An inexpensive wall clock has been devised for displaying solar time or sidereal time as it would be perceived on a planet other than the Earth, or for displaying sidereal time on the Earth. The concept of a wall clock synchronized to a period other than the terrestrial mean solar day is not new in itself. What is new here is that the clock is realized through a relatively simple electronic modification of a common battery-powered, quartz-crystal-oscillator- driven wall clock (which, as unmodified, displays terrestrial mean solar time).
The essence of the modification is to shut off the internal oscillator of the clock and replace the internal-oscillator output signal with a signal of the required frequency generated by an external oscillator. The unmodified clock electronic circuitry includes a quartz crystal connected to an integrated circuit (IC) that includes, among other parts, a buffer amplifier that conditions the oscillator output. The modification is effected by removing the quartz crystal and connecting the output terminal of the external oscillator, via a capacitor, to the input terminal of the buffer amplifier (see figure).
The frequency and amplitude of the external-oscillator signal must be chosen in accordance with the IC design as well as the desired clock speed. Typically, the required amplitude is 0.5 V peak-to-peak and the frequency required for two complete revolutions of the hour hand (two 12-hour cycles) spanning a terrestrial mean solar day is 215 = 32,768 Hz. Examples of other clock cycles and frequencies based on this typical design include the following:
- For one complete revolution of the hour hand (one 24-hour cycle) during a terrestrial mean solar day, the required frequency is 214 = 16,384 Hz.
- For two complete revolutions of the hour hand (two 12-"hour" cycles) during a terrestrial sidereal day, the required frequency is 32,859.27577 Hz.
- For one complete revolution of the hour hand (one 24-"hour" cycle) during a terrestrial sidereal day, the required frequency is 16,429.63788 Hz.
- For two complete revolutions of the hour hand (two 12-"hour" cycles) during a Martian mean solar day, the required frequency is 31,947.1361 Hz.
- For one complete revolution of the hour hand (one 24-"hour" cycle) during a Martial sidereal day, the required frequency is 15,973.568 Hz.
It is worthwhile to note that for the 24- hour or for any of the 24-"hour" clock speeds, the minute hand would complete a revolution in 2 hours or "hours". Therefore, it could be desirable to remove the minute hand to prevent confusion. In addition, in that case, the 12-hour faceplate must be replaced by a 24-hour faceplate.
It is also worthwhile to note that the precision of the clock display depends on the precision of the external oscillator, which can be cheap or expensive, as needed to obtain the precision required for a specific application. For example, the external oscillator could be a battery-powered, fixed-frequency quartz oscillator; a commercially available programmable integrated- circuit frequency synthesizer; or a programmable frequency synthesizer locked to highly stable reference oscillator (e.g., a hydrogen maser).
This work was done by James Lux of Caltech for NASA's Jet Propulsion Laboratory.