What was one of the first devices
used to measure time?
One of the first devices—smaller than the
obelisks mentioned above—to measure
time was a crude sundial. By about 1500
BCE, the true, small sundial (or shadow
clock) was developed in Egypt. It was
divided into ten parts, with two “twilight”
hours marked. But it could only tell time
for half a day; after noon, the sundial had
to be turned 180 degrees to measure the
afternoon hours.
More refinements of measuring time
occurred later. In order to correct for the
Sun’s changing path over the sky
throughout the year, the gnomon—or
object that creates the shadow on the
sundial—had to be set at the correct angle (what we call latitude). Eventually, the sun-
dial was perfected. Multiple designs were used. For example, shortly before 27 BCEthe
Roman architect Marcus Vitruvius Pollio’s (c. 90–20 BCE) De architecturadescribed 13
different designs of sundials.
How does a sundialwork?
The sundial tracks the apparent movement of the Sun across the sky. It does this by
casting a shadow on the surface of a usually circular dial marked by hour and minute
lines. The gnomon—or the shadow-casting, angular object on the dial—becomes the
“axis” about which the Sun appears to rotate. To work correctly, it must point to the
north celestial pole (near the star Polaris, also called the North Star); thus, the gno-
mon’s angle is determined by the latitude of the user. For example, New York City is
located at about 40.5 degrees north latitude, so a gnomon on a sundial in that city
would be at a 40.5 degree angle on a sundial.
The sharper the shadow line, the greater the accuracy; in addition, larger sundials
are more accurate, because the hour line can be divided into smaller units of time.
But the sundial can’t be too large. Eventually, diffraction of the sunlight around the
gnomon causes the shadow to soften, making the time more difficult to read.
What is the definition of a clock?
A clock (from the Latin cloca,or “bell”) is an instrument we use for measuring time.
There are actually two main qualities that define a clock: First, it must have a regular, 57
MATHEMATICS THROUGHOUT HISTORY
A sundial, which uses shadows from the Sun to
mark the passage of time, is one of the oldest time-
keeping devices. National Geographic/Getty Images.