# planet earth

## earths day and night

How long does it take Earth to spin once on its axis? One rotation is 24 hours. That rotation is the length of a day! Whatever time it is, the side of Earth facing the Sun has daylight. The side facing away from the Sun is dark. If you look at Earth from the North Pole, the planet spins counterclockwise. As the Earth rotates, you see the Sun moving across the sky from east to west. We often say that the Sun is rising or setting. The Sun rises in the east and sets in the west. Actually, it is the Earths rotation that makes it appear that way. The Moon and the stars at night also seem to rise in the east and set in the west. Earths rotation is also responsible for this too. As Earth turns, the Moon and stars change position in the sky.

## earths seasons

The Earth is tilted 23 1/2 on its axis (Figure 24.10). This means that as the Earth rotates, one hemisphere has longer days with shorter nights. At the same time the other hemisphere has shorter days and longer nights. For example, in the Northern hemisphere summer begins on June 21. On this date, the North Pole is pointed directly toward the Sun. This is the longest day and shortest night of the year in the Northern Hemisphere. The South Pole is pointed away from the Sun. This means that the Southern Hemisphere experiences its longest night and shortest day (Figure 24.11). The hemisphere that is tilted away from the Sun is cooler because it receives less direct rays. As Earth orbits the Sun, the Northern Hemisphere goes from winter to spring, then summer and fall. The Southern Hemisphere does the opposite from summer to fall to winter to spring. When it is winter in the Northern hemisphere, it is summer in the Southern hemisphere, and vice versa.

## earths motions

Earths axis is an imaginary line passing through the North and South Poles. Earthsrotation is its spins on its axis. Rotation is what a top does around its spindle. As Earth spins on its axis, it also orbits around the Sun. This is called Earths revolution. These motions lead to the cycles we see. Day and night, seasons, and the tides are caused by Earths motions.

## earths rotation

In 1851, Lon Foucault, a French scientist, hung a heavy iron weight from a long wire. He pulled the weight to one side and then released it. The weight swung back and forth in a straight line. If Earth did not rotate, the pendulum would not change direction as it was swinging. But it did, or at least it appeared to. The direction of the pendulum appeared to change because Earth rotated beneath it. Figure 24.9 shows how this might look. A Turn of the Earth In this video, MIT students demonstrate how a Foucault Pendulum is used to prove that the Earth is rotating. See the video at . MEDIA Click image to the left or use the URL below. URL:

## earths gravity

Earth and Moon orbit each other. This Earth-Moon system orbits the Sun in a regular path (Figure 24.4). Gravity is the force of attraction between all objects. Gravity keeps the Earth and Moon in their orbits. Earths gravity pulls the Moon toward Earths center. Without gravity, the Moon would continue moving in a straight line off into space. All objects in the universe have a gravitational attraction to each other (Figure 24.5). The strength of the force of gravity depends on two things. They are the mass of the objects and the distance between them. The greater the objects mass, the greater the force of attraction. As the distance between the objects increases, the force of attraction decreases.

## earths magnetism

Earth has a magnetic field (Figure 24.6). The magnetic field has north and south poles. The field extends several thousand kilometers into space. Earths magnetic field is created by the movements of molten metal in the outer core. Earths magnetic field shields us from harmful radiation from the Sun (Figure 24.7). If you have a large bar magnet, you can hang it from a string. Then watch as it aligns itself in a north-south direction, in response to Earths magnetic field. A compass needle also aligns with Earths magnetic field. People can navigate by finding magnetic north (Figure 24.8).

## earths shape size and mass

As you walk, the ground usually looks pretty flat, even though the Earth is round. How do we know this? We have pictures of Earth taken from space that show that Earth is round. Astronauts aboard the Apollo 17 shuttle took this one, called The Blue Marble (Figure 24.1). Earth looks like a giant blue and white ball. Long before spacecraft took photos of Earth from space, people knew that Earth was round. How? One way was to look at ships sailing off into the distance. What do you see when you watch a tall ship sail over the horizon of the Earth? The bottom part of the ship disappears faster than the top part. What would that ship look like if Earth was flat? No part of it would disappear before the other. It would all just get smaller as it moved further away. In the solar system, the planets orbit around the Sun. The Sun and each of the planets of our solar system are round. Earth is the third planet from the Sun. It is one of the inner planets. Jupiter is an outer planet. It is the largest planet in the solar system at about 1,000 times the size of Earth. The Sun is about 1,000 times bigger than Jupiter! (Figure The outer planets in the solar system are giant balls of swirling gas. Earth and the other inner planets are relatively small, dense, and rocky. Most of Earths surface is covered with water. As far as we know, Earth is also the only planet that has liquid water. Earths atmosphere has oxygen. The water and oxygen are crucial to life as we know it. Earth appears to be the only planet in the solar system with living creatures. You can learn more about the planets in the Our Solar System chapter. Some of the different parts of the Earth are our: Since Earth is round, the layers all have the word sphere at the end (Figure 24.3). All of Earths layers interact. Therefore, Earths surface is constantly undergoing change.

