Today, the sun tells the time -- for 3500 years.
The University of Houston's College of Engineering
presents this series about the machines that make
our civilization run, and the people whose
ingenuity created them.
What was the first
timepiece? Probably a tree, casting a shifting
shadow, long to the west in the morning, short to
the north at mid-day. Sundials clearly began as a
perfectly obvious fact of nature. Then, as we began
asking more of them, they opened into a mire of
My son, home last week for a visit, brought a
computer program he'd written. It predicted
sunrise, sunset, and solar position -- for any
place, any date, any time. The very intricacy of
that program told how hard it is to interpret a
tree's shadow as the sun wheels through the passing
year. The simpler sundials told us only when it was
noon, or they named the summer solstice. Before
they could tell us the time of day throughout the
year, we'd have to learn far better to read their
The oldest known sundial was made in Egypt in 1500
BC. It was L-shaped. The top of its vertical leg
cast its shadow on the horizontal leg. The shadow's
length, not its location, roughly indicated the
time. A vast variety of shadow-casting devices
followed in the ancient world -- L-shaped,
bowl-shaped, step-shaped -- even a light beam cast
by a hole in a darkened room. The great ceremonial
Egyptian obelisks may've doubled as huge sundial
indicators. The Book of Kings tells how Isaiah
called on God to make the shadow of an 8th-century
BC sundial move backward.
But it took the astronomers and geometers of
Hellenistic North Africa to make sundials into
reliable instruments and to regularize the telling
of time. Hellenistic time-keeping devices appealed
to the organized Romans. By the first century BC
they'd erected so many in Rome that one angry
Let the gods damn the first man who invented the
hours, ... who set up a sundial in this city! ...
He has chopped the day into slices. When I was
young, there was no other clock but my belly ...
Now we [eat] when it pleases the sun.
Actually the Romans sliced the day up
differently than we do. They divided day and night
into 12 hours each -- summer and winter alike. Hours
had different lengths in light and dark.
Sundials became the common time-tellers of medieval
Europe. You find vertical dials on the outer walls
of Chartres Cathedral and Cluny Abbey. Complicated
water clocks were the high-tech of medieval
time-telling, but more for show than tell. The
simple clock-like faces of sundials provided all
the social regulation anyone really needed in the
When I was a Boy Scout, as late as the early '40s,
I carried a small flat brass case that held a
compass, a sundial, and a table of corrections. The
sundial was still there, still alive and active in
my own lifetime --after three and a half millennia.
I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds
Rohr, R.R.J., Sundials: History, Theory, and
Practice. Toronto: University of Toronto
Marshall, R.K., Sundials. New York:
the MacMillan Company, 1963.
The computer program that I mention above was
written by John H. Lienhard V, Mechanical
Engineering Department, Massachusetts Institute of
Technology (firstname.lastname@example.org), who also provided
From the 1832 Edinburgh
A 19th-Century Pocket Sundial
The Engines of Our Ingenuity is
Copyright © 1988-1997 by John H.
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