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Clock designed to keep time for 10,000 years

The Clock of the Long Now, also called the 10,000 year clock, is a mechanical clock under construction that is designed to keep time for 10,000 years. It is being built by the Long Now Foundation. A two meter prototype is on display at the Science Museum in London. as of June 2018, two more prototypes are on display at The Long Now Museum & Store at Fort Mason Center in San Francisco. The project was conceived by Danny Hillis in 1986. The first prototype of the clock began working on December 31, 1999, just in time to display the transition to the year 2000. At midnight on New Year's Eve, the date indicator changed from 01999 to 02000, and the chime struck twice. The manufacture and site construction of the first full scale prototype clock is being funded by Jeff Bezos's Bezos Expeditions, with $42 million, and is on land which Bezos owns in Texas.

In the words of Stewart Brand, a founding board member of the foundation, "Such a clock, if sufficiently impressive and well engineered, would embody deep time for people. It should be charismatic to visit, interesting to think about, and famous enough to become iconic in the public discourse. Ideally, it would do for thinking about time what the photographs of Earth from space have done for thinking about the environment. Such icons reframe the way people think."

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:

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 a double dial with digits, 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, The Long Now Foundation sells a soundtrack for the novel with profits going to the project. Musician Brian Eno gave the Clock of the Long Now its name (and coined the term "Long Now") in an essay; he has collaborated with Hillis on the writing of music for the chimes for a future prototype.