Timepiece having a star display
BACKGROUND OF THE INVENTION 
     The present invention relates to an electronic timepiece having a star display. 
     There has been proposed an electronic wristwatch having a star map on a star map disk which is rotatably mounted under a dial of the watch. The dial has an opening through which a part of the star map is disclosed to display constellation figures. The disk is simultaneously driven with hour, minute and second hands by a single power source and is adapted to rotate one revolution per one sidereal day. 
     In such a watch, if an additional function for discerning twilight is provided, it would be very useful not only for astronomical observation, but also for usual outdoor activity, since outdoor activity is possible in the twilight. 
     However, it is difficult to provide a device which discerns the twilight accurately. 
     The interval of time, during which the sun is between the horizon and 6.5 degrees below the horizon, is called a civil twilight. The interval of time, during which the sun is between 12 and 18 degrees below the horizon, is called an astronomical twilight. In the civil twilight, the brightest planet can be observed. 
     In order to discern twilight conditions, an accurate solar position in the celestial sphere (right ascension and declination), the latitude of the observation point, and the local sidereal time are necessary. However, a moving rate of the sun in the ecliptic in the celestial sphere is not constant because the revolution orbit of the earth is an ellipse. Further, the ecliptic crosses the celestial equator at an angle of about 23.5 degrees and the declination of the sun has seasonal variation. Therefore, accurate solar position cannot be indicated, for example, by means of a hand uniformly rotating one rotation per day. 
     Further, in the prior art, stars and constellation figures in a northern part of the northern celestial hemisphere, namely circumpolar stars and constellations around the stars, are not included. Accordingly, such a watch does not meet requirements of users. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an electronic timepiece having star maps for widely displaying stars and constellations in a celestial sphere. 
     Another object of the present invention is to provide an electronic watch having a rotary disk for a star map in which a gear train for driving the disk is simple in structure. 
     A further object of the present invention is to provide an electronic watch by which the twilight can be discerned. 
     According to the present invention, there is provided a timepiece having a case, an hour wheel carrying an hour hand, a minute hand, a dial, means for driving the hands, a star map disk disposed coaxially with an axis of the hour hand wheel and rotatably mounted in the case, gear train means for transmitting rotation of the hour wheel to the star map disk to rotate the disk one revolution per one sidereal day, and a first star map provided on the star map disk. 
     The first star map includes bright and constellation figures in a part of the celestial sphere which are selected from visible stars at an observation point on the earth and are arranged around one of the celestial poles which is disposed at the center of the star map disk. 
     An opening is formed in the dial for disclosing a part of the first star map and for defining a range of the first star map which is visible when observing. Further, a second star map having stars in a range other than that of the first star map is provided in the watch. 
     In an aspect of the present invention, the opening of the dial has a substantially crescent shape including a line showing a horizon and a line defined about the zenith, the first star map includes a part of the southern part of the celestial sphere and the second star map includes circumpolar stars. Further, the dial has two openings including lines for defining a twilight. 
     These and other objects and features of the present invention will become more apparent from the following description with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of an electronic wristwatch having a constellation display according to the present invention; 
     FIG. 2 is a plan view of a rotary disk having a first star map provided in the watch; 
     FIG. 3 is a plan view of a back of the watch in FIG. 1 having a second star map; 
     FIG. 4 is a sectional view partly showing a gear train relating to the star display; 
     FIG. 5 is a schematic plan view of a part of the gear train; 
     FIG. 6 is a plan view showing a second embodiment of the present invention; 
     FIG. 7 is a plan view showing a further embodiment of the present invention; and 
     FIG. 8 is a sectional view of a part of FIG. 7. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, an electronic wristwatch 1 of an analog time-display comprises a watchcase 1a, a star map disk 2 having a first star map printed thereon and rotatably mounted in the watchcase 1a, a dial 6 secured to the watchcase 1a over the star map, and a first star display 7. An hour hand 3, a minute hand 4 and a second hand 5 are coaxially provided in the center of the dial 6. The dial 6 has twelve markers 6a for representing from one to twelve hours by the hour hand 3, an opening 6b for displaying a part of the star map, and a window 6c having a function for discerning twilight. Thus, first star display 7 is composed of the star map on the disk 2, opening 6b and window 6c of the dial 6. Consequently, a part of the star map on the disk 2 disposed under the dial 6 can be seen through opening 6b and window 6c. A marker 6a&#39; represents 12 o&#39;clock and the position of the meridian. 
