The Reason for the Seasons
8-4.4 Explain the motions of Earth and the
Moon and effects of these motions as they
orbit the Sun (including day, year, phases of
the Moon, eclipses, and tides.
VOCABULARY WORDS
Astronomy: The study of the moon,
stars, and other objects in space.
Axis: The imaginary line that passes
through Earth’s center and the North
and South poles.
What is a Rotation?
When earth spins on its axis this is called its rotation.
- A point on the equator will rotate at about 1,600 km per hour.
- Earth’s rotation on its axis causes day and night!
It is day on the side
of Earth facing the
sun.
It takes the earth 24
hours to rotate
ONCE on its axis.
What is a Revolution?
In addition to rotating on its axis, Earth travels around the Sun. The
movement of one object around another object is called revolution.
- Earth’s path as it revolves around the sun is called its orbit.
- Earth’s revolution around the sun is called a year! 365 ¼ days
WHAT ARE SEASONS?
What two factors cause the season?
Seasons on Earth
It is warm near the equator because sunlight hits Earth’s surface directly and
is less spread out. Why is it colder near the poles?
Most direct sunlight
Earth has seasons because its axis
is tilted as it moves around the sun.
What if the Earth’s axis were
straight up and down relative to the
sun, what would the seasons be
like?
EARTH’S AXIS
- When the north end of Earth’s axis is tilted toward the
sun, the Northern Hemisphere has summer
- When the north end of Earth’s axis is tilted away from
the sun, the Northern hemisphere has winter, therefore
getting less direct sunlight.
WHICH ONE OF THE GLOBES WOULD BE SUMMER IN THE NORTHERN HEMISPHERE?
EARTH’S AXIS
Vernal and Autumnal Equinox – they are the half way times
between the solstices, neither hemisphere is tilted toward or away
from the sun.
- One in spring around March 21 – Vernal (Spring) Equinox
- One in the fall around September 21 – Autumnal (Fall)
Equinox
WHICH ONE OF THE GLOBES WOULD BE THE AUTUMNAL
EQUINOX OR THE VERNAL `EQUINOX?
JUNE AND DECEMBER ….SOMETHING TO REMEMBER!!
Solstice – When the sun is overhead at either 23.5 degrees south or 23.5
degrees north of the equator.
- The day when the sun is overhead at 23.5 degrees south is the
WINTER SOLSTICE in the Northern Hemisphere. Around December 21
- The day when the sun is overhead at 23.5 degrees north is the
SUMMER SOLSTICE in the Northern Hemisphere. Around June 21
Review 1. Explain the process that causes day and night?
Earth rotates on its axis once per day. As Earth rotates, half its surface is facing the sun
(day) and half is facing away from the sun (night)
2. What two factors cause the cycle of the seasons?
The cycle of the seasons is caused by Earth’s revolution around the sun AND the tilt of
Earth’s axis.
3. Compare rotation and revolution.
Rotation is turning around a point or an axis; revolution is movement around another
object.
4. What do the words solstice and equinox mean? How are they related to the
position of Earth’s axis?
A solstice is a day when the noon sun is overhead at 23.5 degrees south or 23.5
degrees north. This occurs when one end of Earth’s axis is tilted most directly
toward the sun. Equinox means “equal night,” and occurs when neither pole of
Earth’s axis is tilted toward the sun.
5. Are changes in the distance between Earth and the sun important in causing
the cycle of the seasons?
The tilt of Earth’s axis, not its distance from the sun, is the cause of the cycle of
seasons.
IF YOU KNOW IT…. RAISE YOUR HAND AND SHOW IT!!
When it is winter in the Northern Hemisphere, which areas
on Earth get the most concentrated amount of light?
When it is summer in the Northern Hemisphere, which
areas on Earth get the most concentrated amount of light?