It is provided to indicate more clearly the two sets of faces. Considerable difference in the overall shape and character of the sundial will occur in varying latitudes. In the southern hemisphere, as with conventional sundials, the time scale markings and orientation will be in the opposite direction to the illustration. Figure 6 shows the sundial incorporated into a shell design. The dial is a six- sided inclined tapering shape. Figure 7 shows the geometric principle of the faces of the dial, and is provided to explain the timekeeping feature of the sundial. Figures 8a, b and c show forms of the sundial according to Figure 2 for use at respectively the equator, a latitude in the temperate zone, and at a pole. At the equator, the axis of the vertical helix will be parallel to the surface of the earth, and will be oriented north-south. At a pole the axis of the helix will be perpendicular to the surface of the earth. At an intermediate latitude, the axis will be at an angle to the local horizontal, the angle being determined by the latitude. A point on the earth's surface rotates through 360° (relative to the sun) over 24 hours, i.e. by 15° per hour. A flat surface set with an axis parallel to the earth's axis will rotate with the same properties as the earth, and will move in or out of the sun's light at a regular time of day. The time can be altered by the surface's orientation to the axis. Thus two surfaces both on the given axis, but orientated 15° apart can be used to indicate a time period of one hour. This principle can then be repeated with any number of surfaces to indicate any time interval or divisions over the day, with each face being tangential to the surface of a virtual helix, as the faces, indicating a series of times, are set on the sundial. The time can then be read as the sun's shadow moves up or down the scale of faces and each face in turn moves from shadow to sun or vice versa. Each face of the dial will have an appropriate form which is determined by the shape of the support. What is important is that the geometric relations between each face, and the latitude at which the sundial is intended to be used must have the geometric properties illustrated in Figure 7. In Figure 7, the line A-A represents the axis of the virtual helix and F represents a face. It will be seen that the face F lies in a plane defined by the axes P-P and C-C, the plane extending through a chord of a circle perpendicular to the axis A-A, so that the line P-P is parallel to the axis A-A. Provided these geometric requirements are met, the shape of the face can have any desired form, as decided by aesthetic requirements or by the overall shape of the support. As will be seen from consideration of the Drawings, since the support can have a variety of forms, the face itself can also have a variety of forms. Each face can be anywhere on the surface of the support and be arranged in any way as long as the properties are maintained. The leading edge of the polygon is not acting as a gnomon, so the face can be of any shape. The number of faces can be varied, depending on the time interval set between each face. A 'side' refers to a series of faces that are incorporated on the polyhedron. Each side can indicate time over a segment, or all of the day. The indicated time can follow on, overlap or repeat from one side to the next. Any number of sides or faces can be used. This makes it possible to incorporate the faces in a wide variety of shaped articles whose overall form may be determined by aesthetic considerations. The sundial itself can take the form not only of a helix on a cylinder or cone, but as long as it maintains its properties, can be set over or incorporated in any shape or form of support. For instance, the faces can be cut into a column of any desired cross-section, the cross-section being constant with height, or tapering or even barrel-shaped. The column itself could be able to serve as a support for plant pots, bird baths, etc. The support can take the form of another object and incorporate the sundial on part of the surface, as for instance in a shell form, as shown in Figure 6, or on a rounded object, such as a sphere, or even a fruit, such as a pineapple. Designs can be incorporated between or around the sides and/or between or around the faces of the sundial. For instance, a column of square cross- section could have the faces formed at the corners, or cut into the surfaces of a column of polygonal cross-section. With the particular character of this invention it is clear that all the variations in design, number and configuration of the sides and faces, and the character and shape of the support on which they are placed, are numerous, and a very wide variety of shapes for the support and the faces is possible.

Claims: CLAIMS 1 . A sundial comprising a plurality of planar surfaces mounted on a support, said surfaces constituting a sequence, the plane of each surface being tangential to a virtual helix, extending around an axis adapted to be parallel to that of the earth when the sundial is positioned at a predetermined latitude, solar time being indicated by the presence or absence of shadows cast on each face by the sunwards edge of that face. 2. A sundial according to Claim 1 wherein the axis of said virtual helix extends upwardly from a point outside the base of the support. 3. A sundial according to Claim 1 wherein the axis of said virtual helix extends upwardly from a point inside the base of the support. 4. A sundial according to any one of Claims 1 to 3 wherein each surface constitutes an end face of one of a series of flattened polyhedra. 5. A sundial according to any one of Claims 1 to 3 wherein each surface constitutes an end face of a niche cut into the surface of a three-dimensional object. 6. A sundial according to any one of Claims 1 to 5 wherein a plurality of virtual helices are provided, having the same or different axes, whereby faces of the different helices are displayed at different times. 7. A sundial according to any one of Claims 1 to 6 wherein the virtual helix is a simple helix. 8. A sundial according to any one of Claims 1 to 6 wherein the virtual helix is a cylindroconical helix. 9. A sundial according to any one of Claims 1 to 6 wherein the virtual helix is a spherical helix.

Inventors: ROLFE GARY, ROLFE, GARY
Applicants: ROLFE GARY, ROLFE, GARY
Application year: 1997
Main classification: G04B49
IPC codes: G04B49