WO2005006281A1

WO2005006281A1 - Position indicating device

Info

Publication number: WO2005006281A1
Application number: PCT/GB2004/002942
Authority: WO
Inventors: Adam Nieman
Original assignee: Adam Nieman
Priority date: 2003-07-08
Filing date: 2004-07-08
Publication date: 2005-01-20
Also published as: GB2422240B; GB2422240A; GB0602231D0; GB0314801D0

Abstract

A device (100, 200, 300) having an indicator (102, 202, 306) alignable with an object in space and display means (204, 308) for showing the distance from the Earth or Sun to that object. The device (100, 500, 600) may rotate about a single axis, and have a plurality of indicator arms (102) pointing at a selection of predetermined 'fixed' stars. The device (200, 300, 400, 600) may include an indicator arm (202, 306, 402, 403,606) movable with two degrees of freedom to track a moving celestial object e.g. the Sun. Alternatively, the device (500) may include an array (502) of selectively operable indicia to track said moving celestial object. To make display means (608) mounted on an indicator arm (606) more legible, the lower part of the indicator arm (606) is made heavier so that it always hangs below the upper part due to gravity.

Description

POSITION INDICATING DEVICE

FIELD OF THE INVENTION The invention relates to devices which indicate the position of objects in space. Objects in space can be put into two classes: a first class of objects that are so distant from the Earth that they appear to be in fixed positions relative to one another as the Earth rotates on its axis and orbits the Sun; and a second class of closer objects (e.g. the Sun, planets, satellites) which move relative to this ^λ fixed' background. The invention relates to devices which can remain in alignment with the first class of objects despite the Earth's movement, and to devices which can track the changing position of the second class of objects. Observing the movement of a device whilst it tracks a celestial object helps viewers to locate themselves in the cosmos and aids viewers' understanding of celestial dynamics. The devices have application in educational contexts (museums, observatories, planetaria, schools) , in public display contexts (as public sculpture) and in domestic contexts (as educational toys) .

BACKGROUND TO THE INVENTION Few of us understand the passing of the seasons, let alone the movements of planets or the distribution of galaxies. An intuitive understanding of our place in space eludes most of us. The main reason for this is that our primary reference frame is the surface of the Earth, which is spherical and moves relative to other objects in space, making it difficult to keep track of objects in the sky. Another reason we struggle with ^■•celestial geography' is that it is impossible to judge distances to celestial objects by visual inspection. Devices for tracking celestial objects are known, e.g. for enabling telescopes to remain focussed on an object despite rotation of the Earth. US 4505680 describes a device where the direction of a telescope is linked with the position of an indicator on a spherical star chart. The star chart maps the position of distant celestial objects, i.e. those objects which are so far from Earth that they appear ^Λfixed' in space. By moving the indicator to highlight a particular object on the star chart, the telescope is aimed at that object in the sky. The device contains a motor which rotates the apparatus to compensate for the Earth' s rotation to keep the telescope aligned with the selected object . WO 91/13421 describes a manually adjustable pointer, which allows a user to find certain celestial objects. GB 2120399 describes a device which can track an object which varies in position according to the Earth's position in its orbit around the Sun. The device is used to maintain a solar panel in alignment with the Sun.

