Download Chapter 22 Planet Earth

Chapter 22 Planet Earth
Key Words – define these words on your
paper as you read the following chapter.
Sphere
Axis
Rotation
Revolution
Ellipse
Equinox
Solstice
Earth’s Shape
Space probes and artificial satellites have sent back
images that show Earth is sphere-shaped. A sphere is a
round, three dimensional object whose surface at all
points is the same distance from its center. Tennis balls
and basketballs are examples of spheres. But people had
evidence of Earth’s true shape long before cameras were
sent into space.
Around 350 B.C., the Greek astronomer and
philosopher Aristotle reasoned that Earth was spherical
because it always casts a round shadow on the moon
during an eclipse. Only a spherical object always produces
a round shadow. If Earth were flat, it would as a flat
shadow.
If the earth were flat
It’s shadow would be
Flat.
Physical Properties of Earth
Diameter (pole to pole)
Diameter (equator)
Circumference (poles)
Circumference (equator)
Mass
Density
Average distance from the sun
Period rotation (1 day)
Period of Revolution (1 year)
12,714 km
12,756 km
40,008 km
40,075 km
5.98 X 10 g
5.52 g / cm3
149, 600, 000 km
23 hr. 56 min
365 day, 6 hr., 9 min.
27
Today we know that Earth is sphere-shaped, but is not a perfect sphere.
It bulges slightly at the equator and is some-what flattened at the
poles. The poles are located at the north and south and of Earth’s axis.
Earth’s axis is the imaginary line around which the Earth spins. The
spinning of Earth on its axis, called rotation, causes day and night to
occur.
Earth’s Rotation
As Earth rotates, the sun comes into view at daybreak. Earth continues
to spin, making it seem that the sun moves across the sky until it
appears to set at night. During the night, your area of Earth has spun
away from the sun. Because of this, the sun is no longer visible. Earth
continues to steadily rotate, and the sun eventually comes into view
the next day. One complete rotation takes about 24 hours, or one day.
How many rotations does Earth complete during on year? It completes
about 365 rotations during its journey around the sun.
Earth’s Magnetic Field
Convection currents inside Earth’s mantle power the movement of
tectonic plates. Scientists hypothesize that movement of material
inside Earth also generates a magnetic field.
The magnetic field of Earth is much like that of a car magnet. Earth has
a North and South magnetic pole, just as a bar magnet has opposite
magnetic poles at its ends.
Magnetic North
When you observe a compass needle pointing
toward the north, you are seeing evidence of
Earth’s magnetic field. Earth’s magnetic axis, the
line joining its North and South magnetic poles,
does not align with its rotational axis. The
magnetic axis is inclined at an angle of 11.5
degrees to the rotational axis. If you followed a
compass needle pointing north, you would end
up at the magnetic north pole rather than the
geographic (rotational) north pole.
Earth’s magnetic field and other physical
properties affect us every day.
Seasons / Earth’s Revolution
Earth’s rotation causes day and night to occur. Another important motion of
Earth is the revolution, or yearly orbit around the sun. just as the moon is a
satellite of Earth, Earth is a satellite of the sun. if Earth’s orbit were a circle, and
the sun were at the center of the circle, Earth would maintain a constant distance
from the sun. however, this is not the case. Earth’s orbit is an ellipse, and
elongated closed curve.
The sun is off set from the center of the ellipse. Because of this the distance
between Earth and the sun changes during Earth’s yearlong orbit. Earth gets
closest to the sun – 147 million km away – on January 3. The farthest point in
Earth’s orbit is about 152 million km away from the sun and is reached on July 4.
is this elliptical orbit causing the changing temperatures on Earth? If it were, you
would expect the warmest days in January. You know this isn’t the case in the
northern hemisphere. Something else is causing the change.
Even though Earth is closest to the sun in January, the over all amount of energy
Earth receives from the sun changes little through the year. However, the amount
of energy any one place on earth receives can vary quite a bit.
Earth’s Tilted Axis
Earth’s axis is tilted 23.5 degrees from a line perpendicular
to its orbit. This tilt causes the seasons. Daylight hours are
longer for the hemisphere tilted toward the sun. think of
how early it gets dark in the winter compared with the
summer. The hemisphere tilted toward the sun receives
more hours of sunlight than the hemisphere tilted away
from the sun.
Another effect of Earth’s tilt is that the sun’s radiation
strikes the hemisphere tilted toward it at a higher angle
than it does the other hemisphere. Because of this, the
hemisphere tilted toward the sun receives more
electromagnetic radiation per unit are than the
hemisphere tilted away.
a summer season results when the sun is in the sky longer
and its electromagnetic radiation strikes Earth at a higher
angle. Just he opposite occurs during winter.
Equinoxes and Solstices
Because of the tilt of Earth’s axis, the sun’s
position relative to Earth’s equator
constantly changes. Most of the time, the
sun is North or South of the equator. Two
times during the year, however, the sun is
directly over the equator.
Equinox
When the sun reaches an equinox, it is
directly above Earth’s equator, and the
number of daylight hours equals the number
of night time hours all over the world. At that
time, neither the northern nor the southern
hemisphere is tilted toward the sun. In the
norther hemisphere, the sun reaches the
spring equinox on March 20 and 21 and the
fall equinox on September 22 or 23. in the
southern hemisphere, the equinoxes are
reversed. These dates are the first days of
spring and fall.
solstice
The solstice is the pint at which the sun
reaches its greatest distance north or
south of the equator. In the northern
hemisphere, the sun reaches the summer
solstice on June 21 or 22, and the winter
solstice occurs on December 21 or 22.
just the opposite is true for the southern
hemisphere. When the sun is at the
summer solstice, there are more daylight
hours than during any other day of the
year. When it’s at the winter solstice, on
the shortest day of the year, we have the
most nighttime hours.
Earth Data Review
As you have learned, earth is an imperfect sphere that bulges slightly at
the equator and is somewhat flattened at the poles. The rotation of
Earth causes day and night. Earth’s tilted axis is responsible for the
seasons you experience, and our revolution around the sun marks the
passing of a year.
Review
1. Which earth motions causes night and day?
2. Why does summer occur in Earth’s norther hemisphere when
Earth’s north pole is tilted toward the sun?
3. the physical properties of Earth lists Earth’s distance from the sun as
an average. Why isn’t there one exact measurement of the this
distance?
4. What causes seasons on Earth?
5. What causes winter?
6. What effect on seasons does the sun being closest to Earth in
January have?