     Referring to FIG. 2, the star map shows a southern part of the celestial sphere with respect to the zenith at an observation point at a north latitude of 35°. This part is a range from -55.6 to +35 degrees declination, which takes into account the horizontal refraction of 0.6 degrees caused by the atmospheric refraction. The star map 2 has main bright stars, various constellation figures 2a, the ecliptic 2b indicated by a broken line, and the Milky Way 2c, which are selected from visible stars in the southern part of the celestial sphere and circularly arranged around the center of rotation of the disk 2 (corresponding to the celestial south pole). Namely, visible stars near the celestial south pole are disposed adjacent to the center of the disk 2 and stars passing near the zenith are disposed in a circumferential portion of the disk 2, disposing lines of declination at equidistance. 
     The disk 2 further has numerals 2d consisting of 0 to 23 provided in order of the counterclockwise direction on the outermost periphery thereof for indicating times of the right ascension. In the broken lines of the ecliptic 2b, there are 36 breaks between lines. Each of breaks represents a solar position 2e at noon (Japanese standard time) on the first, eleventh and twenty first of each month in the mean year. That is, three broken lines represent one month, so that each of numerals 2f from 1 to 12 for indicating twelve months is disposed at a position corresponding to the first of each month. 
     As shown in FIG. 1, the opening 6b formed in the dial 6 has a substantially crescent shape and curved edges for defining the opening 6b comprises an arc 6d showing +35 degrees declination of a northern limit in the first star display 7 and a horizon curve 6e showing the horizon in latitude 35° N. Namely, the opening 6b resembles a southern sky in shape at the observation point. Along the horizon curve 6e, characters 6f of NE, E, SE, S, SW, W, NW, each showing direction, are provided on the dial 6. The window 6c is defined by the arc 6g of +35 degrees declination, a small arc 6h and curve 6i. The arc 6h shows -55.6 degrees declination and curve 6i shows the definition of an astronomical twilight at 108 degrees zenith distance at a latitude of 35° N. 
     In the first star display 7, the time of right ascension 2d corresponding to the marker 6a&#39; represents the sidereal time. Further, a solar position on the celestial sphere on a desired date is indicated by a corresponding date on the ecliptic 2b. The twilight is indicated as follows. When the solar position on the desired date on the ecliptic 2b is positioned within the window 6c, the twilight has not yet occurred. It is a day time when the solar position is within the opening 6b. During the twilight, the solar position is hidden under the dial 6 between the horizon 6e and the limit line 6i. 
     Referring to FIG. 3, a second star display 8 having a star map of the northern sky is provided on a back 9 of the wristwatch 1. The second star display 8 comprises a second star map 8a, and peripheral portions for the times of the right ascensions 9c and date 9d. The star map shows the celestial sphere from +35 degrees declination to the north celestial pole, which is not displayed in the first star display 7 and has main bright stars, various constellation figures 9a and the Milky Way 9b in the same manner as the first star display 7. The times of right ascensions 9c are composed of indexes and numerals 0, 3, 6, 9, 12, 15, 18 and 21 disposed between indexes at every angle of 45 degrees. By angularly disposing the watch 1 such that the time of right ascension 9c corresponding to the sidereal time read from the time of the right ascension 2d in the first star display 7 at a time is oriented to the celestial meridian, positions of the constellations in the northern celestial sphere are indicated by the star map. The date portion 9d has numerals showing twelve months, each of which has graduations for days corresponding to the times of right ascensions 9c for indicating the sidereal time at noon in the mean year in the Japanese standard time longitude (at longitude 135° E). 
     Referring to FIGS. 4 and 5, a gear train particularly relating to the first star display 7 comprises a fourth wheel (or second hand wheel) 10 carrying the second hand 5, a center wheel 11 carrying the minute hand 4, and an hour wheel 12 comprising a gear 12a and a pinion 12b and carrying the hour hand 3. The hour wheel 12 is provided between a plate 15 and a bridge 17 so as to be slightly moved in the axial direction and held by a spring washer 20 provided for preventing the influence of the backlash of the gear train. A minute wheel 13 is engaged with a pinion 11a of the center wheel 11, and a pinion 13b of the minute wheel 13 is engaged with the hour wheel gear 12a. The hands 3, 4 and 5 are driven by an electric motor 27 through a gear train 28. 
