Geometrical instruments – Straight-line light ray type – Celestial
Patent
1991-12-19
1993-01-26
Will, Thomas B.
Geometrical instruments
Straight-line light ray type
Celestial
33268, G04B 4902
Patent
active
051813246
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The invention relates to a sundial comprising a body and a base therefor.
BACKGROUND ART
Sundials have been known for a long time, which have an inclining gnomon bar, which in sunny weather casts a shadow on a curved scale body provided with indications of the time. As a result, the time can be directly read from the spot where the shadow falls. The manufacturing of such a sundial involves a good many working steps, and furthermore the parts of the sundial must be assembled rather exactly. In addition, it is difficult to read the time at a long distance, such as a few meters away from it, because the shadow cast by the bar on the scale body is relatively small.
A sundial of this type is known from U.S. Pat. No. 2,754,593. The scale body is supported by a holder which is bolted to a base. This construction is rather complex and expensive to produce.
SUMMARY OF THE INVENTION
The object of the invention is to provide a sundial which is very simple to manufacture and to set up, and which allows an easy reading of the time even at a relatively long distance.
The sundial according to the invention is characterised in that the body is a strip-shaped body having surfaces substantially formed as helicoids, the body being made of a rather thin material, and that at least one of the helicoids is provided with time markings, so that when subjected to sunshine, the body casts a relatively wide sharp-edged time indicative shadow on itself, the shadow edge moving upwards and downwards on the body as the sun moves around the body. In this manner the manufacture of the sundial is very simple. No special gnomon bar is necessary. The time indicated by the sundial is read at the time marking on the helicoid which the front rim of the shadow has reached. When the sundial is set up, care should be taken that the body inclines correctly relative to the path of the sun, i.e. in such a manner that the longitudinal axis of the body is parallel to the earth's axis of rotation, i.e., that the longitudinal axis is pointing towards the North Star.
According to the invention the strip-shaped body may be constituted by a flat bar twisted, for instance 360.degree., about its longitudinal axis, whereby a particularly simple manufacturing of the sundial is obtained.
In addition according to the invention the helicoids of the strip-shaped body may advantageously be right helicoids.
According to the invention, it is preferred that the time markings are non-equidistant and placed near the longitudinal axis of the helicoids.
Moreover according to the invention the time markings may be provided on both helicoids of the strip-shaped body, whereby the sundial is particularly easy to read because it is no longer necessary to move particularly far to find a good reading position. The two sets of time markings normally oppose one another.
According to the invention, the ratio of the thickness to the width of the strip-shaped body may be in the range of 0.01 to 0.1, whereby the sundial is provided with a suitable strength and is rather easy to read.
Furthermore, according to the invention, the ratio of the width to the length of the strip-shaped body may be in the range 0.02 to 0.2, whereby the resulting shadow is of such a width that it is easy to find and read for an observer standing rather far from the sundial.
An embodiment of the sundial comprises a holder for the body, the holder for instance being a circularly curved bar preferably of an arc of measure of at least 180.degree., and a standard for the holder, this embodiment being characterised in that the strip-shaped body is placed along a diameter of the holder, the axis of the strip-shaped body being adjustable parallel to the earth's axis of rotation. The resulting sundial is very easy to adjust.
Moreover, according to the invention, the holder may be an inclining bar, and the strip-shaped body may be arranged in extension thereof, whereby an extremely simple construction of the sundial is obtained.
The strip-shaped body is advantageously made of a weather-proof mater
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Cousins, Frank W., Sundials: A Simplified Approach by Means of the Equatorial Dial, pp. 148-151, no date.
Piet Hein A/S
Will Thomas B.
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