show the hour. By the 13th
century, astronomer monks
were creating complex move-
ments with dials that had an
hour hand and displayed
Moon phases, the solstices,
and equinoxes, and more.
In the Middle Ages, know-
ing the hour was sufficient for
everyday activity. Words like
moment meant the passage of
15 minutes rather than a
blink of an eye. As astronomy
advanced, however, more pre-
cision was needed. This came
courtesy of the regular
motion of swinging pendu-
lums, which Galileo studied
at the beginning of the 17th
century. In 1657, the Dutch
astronomer Christiaan
Huygens applied for a patent
for a clock using the regular
oscillation of a swinging pen-
dulum to regulate the passage
of time. Clocks were now
accurate enough to not only
justify both an hour and min-
ute hand, but a second hand
as well.Science, technology, and com-
merce often complement each
other. In the 17th century, the
three came together to tackle
the difficult challenge of
determining a ship’s longitude
at sea.
In 1676, at the newly built
Greenwich Observatory
located outside of London,
two unique clocks with
13-foot-long (4 meters) pen-
dulums were installed. These
clocks were accurate to within
10 seconds over the course of
a day or better, dramatically
increasing astronomers’ abil-
ity to make accurate
observations.
The first astronomer royal,
Sir John Flamsteed, wanted toknow if Earth’s rotation was
isochronical. In short, did
Earth spin on its axis at a
constant rate? With the new
clocks, Flamsteed showed that
the spinning of our planet was
in fact constant. This provided
the first link in the efforts to
solve the longitude problem.
By the 18th century, navi-
gators were using portable
clocks driven by wound
springs that could accurately
maintain Greenwich time. By
comparing them to their
observed local solar time, they
could determine their longi-
tude hundreds of miles from
land-based observatories.As the 19th century neared,
clocks and pocket watches
had become common and
fashionable. But how did you
set them? With the garden
sundial! When travel was by
foot or horse, differences in
“local” times were negligible
and did not present a prob-
lem. But with the arrival of
trains, all of this changed, and
astronomy came to the rescue
once more.For every 15° of longitude
either west or east of a desig-
nated meridian, the local solar
time decreases or increases by
one hour. That means for
each degree of longitude, time
changes by four minutes. If
there is a sundial that shows
noon at Greenwich, it would
be 11:40 a.m. by a sundial in
Oxford. Across Great Britain,
there is a 30-minute differ-
ence in time. This was hard
enough for railroads in a
small country where every
town adjusted its clocks to
the local sundial. It was worse
in North America, where
time zones spanned three
and a half hours! Each town
used local sundials to set
their clocks, while each rail-
road had a different standard-
ized time for their published
timetables. This made italmost impossible to have rail
schedules that made any sense
at all.
To solve this, astronomers
divided the globe into 24 time
zones. The starting point for
these time zones was based
on the meridian defined by
specific observatories that
made noon day observations.
In England, it was the
Greenwich Observatory,
France used the observatory
in Paris, and the U.S. used the56 ASTRONOMY • DECEMBER 2021
One of the most famous
astronomical clocks (below) can
be found down a side street in the
old city of Prague. Built in 1410, it
has been operating for more than
600 years. The beautiful dial (left)
looks like the front of an astrolabe
and still provides complex
astronomical information.
BELOW: DAVID J. EICHER. LEFT: STEVE COLLIS