From the Sun to Atoms
A Very Brief History of External Clocks
Humans have long valued and been fascinated by the art of measuring time,
producing a diverse array of clever devices as time-keeping aids. The Sumerians
are thought to have produced the first known clocks, and the Egyptians further
developed them producing simple shadow clocks, sundials, and water clocks
perhaps as early as 3500 BC. Over time, clocks have evolved into ever more
precise time-keeping mechanisms that measure time intervals and keep track of
time of day. The next generation of atomic clocks, now in development, is
expected to lose no more than one second in 313 million years.
Before Pocket Watches: The Pocket Sundial.
Sundials show the time of day and thus have to be properly oriented and
calibrated to their location. This portable 18th century European sundial
includes a compass and chart to help calibrate it to local sun time. It is shown
with a 19th century garden sundial. A sundial has no self-contained,
oscillating clock "movement," but instead relies on the rotation of the Earth for
its oscillation. An hourglass similarly lacks an internal, self-sustaining
oscillator. However, if a human were to constantly "reset" it by turning it
over, that turning motion would provide the oscillation.
The Sands of Time.
This 30-second sandglass, like similar hourglasses, measures fixed segments of
time. It was used by sailors to calculate a ship's speed. The ability to
accurately measure time of day, necessary in order to fix longitudinal
(east-west) location, is also critical to navigation. For centuries, sailors
have relied on external clocks to determine longitude. Some migrating animals,
such as birds and butterflies, use internal circadian clocks to maintain their
longitudinal direction by constantly compensating for the movement of the sun
across the sky.
A 19th Century Planetarium Clock.
This 19th century British clock tells time while also showing planetary movements.
This is a prototype model made of wood at 1/4 scale of a 21st century clock
that is designed to last 10 millennia or more. It is largely the creation of
Danny Hillis, an inventor and computer designer who helped pioneer the concept of
massively parallel computers.
An "Atomic" Wristwatch.
This watch receives radio waves from the U.S. Time Scales atomic clocks in
Colorado. Those clocks rely on the oscillations (9,192,631,770 per second) of
cesium atoms for their basic time-keeping mechanisms.
Life is a cyclical chemical process that is regulated in four dimensions.
Jay C. Dunlap, "Molecular Bases for Circadian Clocks," Cell, 1999.
How do we KNOW that biological clocks exist?
The earliest and most convincing evidence that internal biological clocks exist
was obtained by placing organisms in isolation, and then removing environmental
cues, to the greatest degree possible. This chart recording, called an actogram,
shows a typical human pattern of activity (no line) and sleep (shown in line)
with and without environmental cues. When environmental cues are eliminated, the
clock is no longer reset each day at sunrise, and its own pace is revealed. With
a lack of environmental cues, the clock is said to be free-running, and its
endogenous free-running period can be determined. In the case of this record, the
free-running period can be seen to be just a little over 24 hours.
Actogram Showing the Transition to a Free-running System
This actogram is plotted along the horizontal axis in quadruplicate in order to make
the patterns more apparent. The red number 22 on the vertical axis represents
the day at which environmental cues were experimentally withdrawn.