cooler, darker patches called sunspots
appear on the surface. Sunspots
appear and disappear in 11-year cycles.
Layers of gases, called an atmosphere,
surround the sun. Sometimes great
blasts of energy, called solar flares, erupt
from the inner atmosphere. The outer
atmosphere is called the corona. It is
very hot, reaching 3,600,000° F
(2,000,000° C).
The SolarWind
The sun’s corona shoots out streams of
tiny particles that carry an electric
charge. These streams are called the solar
wind. The solar wind moves through the
solar system at about 250 miles (400
kilometers) per second. It can disrupt
radio signals on Earth. It also causes
colorful bands of light called auroras to
appear in the sky in Earth’s far north
and south.
#More to explore
Aurora • Energy • Solar System • Star
U.S. astronauts took a photograph from
space of a huge flare of energy on the sun.
A cutaway view shows the extremely hot
core of the sun. Sunspots dot the surface,
and flares burst from the inner atmosphere.
The corona is the sun’s outer atmosphere.
BRITANNICA STUDENT ENCYCLOPEDIA Sun 203
Sundial
Sundials are the oldest known instruments
for telling time. The surface of a
sundial has markings for each hour of
daylight. As the sun moves across the
sky, another part of the sundial casts a
shadow on these markings. The position
of the shadow shows what time it is.
The flat surface of a sundial is called a
dial plate. It may be made of metal,
wood, stone, or other materials. Numbered
lines on the dial plate, called hour
lines, show the hours of the day.
A part called a gnomon sticks up at an
angle from the dial plate. The sloping
edge of the gnomon is called the style.
As the day passes, the gnomon’s shadow
moves around the dial. Every hour it
falls on a new hour line.
The ancient Egyptians made the earliest
known sundial in about 3500 BC. This
sundial was simply a stick or a pillar that
cast a shadow on the ground. The
ancient Greeks made a sundial with a
bowl-shaped opening cut into a block of
stone or wood. A pointer in the center
cast shadows inside the bowl. Muslims
later invented the modern sundial—the
type with the angled gnomon. Clocks
began to replace sundials in the 1300s.
#More to explore
Clock • Time
Sunflower
Sunflowers are useful plants with large
flower heads, or blooms. They are
named for the way they turn their
blooms from east to west to follow the
sun. Sunflowers were first grown in
North and South America. Today they
are grown in many parts of the world.
Sunflowers are giants among flowers.
The rough, hairy stem grows from 3 to
15 feet (1 to 4.5 meters) tall. The leaves
are wide, rough, and 3 to 12 inches (8 to
30 centimeters) long.
The bloom of a sunflower has two parts.
The center is a disk of small flowers
packed closely together. The disk flowers
are brown, yellow, or purple. The flowers
around the outside are called ray
flowers. They are golden yellow. Some
sunflowers have disks that are 12 inches
(30 centimeters) wide.
Before the invention of the clock, many cultures
around the world used sundials to tell
time.
Sunflowers brighten a field in France.
204 Sundial BRITANNICA STUDENT ENCYCLOPEDIA
Sunflower plants have many uses. The
flowers make a yellow dye. The leaves
are used to feed animals. Seeds that
come from the disk flowers contain a
sweet, yellow oil. Sunflower oil is used in
cooking and in making soaps and paints.
Many people eat dried or roasted sunflower
seeds as a snack.
#More to explore
Flower • Plant • Seed
Sun Yat-sen
Sun Yat-sen helped remove the emperor
of China from power in 1912. Sun
wanted China to be a strong, modern
country with free elections and policies
that benefited all citizens.
Early Life
Sun Yat-sen was born on November 12,
1866, in Hsiang-shan, in southern
China. For several years he studied with
British and U.S. teachers in Honolulu,
Hawaii. Later he studied to be a
medical doctor in Hong Kong, which
was then a British colony. He graduated
in 1892.
Revolutionary Leader
Sun soon found a new career. He
started working to bring down China’s
ruling family. Sun thought that China
had fallen behind other countries in
developing technology. He felt that
changing the government was the only
way to make China a modern country.
In 1895 he took part in an uprising
that failed. He then had to leave the
country.
Sun spent the next 16 years living in
other countries, including the United
States and the United Kingdom. During
this time he gathered followers and
planned several rebellions. The rebellions
failed, but he continued to gain
support.
In 1911 the Chinese people finally rose
up against the emperor. Sun then
returned to China. He became the
leader of a new political group called the
Nationalist Party. The emperor stepped
down on February 12, 1912.
Sun helped establish a new government,
but he disagreed with the man who
became president. He organized another
revolution in 1913, but it failed. He also
tried to gain control of the government
in 1917 and 1921. Finally, in 1923 Sun
made himself the leader of a new Chinese
government.
Last Years
Sun worked hard to strengthen the
Nationalist Party. He also established a
