GB2158618A - Sundial or lamp-dial recording clock time - Google Patents

Abstract

This invention serves two alternative purposes namely as a sundial recording the actual passage of time and as a lamp-dial used in studies of the sunlighting at various times of day and year of, for instance, a model layout of buildings. The shadow of a small object 5, cast into a bowl 1 of a certain defined shape and positioned in a defined manner, marks the time with reference to a system of longitudinal hour-lines and latitudinal bands of the months. A framework 2, 3 holding the bowl in the defined manner allows it to rotate about an axis to positions in which, with periodic adjustments, clock time and certain other times may be recorded. <IMAGE>

Description

SPECIFICATION Sundial or lamp-dial recording clock time This invention relates either to a sundial on which a shadow is cast by the sun or to an instrument herein termed a lamp-dial receiving a shadow from an artificial source of light.

Sundials record the time of day and in some cases the time of year corresponding to the sun's angular position in the sky. Lamp-dials record a simulated time in a simulated situation. It is common practice in drawing and modelling offices to study the sunlighting of buildings by means of a lamp representing the sun which projects shadows in a model layout. If the model is removed and the lamp-dial set in its place the lamp dial will indicate the time of day and time of year at which the sun would cast such shadows. The lamp-dial may also serve as a simple and relatively inexpensive aid in the positioning of the lamp to simulate the sun at pre-selected dates and hours commonly used as data in comparisons of sunlighting performance.

The sundial and the lamp-dial are essentially the same and inter-changeable: a lamp-dial may be used as a sundial and a portable sundial as a lamp-dial; both sundial and lamp-dial translate the angular position of a light-source as time of day and time of year. It is therefore not necessary in what follows herein to give separate explanations and descriptions of the invention as it applies to sundials and lamp-dials; all that applies to a sundial applies to a lamp-dial.However references in this explanation and description to the Celestial Sphere, geographical longitude and latitude, and times of day and year should, in the case of the lamp-dial, be understood to refer not to the here and now of the actual situation but to the place and time simulated by the model, the horizontal plane becoming the base plate of the model, the intersection of the meridian plane with the horizontal becoming the north-south line of the model, the geographical latitude and longitude becoming those of the place modelled, and the times becoming those simulated for the purposes of the model study.

Nevertheless this explanation and description should not be taken as implying that in practice the lamp-dial may not in some respects be designed differently from the sundial. The two instruments have different purposes, the sundial to keep the time, the lamp dial to aid in the model study of sunlighting. For instance the lamp-dial should be small in relation to the distance of the lamp; the sundial may be relatively large to give precise readings; the lamp-dial should indicate the time of year no less plainly than the time of day; the sundial may give priority to the time of day. The different purposes of the instruments have different implications for their design.

Sundials have fallen from favour partly because they usually indicate only solar time of day which is not the same as clock time of day, and partly because the indications they give often lack plainness and clarity. Lamp-dials might be more useful than hitherto if they gave plainer indication of the time simulated, particularly the time of year. The need this invention aims to supply is the telling of clock time and the plain and distinct indication of the time of year.

The geometrical explanation of the method of the invention is as follows. The sun's daily traverse of the sky is the arc of a nearly perfect circle parallel to the Celestial Equator and displaced from it by the angle of declination of the sun. The rate of angular displacement of the sun along the traverse is almost exactly 1 in four minutes or 15 in one hour. Suppose that the shadow of a small object a sphere or ring - fixed to mark the centre of the circle formed by the rim of a hemispherical bowl is cast by the sun into the bowl. The path of such a shadow is a circular arc traced at the same angular rate as the circular path of the sun across the sky.

The small object is at the centre of homothety between the sky and the bowl; its shadow is the image of the sun. If the bowl is disposed so that a diameter of its circular rim forms an axis in the line of the axis of the Celestial Sphere and the corresponding equator of the bowl lies in the plane of the Celestial Equator then the shadow of the small object will trace latitude lines with reference to the equator of the bowl and will cross pole-to-pole lines of longitude spaced 15 apart at hourly intervals. The angle of latitude of the shadow in the bowl, being equal to the declination of the sun, indicates plainly the time of year. A rotation of the bowl about its pole-to-pole axis displaces the longitudinal hour-lines relative to the shadow by four minutes for every degree of rotation without displacing longitudinal lines.A suitable rotation may set the shadow to record clock time without affecting the indication of time of year. As actual distances of the sun and shadow from the centre of homothety are immaterial this explanation may be generalised. The bowl may be the partial interior surface not only of a sphere but of any figure that may be sectioned in co-axial circles by a set of parallel planes i.e. a figure generated by rotation like wood turned on the lathe, such as a spheroid, cone, cone flattened into plane, cylinder etc.

