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

2. Basic Concepts…
(We will refer to the NORTHERN HEMISPHERE.)
1) Not caused by the Earth’s distance 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).

3. 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.

4. 3) Tropic of Cancer
 23.5 N Latitude

5. 3) Tropic of Capricorn
 23.5 S Latitude

6. 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.

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

8. 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. • Dec• 21. equator directly
hits
Hemisphere directly (Tropic
of Cancer 23.5 N). • average
• shortest day. angle of
insolation.
• June 21 – 22. • sun hits @ S. Hemisphere
• longest amount of daylight • equal amts of day of
directly at 23.5 S. (Tropicand
night/
Capricorn).
• high angle of insolation.
• low angle of insolation.

9. Angle of insolation - The angle at which the sun's rays strike a
particular location.
Only latitudes between the tropic of cancer and Tropic of Capricorn
can receive direct rays. The maximum angle of insolation is 90
degrees.
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?

10. Angle of Insolation…
* 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).

11. 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.

12. Annual Motions:
The “Reasons” for
Seasons.

13. The Sun-Earth-Moon System
Objectives
• Identify the relative positions and motions of Earth, the
Sun, and the Moon.
• Describe the phases of the Moon.
• Explain eclipses of the Sun and Moon.
Vocabulary
– ecliptic – synchronous rotation
– summer solstice – solar eclipse
– winter solstice – perigee
– autumnal equinox – apogee
– vernal equinox – lunar eclipse

14. The Sun-Earth-Moon System
The Sun-Earth-Moon System
• The relationships between the Sun, Moon, and
Earth are important to us in many ways.
– The Sun provides light and warmth, and it is the
source of most of the energy that fuels our society.
– The Moon raises tides in our oceans and illuminates
our sky with its monthly cycle of phases.
– Every society from ancient times to the present has
based its calendar and its timekeeping system on the
apparent motions of the Sun and Moon.

15. The Sun-Earth-Moon System
Daily Motions
• The Sun rises in the east and sets in the west,
as do the Moon, planets, and stars as a result of
Earth’s rotation.
• We observe the sky from a planet that rotates
once every day, or 15° per hour.

16. The Sun-Earth-Moon System
Daily Motions
Earth’s Rotation
– There are two relatively simple ways to demonstrate
that Earth is rotating.
1. A Foucault pendulum, which has a long wire, a heavy
weight, and will swing in a constant direction,
appears from our point of view to shift its orientation.
2. Flowing air and water on Earth are diverted from a
north-south direction to an east-west direction as a
result of Earth’s rotation in what is known as the
Coriolis effect.

17. If you sit and watch the Foucault pendulum for an hour,
you will "see" that the plane of the swing of the iron ball
slowly shifts anti-clockwise by about 8.4 degrees per
hour.
This is an optical illusion. The building is actually
shifting "under" the Foucault Pendulum!
The Foucault Pendulum (support + wire + iron ball) is
attached to this building. The building is attached to the
Planet Earth, which rotates on its own axis about once
every 24 hours.
The Earth also goes around the Sun, once every year.
The Sun, in turn, goes around the centre of our galaxy,
The Milky Way, once every 250 million years. These are
all local motions.

19. The Sun-Earth-Moon System
Daily Motions
Earth’s Rotation
– The length of a day as we observe it is a little longer
than the time it takes Earth to rotate once on its axis.
– Our timekeeping system is based on the solar day,
which is the time period from one sunrise or sunset to
the next.

20. The Sun-Earth-Moon System
Annual Motions
• The annual changes in length of days and
temperature are the result of Earth’s orbital
motion about the Sun.
• The ecliptic is the plane in which Earth orbits
about the Sun.

21. The Sun-Earth-Moon System
Annual Motions
The Effects of Earth’s Tilt
– Earth’s axis is tilted relative to the ecliptic at
approximately 23.5°.
– As Earth orbits the Sun, the orientation of Earth’s axis
remains fixed in space.
– At one point, the northern hemisphere of Earth is tilted
toward the Sun, while six months later it is tipped away
from the Sun.
– As a result of the tilt of Earth’s axis and Earth’s motion
around the Sun, the Sun is at a higher altitude in the
sky during summer than in the winter.

22. The Sun-Earth-Moon System
Annual Motions (Not on notes page).
The Effects of Earth’s Tilt
Altitude is
measured in
degrees from the
observer’s horizon
to the object. There
are 90 degrees
from the horizon to
the point directly
overhead, called
the zenith of the
observer.

23. The Sun-Earth-Moon System
Annual Motions
Solstices
– As Earth moves from position 1, through position 2, to
position 3, the altitude of the Sun decreases in the
northern hemisphere.
– Once Earth is at
position 3, the Sun’s
altitude starts to
increase as Earth
moves through
position 4 and
back to position 1.

24. The Sun-Earth-Moon System
Annual Motions
Solstices (Not on Notes Page)
– The summer solstice occurs around June 21 each
year when the Sun is directly overhead at the
Tropic of Cancer, which is at 23.5° N.
– The summer solstice
corresponds to the
Sun’s maximum
altitude in the sky 90°
in the northern
hemisphere.
*The latitude of Virginia Beach is
36.852N. The longitude is
-75.978W.

25. The Sun-Earth-Moon System
Annual Motions
Solstices (Not on Notes Page)
– The winter solstice occurs around December 21 each
year when the Sun is directly overhead at the
Tropic of Capricorn which is at 23.5° S.
– The winter solstice
corresponds to the
Sun’s lowest
altitude in the sky
in the northern
hemisphere.

26. The Sun-Earth-Moon System
Annual Motions
Equinoxes
– When the Sun is directly overhead at the equator, both
hemispheres receive equal amounts of sunlight.
– The autumnal equinox
occurs around
September 21,
halfway between
the summer and
the winter solstices
when the Sun is
directly over the
equator.

27. The Sun-Earth-Moon System
Annual Motions
Equinoxes
– The vernal equinox occurs around March 21, halfway
between the winter and the summer solstices when the
Sun is directly over the equator.
– For an observer at the
Tropic of Cancer or
Tropic of Capricorn,
the Sun is 23.5° from
the point directly
overhead during the
equinoxes.

28. The Sun-Earth-Moon System
Annual Motions
Equinoxes
For a person
standing at the x at
23.5º N, the Sun (at
noon) would appear
in these positions on
the winter solstice,
the vernal equinox,
and the summer
solstice. On the
autumnal equinox,
the Sun would be
at the same
altitude as on the
vernal equinox.