SUMMARY OF THE INVENTION The devices described herein aid the understanding of celestial geography in two ways: 1) they provide an alternative reference frame in which viewers can observe their own motion (as the Earth spins on its axis and orbits the Sun) ; 2) they provide direct reference to the location of celestial objects in three-dimensional space. At its most general, the present invention provides a device having an indicator alignable with a predetermined celestial object, and display means for displaying information relating to said celestial object. The indicator may be an arm. The display means may be provided on the arm. In one aspect, the invention is based on a device which moves with one degree of freedom, a so-called ^Λsingle-axis device' . Such a device can indicate the position of ^λfixed' objects such as stars and galaxies. Several objects can be indicated with a single device, e.g. by having a plurality of indicators extending from a common core. The distance to the objects from the Earth my be displayed, e.g. by having a plurality of indicator arms whose respective lengths are directly proportional on the same scale to the distances from the Earth to their respective objects. The advantage of single-axis devices is that they rotate at a steady rate and therefore require neither computer control nor a complex system of gears to keep their indicator arm(s) aligned with the celestial object (s) - a single motor is all that is needed. The mechanical simplicity of such devices offers significant advantages in terms of cost, reliability and ease of use, especially when compared with dual-axis devices (see below) . In addition, single-axis devices can indicate the position of many objects simultaneously. The present application proposes a modification to the general single-axis device which allows it to track the position of relatively moving objects in addition to the position of ^λfixed' stars and/or galaxies. Previously, this was achieved using a two-axis device because tracking an object that moves in the sky needs an indicator with at least two degrees of freedom. However, the present device achieves this goal with minimal electronic and/or mechanical additions to a single-axis device such that advantages over dual-axis devices are maintained. According to a modification of the first aspect of the invention, there is provided a device having at least one indicator alignable with a respective predetermined celestial object, said at least one indicator being arranged to rotate about an axis to maintain alignment with said predetermined celestial object; and an array of selectively operable display indicia for indicating the position of an object in space which moves relative to said predetermined celestial object. The array may be movable with said at least one indicator. Preferably the predetermined celestial object is a ^Λfixed' star. The array preferably remains aligned with the fixed stars throughout the day. The selectively operable display indicia may be lights, actuators or other indicators which can be switched on or off individually. In use, the relatively moving object's position in relation to the fixed stars is calculated (either electronically or mechanically) and the member of the array that is closest to this calculated position is activated. Preferably, the relatively moving object is the Sun. The array is preferably a line of lights (or LEDs) encircling the rotation axis of the device. When the Sun is the object to be indicated, the line of lights is inclined at 23.5° to the axis, i.e. it is aligned with the ecliptic. The at least one directional indicator may be alignable with the signs of the zodiac. In this way, the movement of the Sun through the signs^' of the zodiac throughout the year could be demonstrated. A device that simultaneously indicates the position of the Sun in relation to the constellations of the zodiac and indicates its position in the sky may be used pedagogically . In addition to displaying the position in real time, such a device could be used to demonstrate other astronomical phenomena e.g. by accelerating its rotation to the extent that the passage of a day could be observed in a matter of seconds. For instance, many aspects of the passage of the seasons become clear when observed in this way. The device is preferably adjustable for different latitudes so that the difference between high and low latitudes is also easily observed. For instance, the long period of twilight in high latitudes in comparison with low latitudes can be understood easily by inspection. When used in real time, the device allows users to see approximately when and where the Sun will rise and set . The device may track a planet instead of the Sun as these, to a close approximation, also follow the ecliptic. In a second aspect, the present invention relates to a dual-axis device. Accordingly, there is provided a direction-indicating device having a directional indicator e.g. arm movable with at least two degrees of freedom for alignment with a predetermined object, and display means for displaying information relating to said object. The display means may show the distance from the device to said object. Preferably, the device includes a base or support on which the directional indicator e.g. arm is movably mounted. The base may contain a motor for driving movement of the indicator. The motor may be computer controlled. The device is preferably computer controlled, and may include one or more of: storage means for holding data about the location of one of more objects relative to a predetermined point, the predetermined point preferably being a point on the Earth's surface with which the location of the device can be compared; input means for instructing the device about its geographical location and/or orientation in order for it correctly to locate said object; selection means for a user to select an object for the device to indicate; and control means operable in conjunction with a motor to cause the indicator arm to be aligned with a predetermined object, the control means including coordinate-supplying means (e.g. a computer program or database) which provides information about the location of said predetermined object at a given time, said information preferably being obtainable from data held by the storage means. The device of the second aspect may be aligned with objects on the Earth's surface which move relative to it, or objects in space which move relative to the ^λfixed' background stars, e.g. solar system objects such as planets, or man-made objects such as satellites. Preferably, the device is arranged to track these relatively moving objects. Preferably, the