Lesson D-9. Cause of Seasonal Changes
Lesson D-9
Cause of Seasonal Changes
Overview:
In Lesson D-6 [Vol. I] students began observing and recording daily temperature(s),
day length, and other parameters of weather in conjunction with the date and the Earth’s
orbit around the sun. By doing so, they correlated seasonal changes with the Earth’s orbit,
but they may not have discerned why those seasonal changes occur as they do. This
lesson will bring them to observe, model, and analyze how the angle at which the sun’s
rays strike the Earth shift as it moves around its orbit. In turn, they will recognize how
this results in the changing environmental parameters and hence the seasons.
This lesson does point out degrees latitude of significant points. However, the
conceptual understanding can be conveyed without doing so, or Lesson D-12, Mapping
the Earth,” may be integrated. Either way, this lesson will give students an understanding
why seasonal changes occur as they do.
Time Required:
Part 1. How Solar Heating Shifts With the Angle of the Sun’s Rays (Demonstration, 1015 minutes, plus discussion 10-15 minutes)
Part 2. Modeling How Day-Length and Height of the Mid-day Sun Vary With Time of
Year (modeling exercise plus interpretive discussion, 60-90 minutes)
Outcomes: Through this exercise, students will be able to:
1. Summarize the changes in physical parameters (day length, height of the sun at noon,
and average temperatures) and changes in the world of living things that occur with
the passing months in your region. (These observations should be evident from
carrying on Lesson D-6.)
2. Explain and model with a flashlight beam how/why heating of a given location varies
with the height of the sun above the horizon.
3. With a globe and a ball representing the sun, model how the Earth maintains its
direction and degree of tilt as it orbits the sun.
4. Explain and show through modeling how this fact (3 above) results in day-length and
height of the noon sun changing over the course of a year from the perspective of any
given location on the globe. Correlate these with seasonal changes experienced.
Bernard J. Nebel, Ph.D.
5. Explain and show through modeling why the seasons in the southern hemisphere are
reversed.
6. Describe the significance of Summer and Winter Solstices in terms the Earth’s orbit
and changes that occur. Give the dates for those events. Describe the significance of
the Spring and Fall Equinoxes.
7. Explain the significance of the Tropic of Cancer, the Tropic of Capricorn, and the
Arctic and Antarctic Circles in terms of how the sun will be observed at those
locations on the solstices. Give the degrees latitude for those circles.
8. Explain why the hottest period of summer and the coldest period of winter occur after
the respective solstices.
9. Give evidence (observations) that support our understanding that the Earth maintains
its direction and degree of tilt as it orbits the sun. Give evidence that the amount of tilt
from the plane of the Earth’s orbit is 23.5 degrees.
Required Background:
Lesson D-6, Seasonal Changes and the Earth’s Orbit (This lesson should ongoing.)
Lesson D-12, Mapping the Earth, Latitude and Longitude, may be integrated. If so,
students should be familiar with measuring angles in degrees.
Materials:
Flashlight with spotlight beam
Globe mounted on a stand
Ball to represent the sun
String
Protractor
Teachable Moments:
In the course of Lesson D-6, students will be observing changes in physical
parameters (day length, height of the sun at noon, and average temperatures) and
correlating these with the movement of the Earth in its orbit about the sun. At any point
that students become aware of the correlation, invite them to explore the reasons in more
detail.
Methods and Procedures:
Lesson D-6, Seasonal Changes And The Earth’s Orbit, (BFSU [Vol. I]) consisted of
modeling the Earth’s orbit around the sun according to current calendar dates in
conjunction with recording and compiling a record of temperature, day length, height of
Lesson D-9. Cause of Seasonal Changes
the noon-day sun, precipitation, and whatever changes/events in the natural biological
world were noted.
Major things, which students should have become experientially familiar with from
that study include:
• During the spring months:
There is a steady increase in length of days (the sun rising earlier and setting later)
The noonday sun is steadily higher in the sky
On average, temperatures are progressively warmer
• During the fall months there is a reverse of these changes.
• Depending on where you live there may or may not have been a significant
difference in amounts of precipitation during different seasons.
• The world of living things responds in harmony with these changes in temperature,
precipitation, and day-length.
Another thing emphasized in the forgoing study is the fact that the Earth maintains
the same direction of tilt as it orbits the sun. This is verified by the fact that the position
of the North Star remains constant (over the North Pole) throughout the year. If the tilt of
Earth changed during the year, we would find the North Pole pointing to different
locations in the sky, not consistently at the North Star.
Hopefully, Lesson D-6 is still ongoing and this lesson will naturally link in.
Otherwise, with a globe of the world in hand and a beach ball representing the sun in the
center of the room, review these observations and trends as necessary. Then pose the
question: Why do these changes in day-length, height of the sun at noon, and average
temperature occur as they do?
Part 1. How Solar Heating Shifts With the Angle of the Sun’s Rays
Some students, likely, will have made the pertinent observations and already have the
answer. Still, guide all students through the following steps of reasoning with Q and A
discussion.