Consequently, according to the present invention there is provided: a partial interior surface, herein termed a bowl, of a figure generated by rotation about an axis, the intersections by planes perpendicular to this axis being circles co-axial with this axis, the interior surface of the bowl being marked with lines of latitude corresponding to such circles and lines of longitude perpendicular to them; an axis about which the said bowl may rotate coinciding with the aforesaid axis and aligned with the axis of the Celestial Sphere; a small opaque object in the said axis casting a shadow into the bowl; means of appropriately positioning and holding the bowl and adjusting and measuring its rotation about the said axis.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which : Figure 1 is a vertical section indicating the lightsource, a ray of light casting a shadow into a hemispherical bowl and the means provided for holding, orienting, inclining and rotating the bowl.

Figure 2 is a full-face view of the hemispherical bowl, of the style tipped by a small opaque sphere and of shadows cast into the bowl.

Figure 3 is a section through the equatorial plane of the bowl showing the small sphere, a part of the bowl and a lateral arm equipped with scales curved round the exterior of the bowl.

Referring to the drawings this embodiment comprises four elements, namely a hemispherical bowl 1 with attachments, an inclinable semi-circular member 2 to hold the bowl at the appropriate inclination, an orientable member 3 on a horizontal base, and the horizontal base 4.

The base 4 is set so that its upper surface is horizontal and, if this embodiment is a sundial, it is fixed in this position by cementing to a fixed block or otherwise. The base 4 also comprises a column of circular section perpendicular to its upper surface. If the embodiment is a lamp-dial the base 4 is dispensable.

The orientable member 3, fitting closely over the vertical column and supported on the horizontal surface of the base 4 (if any), is oriented and fixed by lock-screw or other means (not shown) so that the inclinable member 2 and the style 5 lie in the plane of the meridian with the upper end of the style in the general direction of the visible Celestial Pole.

The inclinable member 2, of rectangular section 6, is closely held by the orientable member 3, its movement being restricted to movement along its own length forming part of the perimeter of a circle the centre of which, marked by the small sphere 7, remains unmoved. The position of this member is set by means of scale 8 and marker 9 and fixed by lock-screw or other means (not shown) so that the style 5 embodying the pole-topole axis of the bowl 1 is inclined to the horizontal at the angle of local latitude and thus lies in the axis of the Celestial Sphere.

The hemispherical bowl 1 is closely but freely held in the said inclination and orientation by the inclinable member 2 by means of projections 10 and 11 so that it may rotate about its pole-to-pole axis. It may be held at any point of rotation by lock-screw or other means (not shown).

Figure 2 shows the interior of the bowl with longitudinal hour-line markings and latitudinal bands of the months, the style 5 tipped with the small sphere 7, and shadows. The spherical latitude of the shadow of the small sphere indicates the time of year of about 12th February and its spherical longitude a time of day of about 10.10 a.m.. A precise reading of the time of year is possible by rotating the bowl to bring a sub-divided scale of months on hour- line X1 under the shadow. This facility enables a confusing network of subdividing lines to be avoided.

Figure 3 shows a lateral arm 12 attached at 13 to the inclinable member 2 (shown also as a rectangular section 6). The lateral arm 12 is inscribed with a scale 15 and bears a transparent moveable sleeve inscribed with scales 16 and 17. The scale 15 is graduated in degrees and scales 16 and 17 in minutes of time.

With the marker 14 set on the zero of scale 15 the bowl faces directly forward from the inclinable member 2 and the hour-line X11 lies in the plane of the meridian. With this setting the shadow of the style and the small sphere records local solar time. The steps by which the record of local solar time is converted into a record of clock time will now be described.

