GB2387675A

GB2387675A - Time and date sundial

Info

Publication number: GB2387675A
Application number: GB0209019A
Authority: GB
Inventors: Michael James Knee
Original assignee: Individual
Current assignee: Individual
Priority date: 2002-04-19
Filing date: 2002-04-19
Publication date: 2003-10-22
Also published as: GB0209019D0

Abstract

A sundial consists of an inverted cone onto which is cast the shadow of a pointed gnomon. Analemmas and lines of constant date are inscribed on the sundial from which both the time and the date can be read by observing the position of the gnomon's shadow. Preferably, the angle of the cone is chosen to increase the sharpness of the gnomon's shadow throughout the day and year. The gnomon may be a vertical pointed stick such as a pencil passing through the apex of the cone. The lines may be printed on paper and made available for assembly into a sundial. The lines may be generated for a specific latitude, longitude and time zone.

Description

TIME AND DATE SUNDIAL

The invention described in this document is a sundial which can tell both the time and date to reasonable accuracy.

Sundials which tell the time are well known. A typical horizontal sundial has a flat plate and an inclined shadow-casting edge known as the gnomon. If the gnomon is inclined and oriented so that it is parallel to the Earth's axis of rotation, then the line of the gnomon's shadow will be independent of the Sun's altitude in the sky and will indicate the Sun's azimuth, which can be read off on a scale on the sundial in units of time. This will give an approximate value of local solar time or, with appropriate modification of the scale, civil time according to the appropriate time zone. A table or graph is sometimes provided by which, with knowledge of the date, the sundial reading can be corrected for the effect of the so-called Equation of Time, a well-known equation giving the difference between sundial time and clock time. This difference is the sum of two sinusoids, one with a period of six months accounting for the tilt of the Earth's axis with respect to the plane of its orbit round the Sun, and another with a period of one year accounting for the eccentricity of the Earth's orbit. An example of a sundial with such a correction graph can be seen at the railway station in Glion, near Montreux in Switzerland. Other sundials have vertical or curved plates, sometimes designed in such a way as to improve or maximize the sharpness of the gnomon's shadow. If the gnomon is replaced by a point, it is possible to tell the date using a sundial. This is because, at a given tithe of day, the altitude of the Sun depends on the time of year. In fact, at a given clock time (noon, for example), the shadow of the point traces out, through the year, a figure-of-eight pattern known as an analemma. This fact is put to good effect in a special sundial called the Noon Sundial, which can be seen at the entrance to the DERA (or QinetiQ) laboratories in Farnborough, Surrey. In that sundial, the shadow of a point is cast onto a vertical plate of ground glass on which is inscribed an analemma. At noon every day, the shadow crosses the analemma at a point from which, with the help of tick marks inscribed on the analemrna, the date can be read. It is only necessary to know which half of the year one is in in order to overcome the ambiguity resulting from the fact that in general the shadow will cross the analemma in two places.

None of the sundials described above is capable of indicating both the time and date.

In one of its aspects, the present invention improves on the Noon Sundial and on traditional sundials by providing an analemma for each hour of the day. The date marks on the noon analemma are replaced by "date

- 2 - lines" crossing the analemmas. It is then possible by interpolation of the shadow's position between analemmas and between date lines to read both the time and the date.

One problem with a sundial having a point gnomon and designed to be read throughout the day is that a flat dial is inappropriate. This is because there will always be times at which the line between the gnomon and its shadow will be far from normal to the surface of the dial and the shadow will be ill-defined. The present invention overcomes this problem by providing a dial whose surface is the inside of an inverted open cone.

Accordingly, the present invention consists in another aspect in: an inverted open cone on whose inside surface are inscribed analemmas corresponding to hourly or other suitable intervals and lines of constant date corresponding to monthly or other suitable intervals À a gnomon consisting of a vertical stick with a sharp point at its top Preferably, À the point of the gnomon is located level with and at the centre of the open top of the inverted cone À the angle of the cone is chosen to be such that the sun's rays at noon at the equinoxes fall normal to the surface of the cone À the portions of each analemma and the lines of constant date corresponding to the first half of the year (or the period from the winter solstice to the summer solstice) are distinguished by colour or other means from those corresponding to the second half of the year (or the period from the summer solstice to the winter solstice) À alternatively, a separate sundial or removable cone insert is provided for each half of the year À the sundial consists of a cone of paper or other suitable material, held in a tumbler or similar structure, and the gnomon consists of a pen or pencil attached to the cone through a small hole in the apex of the cone À the sundial is made available as a kit in the form of a pre-printed sheet suitable for assembly into a cone À the printed sheet may be easily customised for the exact latitude, longitude and time zone where the sundial will be used.

