Clock of the Long Now

Many options were considered for the power source of the clock, but most were rejected due to their inability to meet the requirements. For example, nuclear power and solar power systems would violate the principles of transparency and longevity. In the end, Hillis decided to require regular human winding of a falling weight design for updating the clock face because the clock design already assumes regular human maintenance.

However the clock is designed to keep time even when not being wound: "If there is no attention for long periods of time the Clock uses the energy captured by changes in the temperature between day and night on the mountain top above to power its time-keeping apparatus."[5]

The timing mechanism for such a long lasting clock needs to be reliable and robust as well as accurate. The options considered but rejected as sources of timing for the clock included:[4][6]

Most of these methods are inaccurate (the clock will slowly lose the correct time), but are reliable (that is, the clock will not suddenly stop working). Other methods are accurate but opaque (meaning that the clock is difficult to read or understand).

• gravity pendulum (inaccurate over the long term, and requires many ticks, which creates wear)
• torsion pendulum (fewer ticks, but less accurate)
• balance wheel (more inaccurate than pendulum)
• water flow (inaccurate and wet)
• solid material flow (inaccurate)
• wear and corrosion (very inaccurate)
• rolling balls (very inaccurate)
• diffusion (inaccurate)
• tuning fork (inaccurate)
• pressure chamber cycle (inaccurate)
• inertial governor (inaccurate)
• atomic oscillator (opaque, difficult to maintain)
• piezoelectric crystal oscillator (opaque, difficult to maintain)
• atomic decay (opaque, difficult to measure precisely)

Many of these methods are accurate (some external cycles are very uniform over huge stretches of time) but unreliable (the clock could stop working completely if it failed to track the external event properly). Others have separate difficulties.

• daily temperature cycle (unreliable)
• seasonal temperature cycle (imprecise)
• tidal forces (difficult to measure)
• Earth's rotating inertial frame (difficult to measure accurately)
• stellar alignment (unreliable because of weather)
• solar alignment (unreliable because of weather)
• tectonic motion (difficult to predict and measure)
• orbital dynamics (difficult to scale)
• vandalism (difficult to predict)
• civil disorder (difficult to predict)
• civil war (difficult to predict)
• nuclear war (difficult to predict)
• impact event (difficult to predict)

Hillis concluded that no single source of timing could meet the requirements. As a compromise the clock will use an accurate but unreliable timer to adjust an inaccurate but reliable timer, creating a phase-locked loop.

In the current design, a slow mechanical oscillator, based on a torsional pendulum, keeps time inaccurately, but reliably. At noon, the light from the Sun, a timer that is accurate but (due to weather) unreliable, is concentrated on a segment of metal through a lens. The metal buckles and the buckling force resets the clock to noon. The combination can, in principle, provide both reliability and long-term accuracy.

Many of the usual units displayed on clocks, such as hours and calendar dates, may have little meaning after 10,000 years. However, every human culture counts days, months (in some form), and years, all of which are based on lunar and solar cycles. There are also longer natural cycles, such as the 25,765-year precession of Earth's axis. On the other hand, the clock is a product of our time, and it seems appropriate to pay homage to our current arbitrary systems of time measurement. In the end, it seemed best to display both the natural cycles and some of the current cultural cycles.

The center of the clock will show a star field, indicating both the sidereal day and the precession of the zodiac. Around this will be a display showing the positions of the Sun and the Moon in the sky, as well as the phase and angle of the Moon. Outside this will be the ephemeral dial, showing the year according to our current Gregorian calendar system. This will be a five-digit display, indicating the current year in a format like "02000" instead of the more usual "2000" (to avoid a Y10K problem). Hillis and Brand plan, if they can, to add a mechanism whereby the power source generates only enough energy to keep track of time; if visitors want to see the time displayed, they would have to manually supply some energy themselves.

Options considered for the part of the clock that converts time source (for example, a pendulum) to display units (for example, clock hands) include electronics, hydraulics, fluidics, and mechanics.

A problem with using a conventional gear train (which has been the standard mechanism for the past millennium) is that gears necessarily require a ratio relationship between the timing source and the display. The required accuracy of the ratio increases with the amount of time to be measured. (For instance, for a short period of time the count of 29.5 days per lunar month may suffice, but over 10,000 years the number 29.5305882 is a much more accurate choice.)

Achieving such precise ratios with gears is possible, but awkward; similarly, gears degrade over time in accuracy and efficiency due to the deleterious effects of friction. Instead, the clock uses binary digital logic, implemented mechanically in a sequence of stacked binary adders (or as their inventor, Hillis, calls them, serial bit-adders). In effect, the conversion logic is a simple digital computer (more specifically, a digital differential analyser), implemented with mechanical wheels and levers instead of typical electronics. The computer has 32 bits of accuracy,[2] with each bit represented by a mechanical lever or pin that can be in one of two positions. This binary logic can only keep track of elapsed time, like a stopwatch; to convert from elapsed to local solar time (that is, time of day), a cam subtracts from (or adds to) the cam slider, which the adders move.

Another advantage of the digital computer over the gear train is that it is more evolvable. For instance, the ratio of day to years depends on Earth's rotation, which is slowing at a noticeable but not very predictable rate. This could be enough to, for example, throw the phase of the Moon off by a few days over 10,000 years. The digital scheme allows that conversion ratio to be adjusted, without stopping the clock, if the length of the day changes in an unexpected way.

The Long Now Foundation has purchased the top of Mount Washington near Ely, Nevada, which is surrounded by Great Basin National Park, for the permanent storage of the full-sized clock, once it is constructed. It will be housed in a series of rooms (the slowest mechanisms visible first) in the white limestone cliffs, approximately 10,000 feet (3,000 m) up the Snake Range. The site's dryness, remoteness, and lack of economic value should protect the clock from corrosion, vandalism, and development. Hillis chose this area of Nevada in part because it is home to a number of dwarf bristlecone pines, which the Foundation notes are nearly 5,000 years old. The clock will be almost entirely underground, and only accessed by foot traffic from the east once complete.

Before building the public clock in Nevada, the foundation is building a full-scale clock of similar design in a mountain near Van Horn, Texas. The test drilling for the underground construction at this site was started in 2009. The site is on property owned by Amazon.com founder Jeff Bezos, who is also funding its construction. The lessons learned in the construction of this first full-scale 10,000-year clock will inform the final design of the clock in Nevada.