     A regulating wheel 14 comprises a gear 14a engaged with the hour wheel pinion 12b, a pinion 14b engaged with a star map disk wheel 18, and a friction engaging portion 14c provided between a boss 14d of the pinion 14b and gear 14a. The regulating wheel 14 is rotatably mounted on a pin (or shaft) 16 secured to the plate 15 and supported by the bridge 17. The friction engaging portion 14c is adapted to slip at a load larger than a predetermined torque. The star map disk wheel 18, to which the star map disk 2 is secured, is rotatably mounted on the shaft of hour wheel 12 and supported between the bridge 17 and a washer 19 with a slight axial play. An intermediate correcting wheel 21 comprises a gear 21a meshed with the regulating wheel pinion 14b and a pinion 21b secured to the gear 21a and engaged with a clutch wheel 22. The pinion 21b is rotatably mounted on a pin 23 secured to the plate 15 and supported by the bridge 17. As is well known, the clutch wheel 22 is axially slidably mounted on a stem 25 connected to a crown 24. The crown 24 is normally positioned at the innermost position 24a and is adapted to be axially pulled to the intermediate position 24b and the outermost position 24c as shown in FIG. 1. 
     When the crown 24 is rotated at the normal position 24a, the stem 25 is rotated idle so that the clutch wheel 22 is not rotated. In the intermediate position 24b, the stem 25 is positioned to be coupled with the clutch wheel 22, so that the rotation of the crown 24 is transmitted to the intermediate correcting wheel 21 through the stem 25 and the clutch wheel 22. At the outermost position 24c, the stem 25 is released from the clutch wheel 22. 
     The reduction ratio from the fourth (or second hard) wheel 10 to the center wheel pinion 11a is set to 1/60 as is well known. The reduction ratio from the pinion 11a to the hour wheel 12 through the minute wheel 13 is 1/12, so that the hour wheel 12 rotates one revolution per twelve hours. 
     On the other hand, the star map disk 2 is rotated one revolution per one sidereal day as described below. One mean sidereal day, that is, one mean rotation of the earth with respect to the vernal equinox is equal to the mean solar time of 23 hours(h) 56 minutes(m) 4.091 seconds(s). In order to equalize a rotational speed of the star map disk 2 to an apparent mean rotational speed of the celestial sphere, the star map disk wheel 18 should rotate 1.0027379 (24 h/23 h 56 m 4.091 s) rotation per day. Therefore, the gear ratio of the gear train from the hour wheel 12 to the star map disk wheel 18 must be set so that the wheel 18 may rotate at a speed which is very close to the above speed. In the range of the number of teeth between 12 and 73, when the hour wheel pinion 12b is 3N teeth and the regulating wheel pinion 14b is 61 teeth, the most preferable number of the regulating wheel gear 14 a being 5N teeth and that of star map disk wheel 18 being 73 teeth (wherein N is an integer between 3 and below 14). In this condition, the wheel 18 rotates 1.0027397 per day and an accumulated difference in a year between the disk 2 and the apparent mean rate of the celestial sphere is only 0.2 degrees. 
     In this embodiment, the hour wheel 12 is made by combining the gear 12a and pinion 12b which are separately manufactured. Accordingly, the number of teeth of the pinion 12b can be properly determined regardless of the engagement with the minute wheel 13. Further, as shown in FIG. 5, since the regulating wheel 14 is provided between the hour wheel 12 and star map disk wheel 18, the star map disk 2 secured to the wheel 18 is rotated in the same direction as the hour hand 3. In other words, the star map on the disk 2 rotates in the same direction as the rotation of the southern part of the celestial sphere around the celestial south pole. 
     Describing operations for an initial set of the star map disk, the crown 24 is pulled to the outermost position 24c and rotated to coincide the hour hand 3 and the minute hand 4 with the position of 12 o&#39;clock. Then the difference of time in longitude with respect to a standard time is corrected. For example, the culmination of the fixed atar and sun in Tokyo is about 19 minutes earlier than that of the standard time at longitude 135° E. Thus, the hour and minute hands 3 and 4 are set 11 h 41 m in the morning. Next, the time of the right ascension 9c corresponding to the date 9d of the second star display 8 is read out. For example, on Aug. 30 in Tokyo, the sidereal time is approximately 10 h 32 m at noon in the mean year in longitude 135° E. 
