cotton, processed foods, paper, cigarettes,
and many other products. Most
of Egypt’s important banks, shipping
companies, airlines, and hotels are in
Cairo. The city’s museums and monuments
attract many tourists.
The ancient
Romans
named the
month of July
in honor of
Julius Caesar.
8 Cairo BRITANNICA STUDENT ENCYCLOPEDIA
History
People have lived in the area around
Cairo for thousands of years. In AD 969
Egypt’s rulers at the time established a
new city. It was later named Cairo.
In the 1200s Cairo became the capital of
a large Muslim empire. The city thrived
as a center of trade and Islamic learning
and culture. Most of Cairo’s greatest
buildings went up during that period.
After other rulers took over in the 1500s
the city went into a period of decline.
In the 1860s many European-style
buildings were built in new city districts.
In 1922 Egypt became an independent
country with Cairo as its capital. Since
then the city has grown and grown.
#More to explore
Egypt • Egypt, Ancient • Nile River
• Pyramid
Calendar
A calendar is a tool used to mark the
passing of time. People of ancient times
based their calendars on the most obvious
regular events they knew—the
changing positions of the sun, moon,
and stars. These calendars helped them
figure out when to plant and harvest
their crops. Over time different groups
of people developed other calendars
based on their own needs and beliefs.
Days, Weeks,
Months, and Years
Several units of time are common to
almost all calendars. The day is the most
basic unit. The day measures the cycle of
daytime and nighttime. It is now known
to be the length of time that Earth takes
to spin once on its axis. A group of
seven days is called a week.
The month is about 29 1/2 days long, the
time taken by the Moon to orbit Earth.
The year is about 365 1/4 days long. That
is the time taken by Earth to orbit the
sun. The year measures the seasons.
Fitting months into years is a problem
for calendar makers. The difficulty
comes from the fact that the month is
lunar, or based on the Moon, but the
year is solar, or based on the sun. Twelve
cycles of the Moon take about 354 days,
not 365 1/4. To make the lunar months
fit into the solar year, it is necessary to
add days at certain times. This is why
calendar months differ in length. All
except February are longer than 29 days
to make up for the extra 11 days of the
solar year. For the same reason a day is
added to February every fourth year.
These are called “leap years.”
Modern Cairo’s blocks of apartments, office
buildings, and shops line the Nile River.
Unlike the day,
the month,
and the year,
the week is not
based on any
astronomical
event.
BRITANNICA STUDENT ENCYCLOPEDIA Calendar 9
Early Calendars
The Sumerians of ancient Mesopotamia
(now Iraq) used a calendar of 12 lunar
months. They added an extra month
about every four years. This calendar
served as a model for early Greek and
Jewish calendars.
The early Egyptians also used a calendar
based on the Moon. Later, though, the
Egyptians worked out a calendar that
corresponded almost exactly to the seasons.
The early Romans had a lunar year of
355 days. To make the lunar year agree
with the solar year, priests added extra
months. The calendar became increasingly
out of phase with the seasons.
Julian and Gregorian
Calendars
In 46 BC, the Roman ruler Julius Caesar
decided on the figure of 365 1/4 days for
the length of the year. Every fourth year
had 366 days. This calendar was named
“Julian” after its creator. It had the 12
months at the lengths used today.
The Julian year was still slightly longer
than the solar year, however, and problems
arose as the centuries passed. In
1582 Pope Gregory XIII ordered 10
days to be dropped from the year. To
keep the calendar correct in the future,
Gregory ordered that leap year should be
skipped three times in every 400 years.
His system is called the Gregorian calendar.
It is now used almost everywhere
outside the Islamic world.
Native American Calendars
The Maya and the Aztec had the best
Native American calendars. Their calendars
each had a cycle of 260 named days
related to their religious rituals and a
year of 365 days. The year was divided
into 18 months of 20 days each, with
five days added to fill out the years.
Other Calendars
Several calendars other than the
Gregorian calendar are still in use. The
Muslim calendar, for example, is used
by most Arab countries. It is a lunar
calendar with 12 months of 30 or 29
days that add up to a year of 354 or
355 days.
The Chinese calendar is still used along
with the Gregorian calendar in China,
Taiwan, and neighboring countries. The
traditional Hindu and Jewish calendars
continue to be used as well for religious
purposes.
#More to explore
Aztec • Caesar, Julius • Maya • Season
Great Britain
and its colonies
lost 11
days when
they switched
from the Julian
to the Gregorian
calendar
in 1752. The
day after September
2 was
September 14.
A Jewish calendar from the 1800s is written
in Hebrew.
10 Calendar BRITANNICA STUDENT ENCYCLOPEDIA
California
More people live in California than in
any other U.S. state. About one eighth of
the country’s entire population lives in
