  The Long Now Foundation is building a mechanical clock that is designed to
keep time for the next 10,000 years. The clock maintains its long-term accuracy
by synchronizing to the Sun. The 10,000-Year Clock keeps track of five
different types of time: Pendulum Time, Uncorrected Solar Time, Corrected Solar
Time, Displayed Solar Time and Orrery Time. Pendulum Time is generated from the
mechanical pendulum and adjusted according to the equation of time to produce
Uncorrected Solar Time, which is in turn mechanically corrected by the Sun to
create Corrected Solar Time. Displayed Solar Time advances each time the clock
is wound, at which point it catches up with Corrected Solar Time. The clock
uses Displayed Solar Time to compute various time indicators to be displayed,
including the positions of the Sun, and Gregorian calendar date. Orrery Time is
a better approximation of Dynamical Time, used to compute positions of the
Moon, planets and stars and the phase of the Moon. This paper describes how the
clock reckons time over the 10,000-year design lifetime, in particular how it
reconciles the approximate Dynamical Time generated by its mechanical pendulum
with the unpredictable rotation of the Earth.