indicator e.g. arm is rotatable about two perpendicular axes. Any direction may then be defined by two coordinates representing angles from a predetermined origin about these axes. Preferably, the coordinate supplying means provides two angular coordinates to the control means. By supplying coordinates to the control means at regular intervals, and updating the position of the indicator accordingly, the indicator is able to track and show the movement of objects in space. The display means may be an electronic display, e.g. an array of LEDs . Information relating to the object being indicated is provided to the display means e.g. from a computer program. The control means may be adapted to provide information to the display means at the same time as driving the motor. Preferably, the information is or includes the distance from the Earth to the object in space. The name of the object pointed to may be displayed. Typically, this distance is continuously changing. Updates, e.g. regular updates of the display means are preferred. Information may be given alphanumerically . Alternatively or additionally, the display means can indicate distance by altering the length of the indicator arm. The device may have a plurality of directional indicators e.g. arms to point at a plurality of objects in space. Such a device gives information about the relative movement between the two indicated objects. The control means preferably includes a central processor programmed to control the drive of each arm in a way that tracks the object according to stored information about its path. The processor may be updated or re-programmed periodically or irregularly, e.g. by regularly downloading orbital elements of satellites from the Internet or by receiving location information from a moving object on Earth by mobile phone. The control means is preferably housed in the base. The processor may also hold the data corresponding to the distance of the object from the Earth. For the avoidance of doubt, it is emphasised that the arm preferably operates as a display pointer for the purposes mentioned above rather than as a light collector, i.e. the indicator arm does not constitute or contain a telescope, heliostat or similar device. The size of the device is not particularly limited, but small versions, e.g. for use on a desktop, are envisaged. In development of the above-described devices, the inventor has become aware of problems associated with using display pointers which both indicate the position of an object and display a sign labelling said object. The legibility of a sign depends on its orientation with respect to the viewer. Signs are most legible when viewed face on. The quantity that determines legibility is the area of the projection of the sign in the viewer's visual field. The area of the sign itself does not change but the area of its projection varies according to the sign's orientation with respect to the viewer. Two angles relating the sign to the viewer's gaze affect the area of the projection. The first angle is the direction that the sign is pointing towards, relative to the -viewer's gaze. The second angle is the rotation of the sign about its own axis, relative to the viewer's gaze. In Fig. 1 these angles are labelled as α and β respectively relative to two axes x, y that are mutually perpendicular to one another and to the direction of the viewer's gaze. If α and β are both 0, the viewer sees the sign face on, i.e. the sign is pointing at 90° to the viewer's gaze, and its orientation along its axis is such that the plane face of the sign is perpendicular to the viewer's gaze. The problem occurs when the absolute value of either α or β is close to 90°. In this case, the area of the projection of the sign is at a minimum and so the sign will be unreadable. When the absolute value of both α and β is close to either 0° or 180°, the area of the projection of the sign is at a maximum and so the sign will be easy to read. For signs that indicate compass headings to terrestrial locations, the angle β is constant for every direction. However, the angle β may vary in signs that indicate elevation (angle above or below the horizon) as well as azimuth (compass heading) . For signposts that rotate throughout the day, such as the single-axis devices described in this patent application, the variation may become a significant problem. Signs may look ^Λtwisted' or upside-down much of the time. The effect is particularly pronounced for signposts located at low latitudes. For some combinations of celestial object, signpost location and viewer location, signs may be illegible for long periods. A solution to the above problem is proposed by a further aspect of or modification to the present invention, which provides a device having a base with an indicator arm mounted thereon, the indicator arm being movable relative to the base to be alignable with a predetermined celestial object, and display means on said arm for displaying information relating to said celestial object, wherein the device is arranged to move said arm to track said predetermined celestial object, and said arm is rotatable about its own axis to maintain the display means in an upright position. Rotating the signs that make up a single-axis device about their own axis can counteract the rotation of the assembly itself throughout the day. Such rotation can maintain the sign's correct ^Λup/down' orientation and maximise the average area of projection. Preferably, the base includes a hub, e.g. a spherical hub, on which the indicator arm is preferably mounted via a radially projecting shaft. The arm preferably contains bearings to allow it to rotate around the shaft. The correct up/down' orientation may be maintained by making the torque due to gravity (about the shaft) from the ^Λbottom' half of the indicator greater than the torque from the ^Λtop' half of the sign, i.e. the lower part of the display means is weighted down. This way, gravity keeps the sign ^upright' . In a preferred embodiment of the device, the top half and the bottom half of the indicator are geometrically symmetrical, but the bottom half is made more massive e.g. by using a denser material. Alternatively or additionally, the difference in torque between the top of the sign and the bottom may be achieved by locating the axis of the shaft above the indicator's longitudinal axis, i.e. having the point where the shaft meets the indicator arm offset from the indicator arm's longitudinal axis. In a preferred embodiment, the end of the indicator arm nearest the hub is moulded so as to have the same curvature as a surface of the hub. This way it remains flush with the hub throughout its revolution.