First a correction is made for the difference between the local longitude and the longitude of the standard meridian of the Time Zone. The zero of scale 16 is set on scale 15 at the value of this difference; the marker 14 is brought to the zero of scale 16. With this setting the shadow of the style and small sphere records the solar time of the standard meridian of the Time Zone. If for example an embodiment of this invention is installed as a sundial in the UK at a place of longitude 7 1/2"W and the scales and marker are set as shown in Figure 3, indicating a difference of 7 1/2" between local longitude and the longitude 0 of the standard meridian of Greenwich, then the shadow of the style and small sphere will record Greenwich solar time.Greenwich solar time being 30 minutes ahead of local solar time the bowl has been turned eastwards 7 1/2" to add 30 minutes to the time recorded. For local longitudes east of Greenwich the sleeve and marker are moved the other way from the zero of scale 15. The sleeve may be locked in this setting by a lock-screw or other means (not shown). Then a correction is made to convert solar into mean time. The marker 14 is set to indicate the value of the Equation of Time for the day in minutes plus or minus on scale 16. With this setting the shadow of the style and small sphere records the mean time of the standard meridian of the Time Zone. Finally a correction is made for Summer Time when this is in force by transferring the marker 14 to the corresponding position on the scale 17, the zeros of scales 16 and 17 being 15 or one hour apart. Subsequent corrections according to the Equation of Time are made on scale 17 so long as Summer Time remains in force. With these settings the shadow of the style and small sphere records Summer Time.

As a sundial this embodiment of the invention might keep time within two minutes of precise clock time if settings on scales 16 and 17 were made forty times during the full year and within one minute if 81 such settings were made. If the local longitude or the value of the Equation of Time or both were not accurately known the clock time setting could nevertheless be made at a moment of sunshine by rotating the bowl and fixing it at the point at which the shadow indicated clock time as given by an accurate watch and by periodic resetting in the same fashion whenever the time recorded had again become perceptibly different from clock time.

Claims (7)

1. A sundial or lamp-dial in which the time of day indicated by a shadow can be adjusted to be clock time by rotation,about an axis, of the surface receiving the shadow in such a way that the indication by the shadow of the time of year is unaffected by the rotation and the time of day indicated is altered to the same extent by a given degree of rotation whenever it may occur and in which the record of clock time can be made continuous, within a margin of error of about two minutes, by forty such rotations in the course of a year; the said axis being aligned with the axis of the Celestial Sphere; the said shadow being cast by a small object in the line of the said axis; the said surface receiving the shadow being such that all planes perpendicular to the said axis if they intersect the said surface do so in arcs of circles whose centres lie on the said axis, or being itself a plane surface perpendicular to the said axis.

2. A sundial or lamp-dial as claimed in Claim 1 in which the shadow indicating the time is received on a concave hemispherical surface or bowlmarked with hour-lines of spherical longitude at equal intervals of 15 running between two diametrically opposed poles on the rim of the said bowl to indicate twelve successive hours, numbered and with intermediate divisions; the said shadow being cast by a small object at the centre of the circle formed by the rim of the bowl; the axis about which the bowl may rotate running between the said two poles and being aligned as in Claim 1; the record of clock time being obtained by an appropriate rotation of the bowl and maintained by periodic rotational settings as in Claim 1.

3. A sundial or lamp-dial as claimed in Claims 1 and 2 in which the indication of the time of year given by the shadow is distinguished clearly from the indication of the time of day by being a spherical latitude in contrast with the spherical longitude which indicates the time of day and by remaining unchanged by rotation of the bowl as described in Claim 1.

4. A sundial or lamp-dial as claimed in Claims 1, 2 and 3 in which the possibility of confusion implied in a multiplicity of temporal lines (hour-lines and date-lines) is reduced by the omission of datelines subdividing the monthly bands and provision for precise reading of the time of year on subdivided scales which may be rotated with the bowl, without latitudinal dispiacement, so that the reading may be taken.

5. A sundial or lamp-dial as claimed in all previous Claims which records any one of four times namely local solar time, of the longitude on which the Time Zone is based (the Time Zone longitude), the mean time of the Time Zone longitude, and clock time according to the setting which may eliminate none, one, two or all of the three components of the difference between local solar time and clock time namely the components due to the difference between local longitude and Time Zone longitude, the component of the Equation of Time and the component due to the being in force or not being in force of a Summer Time convention; the settings being made by means of a rotation marker and scales.

6. Apparatus according to any of the preceding Claims comprising a base providing a horizontal datum plane, an orientable member resting on the horizontal base, an inclinable member held in the appropriate orientation by the orientable member, a rotatable bowl as in all preceding Claims held by the inclinable member and to which the inclinable member imparts an orientation and inclination as implied in Claims 1 and 2, and comprising also a lateral arm fixed to the inclinable member and curved round the exterior of the bowl along its equator and equipped with one fixed and two sliding scales by which a marker fixed to the exterior of the bowl indicates the degree of rotation of the bowl and may be pre-set according to the components and total of the rotation required for the purposes indicated in any of the preceding Claims.

7. Apparatus according to Claim 6 substantially as described with reference to the accompanying drawings.