1) The seasons are caused by the tilt of the Earth's axis of rotation as it revolves around the sun, not by variations in distance from the sun. 2) As the Earth revolves around the sun, different parts of the planet are tilted either toward or away from the sun, receiving different amounts of sunlight and experiencing different seasons. 3) The tilt of the Earth's axis remains fixed at 23.5 degrees, causing the northern hemisphere to be tilted toward the sun during northern hemisphere summer and away from the sun during winter.

1. Annual Motions:
What causes the
seasons?
Earth Science

2. Cornell Notes
Rotation
2) Revolution
3) Earth’s orbit
1)

3. Astronomy Basics…

Rotation –
when a planet
spins on its
axis. It takes
24 hours for
Earth to spin
on its axis
once.

4. Astronomy Basics…



Revolution –
The time it takes
when an object
goes around a
sun or planet.
It takes 365 days
for Earth to
revolve once
around the Sun.
It takes 30 days
for the moon to
revolve around
Earth.

5. Astronomy Basics…



Earth’s orbit
– The Earth’s
orbit is
Eccentric (oval-shaped).
Perihelion –
closest to the
sun.
Aphelion –
farthest from
the sun.

6. Basic Concepts…(We will refer to the NORTHERN HEMISPHERE.)
1) Not caused by the Earth’s distant from the sun!!!

Aphelion – Earth is 94.5 million miles away from
the sun (IN JULY).

Perihelion – Earth is 91.5 million miles away from
the sun (IN JANUARY).

7. 2) Caused by the Earth’s tilt of the AXIS: 23.5°.

The Earth’s Axis remains at a fixed place (as it orbits around the
sun), so that, at one point, the northern hemisphere of Earth is
tilted toward the sun, while at another point six months later, the
northern hemisphere is tipped away from the sun.

8. 3) Tropic of Cancer

23.5 N Latitude

9. 3) Tropic of Capricorn

23.5 S Latitude

10. How does the angle affect our
seasons?

As the winter solstice approach, the amount of
daylight is decreased. Why? During the winter, the
sun’s rays will not hit our area directly. The sun
appears to be at a lower altitude.

11. 
In contrast, during the summer, the sun
hits our area more directly and the sun
appears to be at a higher altitude.

12. Vernal Equinox:
• Spring
Vernal Equinox
• March 21.
• hits equator directly
• average angle of insolation.
• equal amts of day and night
Summer Solstice
Winter Solstice
Autumnal Equinox:
• Solstice:
Autumnal Equinox WinterAutumn/Fall
Summer Solstice:
• Sept 21.
• winter
• Sun hits the N.
Hemisphere directly (Tropic
of Cancer 23.5 N).
hits
• Dec• 21. equator directly
• June 21 – 22.
• longest amount of daylight
• high angle of insolation.
• average
• shortest day. angle of
insolation.
• sun hits @ S. Hemisphere
• equal amts of day of
directly at 23.5 S. (Tropicand
night/
Capricorn).
• low angle of insolation.

13. Angle of
insolation
- The angle
at which the
sun's rays
strike a
particular
location.

14. Only latitudes
between the
tropic of cancer
and Tropic of
Capricorn can
receive direct
rays. The
maximum
angle of
insolation is 90
degrees.

15. •
The angle of insolation
varies with:
•
1) Season - The
maximum angle of
insolation, maximum
duration, and maximum
intensity occur during the
summer.
•
2) Latitude - As latitude
increases, the angle of
insolation decreases
•
3) Time of day- From
morning to noon the angle
of insolation increases
and the from noon to
evening the angle of
insolation decreases.
Q: When is the best time to get a tan during the summer? 12 noon or 6 pm? Why?

16. •
As the angle of insolation increases, the sun gets higher in the
sky. As the sun gets higher in the sky the length of the shadow
decreases.
Effect 1:
a) Winter – Our shadows are longer at noon (angle in low).
b) Summer – Our shadows are shorter at noon (angle is high).

17. Effect 2:
a) Winter – The duration (amount of time) of daylight is shorter.
b) Summer – The duration of daylight is longer.
Effect 3:
a)
Winter – intensity (heat) of insolation is low.
b)
Summer – intensity of insolation is higher.