BRIEF DESCRIPTION OF THE DRAWINGS Examples will now be described with reference to the accompanying drawings, in which: Fig. 1 is a diagram illustrating the two angles at which a sign may be oriented relative to the direction of a viewer's gaze, as discussed above; Fig. 2 shows a device with one degree of freedom that indicates the position of fixed stars', i.e. objects beyond the solar system such as stars and galaxies; Fig. 3 shows a device with two degrees of freedom that indicates the location in space of an object that moves with respect to the fixed stars, e.g. objects in the solar system and man-made objects; Fig. 4 shows a schematic picture of the device of Fig. 2 connected to a remote user terminal; Fig. 5 shows an alternative example of the device shown in Fig. 3; Fig. 6 shows a side view of a single-axis device with an array of lights distributed around its axis; Fig. 7 shows a perspective view of the device of Fig. 6; Fig. 8 shows a single axis device with a rotatable sign post; Fig. 9 shows a close up of the connection between the sign post and hub in the device of Fig. 8.

DETAILED DESCRIPTION; FURTHER OPTIONS AND PREFERENCES Fig. 2 illustrates a typical example of a fixed stars' single-axis device 100. A plurality of elongate rods 102 (made transparent material, e.g. Perspex^®) of various lengths project radially from a central sphere 104, which has a light-source (not shown) at its centre. Light is guided through the rods making their ends glow brightly. The central sphere 104 is mounted on a support rod 106 which extends from a base 108 at an angle φ. The base 108 includes a motor assembly 110 which drives the support rod 106 to rotate the sphere 104. In use, the device is arranged so that the support rod 106 points north and the angle φ with the horizontal is the same as latitude of the location (e.g. at the Equator the shaft is parallel to the ground, at the North Pole it points straight up, in London the angle is 51°) . The orientation of the rods 102 is then adjusted to correspond to fixed stars, and then the motor assembly 110 is started so that the rods 102 remain aligned with their respective stars. To achieve this, the period of rotation is 23 hours 56 minutes (one sidereal day) . This can be thought of as the central portion of the kinetic sculpture. The outer portion consists of predetermined star systems. Each rod 102 corresponds to a star system and points towards it. The location on the central sphere 104 from which each rod 102 radiates corresponds to the equatorial coordinates (right ascension and declination) of the object it is pointing to. The length of each rod 102 corresponds (in linear proportion) to the distance between the predetermined star and the Sun. As the central sphere 104 rotates, the rods 102 remain in line with their corresponding stars. Thus, the sculpture does not rotate with respect to the galaxy as a whole. In Fig. 2, there are 48 rods pointing to the 48 closest star systems to Earth that are visible to the naked eye. The central sphere 104 is hollow to contain lights or other electronic devices, which served by power transmitted through the support rod 106. The motor assembly 110 consists of bearings, gears and a motor. A variety of motor configurations could serve this purpose including ^xoff the shelf commercial telescope drives. Driven by the motor via the support rod 106, the assembly of rods 102 rotates once every sidereal day as indicated by the arrow . Any ^vfixed stars' could be indicated by such a device, e.g. stars, nebulae, or galaxies. In another embodiment of the device, for instance, it is the Local Group of galaxies that is referenced. The Local Group constitutes the 40 or so galaxies closest to our own Milky Way that form a unit through mutual gravitational attraction. The length of the arms again corresponds to the distance but this time each arm is terminated by a disc representing the galaxy it corresponds to. Fig. 3 shows an embodiment of a dual-axis device 200. The device 200 consists of an indicator arm 202 with an electronic display 204 built into it. The electronic display is made up of an array of LEDs, and indicates the name of and the distance to the object in space being pointed to. The direction of the arm 202 is controlled by two motors 206, 208. For example, one motor 206 adjusts altitude and one motor 208 adjusts azimuth. Alternatively, with an equatorial arrangement, one adjusts declination and the other right ascension.