     The crown 24 is pushed to the intermediate position 24b, the stem 25 is coupled with the clutch wheel 22. The clutch wheel 22 is rotated in cooperation with the crown 24 through the stem 25 and the pinion 14b of the regulating wheel 14 is rotated through the intermediate correcting wheel 21. During the transmission, the friction engaging portion 14c slips, so that the regulating wheel gear 14a is not rotated and hour and minute hands 3 and 4 continue to rotate. Thus, the star map disk 2 mounted on the wheel 18 which is meshed with the regulating wheel pinion 14b is rotated to a desired position. In the case of the above example, the 10 h 32 m position of the right ascension 2d on the disk 2 coincides with the marker 6a&#39; by rotating the crown 24. 
     The crown 24 is again pulled to the outermost position 24c. By rotating the crown, the hour and minute hands 3 and 4 and the star map disk 2 are rotated, and the hands are positioned for indicating the actual time. Then the crown 24 is pushed to the innermost position 24a to start the watch. As shown in FIG. 1, when the actual time is at 10 h 9 m 39 s in the morning on Aug. 30 in Tokyo (corrected difference of time in longitude is +19 m), the local sidereal time is approximately at 9 o&#39;clock. 
     Thus, the star map disk 2 rotates together with hour hand 3, thereby indicating positions of stars and sun in the sky within the opening 6b of the dial 6. 
     Further, by rotating only the star map disk 2 with the crown 24 at the intermediate position 24b, the time period from sunrise to sunset and twilight is indicated without stopping the hour, minute and second hands 3, 4, 5. More particularly, the star map disk 2 is rotated to coincide with the solar position 2e of a particular day with the horizon curve 6e at the east side, which indicates the sunrise condition. Nominal sidereal time is read, for example at the marker 6a&#39;. Thereafter, the disk 2 is rotated to coincide the solar position 2e with the west side line of the horizon curve 6e, so that the nominal sidereal time at the sunset is indicated at the marker 6a&#39;. Thus, the difference of time between both the nominal sidereal times corresponds to the time of period from sunrise to sunset of the particular day. Further, the difference of time between the nominal sidereal time when the solar position 2e passes the west side line of the horizon curve 6e and the nominal sidereal time when the solar position passes the astronomical twilight definition line 6i corresponds to the astronomical twilight of the day. 
     It will be understood that the astronomical twilight definition line may be replaced with a line defining the civil twilight. Although the star map shows a southern part of the celestial sphere around the celestial south pole, a star map showing a northern part of the celestial sphere around the celestial north pole may be provided. In such a watch the rotational direction of the star map disk 2 is inverted by inserting an idler in the gear train from the hour wheel 12 to the star map disk wheel 18. Further, it is possible to include stars which are visible at a portion in the southern hemisphere using an arrangement of declination different from the above described arrangement, if mechanical conditions of the watch coincide with actual celestial conditions. The second star display may be provided on a member other than the back 9, such as a glass of the watch, watch band, and other members. 
     FIG. 6 shows an example of the second star display. In the second embodiment of the present invention, the dial 6 is not provided with a window corresponding to the window 6c in the first embodiment, instead a second star display 8b corresponding to the second star display 8 of the first embodiment is provided on the dial 6. The second star display 8b indicates the times of the right ascensions 9e in the same manner as the first embodiment. Other components are the same as the first embodiment and identified by the same reference numerals, and the operation of the watch is the same as the first embodiment. 
     Referring to FIGS. 7 and 8 showing another embodiment of the present invention, the same parts as the first embodiment are identified to the same reference numerals as those of corresponding Figures. In this embodiment, a transparent disk 30 is provided above the star map disk 2. An annular peripheral ring 31 is disposed along the inside wall of the watchcase 1a. Markers 6a for representing one to twelve hours are provided on a slant of the ring 31. A line 30d corresponding to the arc 6d of FIG. 1, line 30e corresponding to the horizon curve 6e, line 30i corresponding to the curve 6i for defining an astronomical twilight and characters 30f are printed on the transparent disk 30. Other portions and members are the same as those of the first embodiment. In this example, since lines for defining necessary ranges in the star map are provided on the transparent disk 30, all stars on the star map disk 2 are visible. 
     The defining lines may be provided on the underside of a case glass 32 as shown by chain lines 32a. 
     While the invention has been described in conjunction with preferred specific embodiments thereof, it will be understood that this description is intended to illustrate and not limit the scope of the invention, which is defined by the following claims.