An example of the invention is illustrated in the accompanying drawings, in which: - Figure 1 shows an example of a sundial according to the invention.

The inscriptions are not shown.

Figure 2 shows an exernple of a pre-printed sheet for an all-year sundial according to the invention, which may be cut out and assembled into an inverted cone with the printing on its inside surface.

Figure 3 shows an example of a pre-printed sheet for a half-year insert _

- 3 A particularly simple form of sundial according to the present invention can be constructed (as shown in Figure 1) from a printed sheet formed into a cone, resting in a cylindrical support such as a glass tumbler and using a ballpoint pen as a gnomon. It is estimated that this sundial will tell the time to an accuracy of about five minutes and the date to an accuracy of about three days near the equinoxes. Inevitably, the date accuracy falls off considerably near the solstices, an inevitable consequence of the sinusoidal variation in the Sun's declination through the year.

The printed sheet can represent an entire year or take a simpler form which covers one half of the year. Figure 2 shows an example of a printed sheet for an all-year sundial according to the invention whilst Figure 3 shows an example of a printed sheet for a half-year sundial.

A sundial according to the invention can of course take many mechanical forms. Whilst there is attraction in the notion of a printed insert which can be distributed as a flat sheet and then assembled by the user with everyday objects serving as the support and gnomon, the invention encompasses structures having a dedicated support and gnomon. The surface bearing each analemrna and the lines of constant date can be integral with the sundial support and technologies other than printing can of course be employed. It should be understood that this invention has been described by way of example only and a wide variety of modifications are possible without departing from the scope of the invention. For example, this description

assumes that the sundial is in the northern hemisphere. Modifications for the southern hemisphere are obvious to the skilled person.

Claims

1. A sundial on whose surface are inscribed analemmas corresponding to hourly or other suitable intervals and lines of constant date at monthly or other suitable intervals from which both the time and the date can be read by observing the position, relative to the inscribed lines, of the shadow of a pointed gnomon.

2. A sundial according to Claim 1, wherein the lines are inscribed on the

inside surface of an inverted cone or other shape designed to increase the sharpness of the gnomon's shadow throughout the day and year.

3. A sundial according to Claim 2, wherein the gnomon is a vertical stick with a sharp point located level with and at the centre of the open top of the inverted cone.

4. A sundial according to Claim 2, wherein the angle of the cone is chosen to be such that the sun's rays at noon at the equinoxes fall approximately normal to the surface of the cone.

5. A sundial according to Claim 1, wherein the portions of each analemma and the lines of constant date corresponding to the first half of the year (or the period from the winter solstice to the summer solstice) are distinguished by colour or other means from those corresponding to the second half of the year (or the period from the summer solstice to the winter solstice).

6. A sundial according to Claim 1, wherein the inscribed lines correspond to one half of the year only.

7. A sundial according to Claim 1, wherein separate inserts are provided for each half of We year.

8. A sundial according to Claim 2, wherein the cone is made of paper and is held in the mouth of a tumbler or similar structure.

9. A sundial according to Claim 3, wherein the gnomon consists of a pointed stick attached to the cone through a hole in the apex of the cone.

10. A sundial according to Claims 8 and 9, which is made available as a kit in the form of a pre-printed sheet suitable for assembly into a cone.

11. A sundial according to any of the preceding claims, wherein the inscribed lines are generated for a specific latitude, longitude and time zone.

- 5

12. A sheet adapted to be assembled with an object or indicia serving as a gnomon, to form a sundial, the sheet bearing analemmas corresponding to hourly or other suitable intervals and lines of constant date at monthly or other suitable intervals from which in the assembled sundial both the time and the date can be read by observing the position, relative to the inscribed lines, of the shadow of the gnomon.