As with the ^λfixed' objects device, there is a wide range of options for configuring and controlling the motors including ^Λoff the shelf solutions in the form of commercial telescope drives. The electronic display 204 is driven by an embedded system with code for calculating distances to a variety of predetermined celestial objects stored on an EPROM or similar device. To calculate the direction to a given object, the device needs to know where it is on Earth and what time it is. These can be programmed in e.g. for devices whose geographical location is fixed, or they can be input manually by the user, thereby allowing the possibility of a portable device. The geographical location of the device 200 can be input e.g. with a keypad, and the device 200 may also have a clock signal (e.g. an atomic clock transmission) to keep track of time . The device shown in Fig. 3 may be used in a system 300 schematically illustrated in Fig. 4. The system 300 consists of a operating station 302 with a touch screen Windows PC 304 running an application that displays information regarding the sizes and positions of various celestial bodies. The operating station 302 is mounted on the ground and shaped so that the touch screen 304 is easy to operate for a standing viewer. The operating station 302 communicates with a pointer 306, which is movable, mounted on base 310. The pointer 306 is of the type illustrated in Fig. 3, i.e. it is movable about two axes to indicate the approximate direction to a predetermined celestial body. The pointer 306 includes a display 308 on both of its sides which shows the name of and distance to the predetermined celestial object. The display 308 comprises a plurality of LEDs . The operating station 302 contains a processor that works with the application such that when a user selects a celestial body e.g. from choices presented using the touch screen, information is sent from the operating station 302 to the pointer 306 to instruct the pointer

