Eric Vittoz - IEEE

TECHNICAL LITERATURE
It’s about Time: A Brief Chronology of Chronometry
Thomas H. Lee, Stanford University, tomlee@ee.stanford.edu
Prologue
The lobby clock’s label boasted, in HP’s characteristically
understated way, that a cesium standard controlled
the displayed time. While waiting for his host
to greet him, Max Forrer reflexively checked his wristwatch,
and noted a three-second disagreement.
Although most people in 1968 would have dismissed
the difference as trivial (if, indeed, they had noticed
one at all), the discrepancy bothered the new director
of the Centre Electronique Horloger (CEH). Since
CEH’s founding in 1962, engineers had toiled in secret
at the Centre’s labs in Neuchâtel, Switzerland to
develop the world’s first quartz-controlled wristwatch.
They had succeeded brilliantly: In December of 1967,
ten “Beta 2” prototypes had swept the top ten spots
in the annual Concours held at the Observatoire de
Neuchâtel, smashing all previous records by exhibiting
drifts of only a few hundred milliseconds per day.
Given the magnitude and consistency of that triumph,
Forrer could not accept that his wristwatch was off by
three seconds. Of logical necessity, then, the lobby
clock had to be wrong, cesium notwithstanding. Once
his host appeared, Forrer shared his reasoning, and a
subsequent investigation revealed that the lobby
clock was indeed off by about two seconds [1]. Word
spread quickly among HP engineers that a wristwatch
had caught an error in their atomic-controlled clock.
The times – and certainly timepieces – they were “achangin.”
The Age of Continuous Time
Rejoice at simple things; and be but vexed by sin and
evil slightly.
Know the tides through which we move.
– Archilochos, c. 650BCE
The path to Forrer’s moment of rejoicing was anything
but linear and predictable. Simple things – natural,
continuous processes – such as the regular
motions of the sun, moon and stars, had marked the
tides through which we move for most of human
existence. Sundials had been used since at least 3500
BCE, when obelisks appeared for this purpose in
Egypt [2]. The 3300-year-old Luxor Obelisk is a beautiful
example of how far the ancients were able to
develop this technology (Fig. 1). Standing 23 meters
tall, the 200,000-kilogram pink-granite monolith still
tells time as accurately as it did when it was built.
Portable sundials appeared about two millennia
later, contemporaneously with the first timekeepers
based on an artificial (but still continuous) process:
the flow of water. The oldest surviving example dates
to the reign of Amenhotep III, over a century after the
Figure 1 – The Luxor Obelisk at the Place de la Concorde
in Paris, where it was moved from Egypt in 1829 (photo
credit: David Monniaux)
first written description. Consisting of a leaky bowl
with graduations on the inner surface, sloping sides
helped compensate for a varying drain speed as the
bowl emptied. This early clock represents the first in
a line that would see considerable refinement in the
hands of the Greeks, who called them clepsydras
(“water thieves,” because of the outflow of water),
starting around 325 BCE. Water clocks reached their
evolutionary peak with a succession of Chinese water
towers (200 CE to 1300 CE), the last of which operated
bells and other mechanical indicators [2].
A Little Relaxation is Good
The rather long periods (e.g., a day or a lunar month)
of the natural processes accessible to the ancients
made it difficult to mark time accurately in fine increments.
Besides possessing periodicities that aren’t
directly traceable to any fundamental time constants,
water clocks also suffer from the temperature- and
impurity-sensitive characteristics of water, to say nothing
of freezing. Partial solutions devised over the centuries
include sundials surrounded by an array of
marker stones to facilitate resolving shorter intervals,
as well as the substitution of mercury for water in at
42 IEEE SSCS NEWS © 2008 IEEE Summer 2008