## earths revolution

Earths revolution around the Sun takes 365.24 days. That is equal to one year. The Earth stays in orbit around the Sun because of the Suns gravity (Figure 24.12). Earths orbit is not a circle. It is somewhat elliptical. So as we travel around the Sun, sometimes we are a little farther away from the Sun. Sometimes we are closer to the Sun. Students sometimes think the slightly oval shape of our orbit causes Earths seasons. Thats not true! The seasons are due to the tilt of Earths axis, as discussed above.

## instructional diagrams

No diagram descriptions associated with this lesson

## questions

An imaginary line running through the poles of Earth is called

``````a. orbit

b. pole

c. magnetic pole

-->  d. axis
``````

How long does it take the Earth to make one rotation on its axis?

``````-->  a. one day

b. one month

c. one year

d. one week
``````

How many degrees does the Earth turn in one day?

``````a. 90 degrees

b. 180 degrees

-->  c. 360 degrees

d. 1080 degrees
``````

What causes Earths seasons?

``````a. the planets rotation

-->  b. the planets tilt

c. the planets revolution

d. the planets orbit
``````

Earths magnetic field is due to

``````-->  a. the movement of metal in Earths outer core

b. a bar magnet that is lodged between the magnetic poles

c. the force of the Suns magnetic field

d. mantle convection
``````

Earth is the only planet in the solar system known to have liquid water.

``````-->  a. true

b. false
``````

All objects in the solar system have gravitational attraction to each other.

``````-->  a. true

b. false
``````

If you look at Earth from the North Pole, the planet spins in a clockwise direction.

``````a. true

-->  b. false
``````

Earth is divided into four hemispheres.

``````a. true

-->  b. false
``````

The moon appears to rise in the east because Earth rotates from east to west.

``````a. true

-->  b. false
``````

Leap year is needed because Earth takes more than 365 days to orbit the Sun.

``````-->  a. true

b. false
``````

Earth tilts about 3 degrees on its axis.

``````a. true

-->  b. false
``````

Winter begins in the Southern Hemisphere on June 21.

``````-->  a. true

b. false
``````

Earth gets more gravitational pull from the Sun than the Moon because it is larger.

``````a. true

-->  b. false
``````

The longest day in the Northern Hemisphere occurs on June 21.

``````-->  a. true

b. false
``````

The planets move in fixed paths around the Sun.

``````-->  a. true

b. false
``````

The Earths mostly iron crust produces the magnetic field.

``````a. true

-->  b. false
``````

To make one complete rotation takes Earth 365.24 days.

``````a. true

-->  b. false
``````

Planets closer to the sun that Earth have shorter years.

``````-->  a. true

b. false
``````

The hemisphere that receives less direct rays of sunlight is cooler.

``````-->  a. true

b. false
``````

all of the water on Earth

``````a. axis

b. biosphere

c. hemisphere

-->  d. hydrosphere

e. lithosphere

f. revolution

g. rotation
``````

half of a sphere

``````a. axis

b. biosphere

-->  c. hemisphere

d. hydrosphere

e. lithosphere

f. revolution

g. rotation
``````

spinning motion around an axis

``````a. axis

b. biosphere

c. hemisphere

d. hydrosphere

e. lithosphere

f. revolution

-->  g. rotation
``````

all of the living organisms on Earth

``````a. axis

-->  b. biosphere

c. hemisphere

d. hydrosphere

e. lithosphere

f. revolution

g. rotation
``````

imaginary line passing through the poles of a planet

``````-->  a. axis

b. biosphere

c. hemisphere

d. hydrosphere

e. lithosphere

f. revolution

g. rotation
``````

all of the solid rock of Earth

``````a. axis

b. biosphere

c. hemisphere

d. hydrosphere

-->  e. lithosphere

f. revolution

g. rotation
``````

movement in an orbit around a larger body

``````a. axis

b. biosphere

c. hemisphere

d. hydrosphere

e. lithosphere

-->  f. revolution

g. rotation
``````

The outer planets of the solar system are giant balls of

``````a. solid ice.

b. dense rock.

c. liquid water.

-->  d. swirling gas.
``````

Without gravity, the moon would

``````a. drop to Earths surface.

b. start orbiting the sun.

-->  c. fly off into space.

d. shrink in size.
``````

The strength of the force of gravity between two objects depends on their

``````a. mass.

b. speed.

c. distance apart.

-->  d. two of the above
``````

Which statement about Earths magnetic field is false?

``````a. It extends into space.

-->  b. It is caused by gravity.

c. It has north and south poles.

d. It shields Earth from harmful radiation.
``````

Earths motions contribute to

``````a. tides.

b. seasons.

c. day and night.

-->  d. all of the above
``````

The side of Earth facing away from the sun is always

``````-->  a. in the dark.

b. experiencing winter.

c. cooler than the other side.

d. two of the above
``````

Earth has seasons because

``````-->  a. it tilts on its axis.

b. its orbit is elliptical.

c. it rotates on its axis.

d. its distance from the sun varies.
``````

## diagram questions

No diagram questions associated with this lesson