306 to move to a certain orientation, and to instruct the display 308 to show certain information. To track the selected object, the application produces a serial data stream containing information to adjust the position the pointer and update the display 308. The information for instructing the pointer 306 includes two angular positions defined with respect to a predetermined origin. The angles are calculated using an algorithm appropriate to the object selected. The algorithms are similar in that all of them require just two inputs: the time and the location of the device. The location of the device constitutes three inputs, e.g. a longitude, latitude and altitude. The algorithms then used stored values of various physical quantities associated with the object to calculate the position of the object relative to the device. For planets the algorithms used .are derived from a theory of celestial dynamics known as VSOP87. For satellites, the position is calculated more simply using only the satellite's keperlian elements. These elements need updating regularly to account for limitations of the algorithm used and spacecraft manoeuvres in orbit. For interplanetary craft, the position is calculated by interpolating from an array of known positions and predicted positions in a heliocentric coordinate system. In general, there is a wide variety of algorithms and other methods of returning an object's position at a given time and location. The actual method employed is determined by convenience, the requirements of precision and the availability of data. Since the speed of motion required for tracking objects in space is relatively slow, stepping motors provided in base 310 drive the pointer 306 via a gearbox. Stepping motor controllers are simpler to design than servo motors. The power supply to the pointer 306 is low voltage DC. A single power supply can serve both the motor in the base 310 unit and the display 308. An off-the-shelf mains power supply is mounted in the operating station 302. System 300 can operate outdoors. If it is, the base 310 needs to enclose the motor in an appropriate weather- proof enclosure. For maintenance purposes the motor control unit should be readily demountable. Fig. 5 shows an alternative embodiment of the device of Fig. 3, including a schematic view of the working of the motor. The device 400 of Fig. 5 has twin arms 402, 403 which are pivotable about an axle 404 mounted in housing 406. The housing 406 comprises a base 408 with a tip 410. The axle 404 is provided in the tip 410. The tip 410 is rotatable relative to the base 408. This rotation is controlled by a first motor 412 mounted in the base 408. The first motor 412 drives a shaft 413, which is connected to the tip 410 via a gearbox 414 and clutch 416. A bearing 418 is provided between the tip 410 and the base 408. The first motor 412 therefore controls the azimuth (bearing) of the pointers 402, 403. The tip 410 encloses a second motor 420. The second motor 420 drives a shaft 421, which is connected to the pointers 402, 403 via a gearbox 422 and clutches 424. The second motor 420 therefore controls the elevation of the pointers 402, 403. In a further, unillustrated embodiment, two single- armed pointers are placed side by side, or one system with two independent indicator arms is provided. One arm indicates the direction and distance to the Moon and the other points to the Sun. The arms in this example are long tetrahedra (three sided pyramids) . Each of the arms' three sides carries an electronic display allowing the distance to be read from any angle. If placed near tidal waters, this arrangement would help to elucidate the mechanism of the tides. In another envisaged embodiment, a single arm indicates the distance and direction of a moving Earth based vehicle such as a ship. A GPS (Global Positioning System) receiver on the ship records its location. The data are then transmitted electronically to a pointing device, which may, for instance, be located in the ship owners' headquarters. The direction indicated is the heading of the great circle linking the ship and the location of the pointing device. (A ^λgreat circle' is the path of the shortest distance between two points on a sphere.) The distance indicated is the distance between the ship and the device along this heading. Because the ship is confined to the surface of the Earth, only one motor is needed to orient the pointer. Alternatively, the pointer could be located on the ship itself. The distances and directions indicated could be those to cities and ports chosen by passengers from a list. Figs. 6 and 7 illustrate another example of the present invention. These drawings show a device 500 which is rotatable about a single axis but has an array of lights 502 which are individually illuminable to indicate the position of the Sun. A motor 508 drives an axle 510 at a rate of one revolution per sidereal day.

The motor 508 and axle 510 are mounted on a first annular disc 503, which is mounted on a upright rod 506 attached to a base 504. As with all single-axis machines described herein, the axle 510 is oriented north-south and inclined at an angle φ equal to the latitude of the location. Attached to the axle 510 is an assembly 512 consisting of a representation of the zodiac constellations. The assembly 512 is rotatable in the centre of the first annular disc 503. The array of lights 502 is arranged through the middle of the assembly 512 as a line of lights. At any time, only one of these lights is on. This light corresponds to the position of the sun in the zodiac and in the sky at the device's location. The assembly 512 is attached to the axle but is rotated by 23.5° about a line joining the vernal and autumnal equinox on the representation of the zodiac. This angle of inclination is indicated as θ in Fig. 6. The first annular disc 503 indicates the position of the zenith. A second annular disc 514 may also be provided to indicate the position of the horizon. These discs that make the process of relating the assembly 512 to the objects it represents easier. ^' In the embodiment shown in Figs. 6 and 7, the object chosen is the Sun, which takes a year to cycle through the fixed stars, i.e. the zodiac constellations. From Earth, the Sun is seen to move through each constellation of the zodiac in turn along a path called the ecliptic, which is also the path that the sun appears to travel across the sky each day. On the device 500, the ecliptic is indicated by the array of lights 502, which encircles the axis of rotation along a path inclined at 23.5° to the plane perpendicular to the axis. 23.5° is the angle at which the Earth's axis is inclined with respect to the ecliptic. As the assembly 512 rotates throughout the day, this line of lights remains aligned with the ecliptic. The Sun's position along the ecliptic (its ^■•ecliptic longitude' ) is indicated by switching on the light whose position on the circle (relative to the rotational centre) most closely indicates the position of the Sun. As the assembly 512 rotates throughout the day, this light will remain aligned with the sun. As the sun continues on its journey throughout the year lights are turned on in turn to indicate its current position in the both the zodiac and the sky. The array could contain 365 lights so that a new light would be switched on approximately every day.

