BRITANNICA STUDENT ENCYCLOPEDIA Comet 165
called Halley’s Comet. It can be seen
from Earth about every 76 years.
Orbit
Most planets orbit the sun in a path
shaped like a circle. On the other hand,
a comet’s orbit is shaped like a long oval.
Most comets travel very far into the
outer solar system as they orbit.
Depending on the comet, it can take
from several years to many thousands of
years to complete one orbit.
Earth passes through the orbit of some
comets. Bits of dust left behind by the
comets crash into the gases surrounding
Earth. They burn up in the sky in what
is called a meteor shower.
Parts of a Comet
The main part of a comet is called the
nucleus. It is made of frozen water and
gases plus dust and rock. The nucleus
usually measures a few miles across.
Most of the time a comet consists of
only its nucleus. However, when the
comet gets close to the sun, the sun’s
heat causes it to shed gas and dust. A
huge cloud of gas and dust called a coma
builds up around the comet. The coma
is sometimes larger than Earth. It makes
the head of the comet look blurry.
When a comet moves even closer to the
sun it grows tails. This happens because
the sun gives off energy and streams of
particles. They push some of the coma’s
dust and gases out into long streams.
Comet tails can be very long—as long as
the distance from Earth to the sun. The
coma and tails last only while the comet
is fairly close to the sun.
#More to explore
Meteor and Meteorite • Solar System
• Sun
Communication
People learn and share information
through communication. All forms of
communication have one thing in common:
the sending and receiving of a
message. Using different kinds of communication,
people can pass along their
ideas and feelings to just one person or
to millions.
Communication is not only a human
activity. Animals communicate through
sounds, scents, and acts. Computers
communicate with other computers
through electronic networks.
People can quickly and easily
communicate with one another
over long distances using computers
and cell phones.
Halley’s Comet
passes by
Earth every
76 years.
It is named
for Edmond
Halley, an
English
astronomer
who predicted
one of its
appearances.
166 Communication BRITANNICA STUDENT ENCYCLOPEDIA
Kinds of Communication
People communicate through language,
both spoken and written. Spoken language
includes conversations and
speeches.Written language includes
books, newspapers, and e-mail.
People also communicate without
words. Body and face movements show
feelings. Deaf people use hand movements
called sign language.
The arts are also a form of communication.
Photographs, paintings, music, and
movies can communicate feelings as well
as facts.
History
The first modern humans appeared
about 200,000 years ago. They likely
used sounds, movements, and then
speech. About 30,000 years ago humans
started making paintings and sculptures
on the walls of caves.
Humans invented writing more than
5,000 years ago. The ancient Sumerian
people developed the first known script,
called cuneiform. Then the ancient
Egyptians developed a picture-based
writing called hieroglyphics. The invention
of the printing press in the 1400s
was a major advance in written communication.
It let printers quickly make
copies of a text.
Telecommunication is communication
over long distances. In ancient times
pigeons carried written messages. Before
trains and cars, messengers on foot or on
horseback carried letters. Public postal
services started in the 1500s and 1600s.
Since then, telecommunication has
become faster thanks to television, telephones,
and the Internet.
#More to explore
Arts • Language • Sign Language
• Telecommunication •Writing
Communism
Communism is a type of government as
well as an economic system (a way of
creating and sharing wealth). In a Communist
system, individual people do not
own land, factories, or machinery.
Instead, the government or the whole
community owns these things. Everyone
is supposed to share the wealth that they
create.
Karl Marx
By the 1800s many countries followed
the economic system called capitalism.
Under capitalism, individual people,
called capitalists, own property and run
companies. Some capitalists became rich
but paid their workers very little. In
response, many workers began supporting
the ideas of socialism. In a socialist
Two songbirds—a female and a male—
communicate with one another by singing.
Birds can send a variety of messages, such
as interest in mating, through their songs.
BRITANNICA STUDENT ENCYCLOPEDIA Communism 167
system, the government owns companies
and divides wealth more fairly among
the people. Karl Marx, a German
thinker of the 1800s, took socialist ideas
one step further. Marx’s ideas became
the basis of Communism.
Marx called the workers the proletariat.
Marx thought that some day the workers
would lead a revolution and overthrow
the capitalists. After taking
control, the proletariat would become
the new ruling class. At first there would
be a “dictatorship of the proletariat” to
defeat all opposition. Then a true Communist
society would develop. This
would include common ownership of
property and no government. People
