employed Sosigenes to fix the
old Roman lunar calendar. By
Caesar’s time, the lunar calen-
dar had become so out of sync
with the seasons that it
required a decree from the
emperor to remedy the situa-
tion. At Sosigenes’ suggestion,
the old lunar calendar was
replaced with one which used
only the Sun to delineate the
year. The Moon was left to
drift through the 12 months
of Caesar’s new calendar.This Julian calendar also
implemented leap years, add-
ing one extra day every four
years. But this was not quite a
perfect fix, as the last fraction
of a day in a year is slightly
less than one-quarter of a day.
By the 16th century, the Julian
calendar was also out of step
with the seasons. This led
Pope Gregory XIII to imple-
ment updates in 1582 that
dictated leap years be skipped
on years divisible by 100except when divisible by 400.
So while 1900 was not a leap
year, 2000 was. Two thousand
years later, the whole world
still uses the modified calen-
dar of Sosigenes.Cultures the world over devel-
oped methods for tracking
the hours using water clocks,
hourglasses, and sundials,
often for the purpose of
scheduling religious rituals.Of course, there were limita-
tions to these methods. Water
froze in the winter, hourglass-
es needed to be turned over,
and sundials were of no use
after sunset.
Around the year a.d. 1000,
mechanical devices that could
ring bells to tell time began to
appear in western Europe. In
fact, some think the word
“clock” derives from the
French, cloche, meaning bell.
These early mechanisms had
no dials and only rang bells.
Within a few hundred years,
dials were added to visuallyThe Samrat Yantra (above) is the largest purpose-built sundial in the world, with its gnomon — or tower — standing
73 feet (27 m) high. Built in the early 18th century, the massive instrument is part of the Jantar Mantar observatory in
Jaipur, India. The gnomon (left) casts a shadow onto the flanking quadrant arcs, which can indicate the time to an
accuracy of two seconds. ABOVE: JORGE LÁSCAR, CC BY 2.0. LEFT: JAKUB HAŁUN CC BY-SA 4.0WWW.ASTRONOMY.COM 55
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