Alternatively, the array could contain 52 lights so that a new light would be switched on approximately every week. A circuit (not shown) is used to determine which light is to be activated. The circuit is based on a simple calendar chip of the type used in many electronic devices. Added to this is some logic circuitry that closes a circuit for the appropriate light according to the date. The implementation may involve a microprocessor, although the actual implementation of the circuit would depend on the protocol of the calendar chip and the type of logic circuitry used. As an example of the type of logical test involved, we can consider an implementation in which the calendar chip passes an index corresponding to the day number (days since the first of January) . Each light is then controlled by a circuit that performs the following logical test: [day index * (number of lights ÷ 365)] AND [light-index] . This closes the circuit for the light whose index number corresponds to the correct fraction of ~ the year. For example, if the number of lights were 52 and the date were 5 February, then the day index would be 36 and therefore [day index x (number of lights ÷ 365)] = 5, so light number 5 would be on and all the others off. In principle, the same result could be achieved with a clockwork calendar in which the rotation of a metallic object about an armature makes and breaks individual light circuits in turn. Because the Earth's orbit is not very eccentric (i.e., it approximates to a circle) the rate at which the Sun appears to move through the zodiac is constant, to a first level of approximation. This means that the Sun's position can be indicated fairly accurately by equally spaced lights switching after equal time intervals (as described above) . However, if higher precision were needed, it is possible to account for the yearly variation in the Earth's angular velocity by one of two methods, or by a combination of both: 1. The lights could be spaced around the circle in such a way that their location reflects the position of the Sun at equal time intervals. 2. The time interval between each switch could be varied to reflect the time at which the Sun will be at a fixed position. Figs. 8 and 9 show a further example of the present invention. This example is a further alternative to the single-axis device shown in Fig. 2. Here, device 600 comprises a shaft 602 attached to the ground and oriented to point due north whilst making an angle φ with the ground. Angle φ is the same as the angle of latitude of the geographical location of the device 600. A spherical hub 604 is rotatably mounted on the shaft 602. A motor (not shown) enclosed in the hub 604 drives it to rotate about an axis defined by the shaft 602. A signpost 606 is attached to and extends away from the hub 604 to point at a fixed celestial object. The signpost 606 has a plane surface 608 on which information relating to the object being pointed at is displayed. The signpost 606 is attached to the hub 604 by a rod 612 which protrudes radially from the hub 604. The signpost 606 is freely pivotably mounted on the rod 612. The location at which the rod 612 is attached to the end of the signpost 606 is offset from the longitudinal axis of the signpost 606 so that the centre of gravity of the signpost is below the pivot axis. This means that the signpost 606 will always hang so that the plane surface 608 is substantially upright. In other words, the information displayed on the plane surface 608 is always at its most readable orientation, i.e. the bottom part of the display is below the top part. The edge 610 of the signpost 606 closest to the hub 604 is curved to fit the shape of the hub's surface. The invention may include any variations, modifications or alternative applications of the above embodiment, as would be readily apparent to the skilled person without departing from the scope of the present invention in any of its aspects.

