Earth in SpaceCHAPTER 19 – LESSON 1

What causes the cycle of seasons

on Earth?

Your prior knowledge is what you already know

before you read about a topic. Before you

read, write what you know about seasons on

Earth in the graphic organizer. As you read write

down what you learn.

Share ideas.

Building Background

What time does the sun rise in the morning

and set at night?

Describe how the number of hours of

daylight changes during the winter and

summer.

Why does all of this happen?

Earth’s Rotation

Axis: the imaginary line that passes through Earth’s center and the

North and South poles.

Rotation: the spinning of Earth on its axis.

Earth’s rotation is what causes night and day.

The Earth rotates from West to East.

It takes the Earth 24 hours to rotate once – this is

Earth’s Revolution

Earth also travels around the sun. This movement is called a

Revolution: the movement of one object around another.

It takes the Earth 365.25 days (or 1 year) to make one revolution

around the Sun.

Earth’s orbit around the sun is shaped like an ellipse (oval).

Earth’s Revolution

What is a leap year?

Why do leap years occur?

Earth’s Seasons

Sunlight hits the Earth at different angles.

Near the Equator gets the direct sunlight and the further you move

away from the Equator (toward the Poles) the less direct sunlight you get).

This means that the closer you are to the Equator the warmer the

climate is and the farther away you are the colder the climate is.

Earth’s Seasons

Based on the information that was just presented, what do you think

causes Earth’s seasons?

Earth’s Seasons

Earth has seasons because its axis is tilted as it revolves around the

sun.

As Earth revolves around the sun, the north end of its axis is tilted away from the sun for part of the year and toward the sun for part

of the year. The same is true for the south end of its axis.

Summer and Winter are caused by Earth’s tilt as it revolves around

the Sun. The change in seasons is NOT caused by the change in

Earth’s distance from the sun.

Earth’s Seasons

Earth’s Seasons - Solstices

Solstices: when the sun is farthest north or south of the equator

Occurs twice a year

Summer Solstice

Occurs June 21

Longest day of the year (daytime longer than nighttime)

Northern hemisphere is tilted toward the sun.

Winter Solstice

Occurs December 21

Shortest day of the year (daytime shorter than nighttime)

Southern hemisphere is tilted toward the sun.

Earth’s Seasons - Equinoxes

Equinox – when neither hemisphere is tilted toward or away from the

sun.

Occurs twice a year.

Vernal (Spring) Equinox

March 21

Daytime = Nighttime

Autumnal Equinox

September 22

Daytime = Nighttime

Earth’s Seasons

How are the solstices and equinoxes

related to the seasons?

How would the seasons be different if

Earth were not tilted on its axis?