Claims

CLAIMS :

1. A directional display device having a directional indicator alignable with a predetermined celestial object, and display means for displaying information relating to said celestial object.

2. A device according to claim 1, wherein the indicator is an elongate arm.

3. A device according to claim 2, wherein the display means is provided on the arm.

4. A device according to any one of claims 1 to 3, wherein the information includes the distance to and/or the name said object.

5. A device according to any one of claims 1 to 4, wherein said object is selectable from a plurality of celestial objects, the device having a stored menu of information relating to each of said plurality of objects .

6. A device according to any one of claims 1 to 5 which is programmed to maintain alignment with said object over time.

7. A device according to any one of claims 1 to 6 having a plurality of indicators extending from a common core.

8. A directional display device having at least one indicator alignable with a respective predetermined celestial object, said at least one directional indicator being arranged to rotate about an axis to maintain alignment with said predetermined celestial object; and an array of selectively operable display indicia for indicating the position of an object in space which moves relative to said respective predetermined celestial object .

9. A device according to claim 8 having control means for controlling which of the selectively operable indicia is to be activated.

10. A device according to claim 9, wherein the control means includes a processor for calculating the relatively moving object's position in relation to the fixed stars, the control means being arranged to activate the one of the selectively operable indicia that is closest to said calculated position.

11. A device according to any one of claims 8 to 10, wherein the array is movable with said at least one indicator .

12. A device according to any one of claims 8 to 11, wherein the respective predetermined celestial object is a fixed' star.

13. A device according to any one of claims 8 to 12, wherein the selectively operable indicia are lights.

14. A device according to any one of claims 8 to 13, wherein the relatively moving object is the Sun.

15. A device according to any one of claims 8 to 19, wherein the at least one indicator is alignable with the signs of the zodiac.

16. A direction-indicating device having an indicator arm movable with at least two degrees of freedom for alignment with a predetermined object, and display means for displaying information relating to said object .

17. A device according to claim 16, wherein the display means shows the distance from the device to said object .

18. A device according to claim 16 or 17 including a base or support on which the indicator arm is movably mounted, said base containing a motor for driving movement of the indicator arm.

19. A device according to any one of claims 16 to 18 including storage means for holding data about the location of one of more objects relative to a predetermined point .

20. A device according to claim 19, wherein the predetermined point is a point on the Earth's surface.

21. A device according to claim 19 or 20 including input means for instructing the device about its geographical location in order for it correctly to locate said object.

22. A device according to any one of claims 19 to 21 including control means operable in conjunction with a motor to cause the indicator arm to be aligned with a predetermined object, the control means being arranged to compare the location of the device with said predetermined point.

23. A device according to claim 22, wherein the control means including coordinate-supplying means which provides information about the location of said predetermined object at a given time, said information being obtainable from data held by the storage means.

24. A device according to any one of claims 16 to 23 including selection means for a user to select an object for the device to indicate.

25. A device according to any one of claims 16 to

24, wherein the indicator arm is rotatable about two perpendicular axes.

26. A device according to any one of claims 16 to

25, wherein the display means is an electronic display.

27. A device according to claim 26, wherein the display means is an array of LEDs.

28. A device having a rotatable hub with an indicator arm mounted thereon, the hub being rotatable to allow the indicator arm to be alignable with a predetermined celestial object, and display means on said arm for displaying information relating to said celestial object, wherein the device is arranged to move said arm to track said predetermined celestial object, said arm is pivotable relative to the hub, the centre of gravity of said arm being below the pivot axis.

29. A device according to claim 28, wherein the hub is spherical and the indicator arm is attached to it via a radially projecting shaft.

30. A device according to claim 29, wherein the point at which the radially projecting shaft connects to the arm is offset from the longitudinal axis of the arm.

31. A device according to any one of claims 28 to 30, wherein the lower part of the arm is heavier than the upper part.