First: What would happen if the sun were to suddenly go out? What is the temperature
far out in space, e.g., at the distance of Pluto or beyond? Students should conclude that
without the sun, the Earth would cool to a temperature like that of far outer space. In
short, the Earth would become a frozen ball making all life impossible. Livable
temperatures on Earth depend on the sun.
Most conspicuously, we experience the sun as providing light. Therefore, some
students may be puzzled by the connection between the sun and heat. Remind them that
light is a form of energy (Lesson C-1). As light is absorbed by water, land, or vegetation
it is converted to heat. Ask students to speak of their experience of sitting in the sun and
feeling its warmth, even as the air may be cool. This is the effect of sunlight being
absorbed by our skin and being converted to heat. In the summer, it is essentially
Bernard J. Nebel, Ph.D.
automatic to seek shade to avoid such heating. Another example is experienced as we
step barefoot on pavement or sand that has been in the sun.
Students may question why it is that we don’t feel such heat from room lights.
Explain that it is simply a matter of intensity. Sunlight contains in the order of 10,000
times more energy that normal room light. Our eyes are very good at compensating so
that we don’t “see” this magnitude of difference.
Note: An additional point that is worth inserting here is that air is transparent to light.
Hence, sunlight comes through the atmosphere without heating it. It is absorption of
light energy by the Earth’s surface and its consequent conversion to heat that, in turn,
heats the atmosphere. Thus, the air is effectively heated from ground level up. This bit
of information is very important when we come to discussing climates and weather
(Lesson D-13 and further).
Second: All of the Earth receives light. Why should equatorial regions be warm while
polar regions are cold? Again, some students may already recognize that the heating
effect from the sun is most intense when it is closest to overhead. But why? It will be
well to take them through the following demonstration to see the logic behind this result.
Hold a flashlight two to three feet above a piece of paper and shine its beam straight
down on the paper. Have students pencil a circle around the image of the beam and mark
its center point. Next, hold the flashlight at the same distance from the paper, but at an
angle to the side (20-30 degrees from the horizontal). Point the beam at the center of the
previous image, and again, have students pencil a line around the image. It will be the
same width as before but stretched into an oval. Guide students through the following
steps of reasoning.
• The beam from the flashlight represents a constant stream of light energy. The spot
of light on the paper is the area receiving all of that energy.
• As the beam is moved from the vertical toward the horizontal, the area receiving
that stream of light energy gets larger. (The oval has a larger area than the circle.) In
other words, the same amount of light energy is spread over a larger area.
Therefore, any given area (square inch, square centimeter) will receive less input of
energy.
• Exactly the same idea holds for the sun’s rays striking the earth. When the sun is
directly overhead (vertical to the surface of the earth) there is maximum energy
received (maximum heating) per unit area. When the sun is lower in the sky, its rays
are spread out over a larger area and heating is reduced accordingly.
Lesson D-9. Cause of Seasonal Changes
Part 2. Modeling How Day-Length and Height of the Mid-day Sun Vary With Time
of Year
The remainder of this exercise is to model how day-length and the angle at which the
sun’s rays strike the Earth change throughout the year at any given location. Both of these
effects occur because of the Earth maintaining its degree and direction of tilt as it orbits
the sun.
For this modeling, have a ball, or even better a naked lamp in a dimmed room, on a
table in the center of the room to represent the sun. Place a globe on a stand so that it can
be moved around the “sun” accurately maintaining its degree and direction of tilt. (For
Lesson D-6, you may have attached a ball representing the sun to a position near the
ceiling to keep it out of the way. For this modeling you need to have the “sun” and the
“Earth” on the same level.) As in Lesson D-2, stick a rice grain on the globe with
toothpaste to model “Joe” observing the sun from your location.
Have students recall or review how the Earth, by maintaining its degree and direction
of tilt, has the northern hemisphere tipped toward the sun during the summer months and
away from the sun during the winter months. With the globe modeling a given time of
year, e.g., the Summer Solstice when it is tipped toward the sun, slowly rotate the globe
and have students estimate the fraction of the 24-hour day that Joe would be able to see
the sun from his location on Earth. If Joe is in the United States, Europe, or elsewhere in
the northern hemisphere, student will observe that Joe is in the lighted portion for
considerably more than half of the Earth’s rotation. That is, he will be experiencing days
longer than 12 hrs and nights proportionally shorter than 12 hours.
Likewise model the angle at which Joe would see the sun above the horizon at noon.
You may have students do this by holding a piece of string with one end at Joe and the
other end toward the center of the “sun” and estimating the angle between the string and
the surface of the globe at that point.
Note: Students may actually measure this angel using a protractor. Hold the flat edge
of the protractor so its flat edge is centered at Joe’s location and tangential to the
surface of the globe. Where the string from Joe’s location (the center of the
protractor) crosses the protractor is the angle of the sun above the horizon. Note that
the sun is so much larger than the Earth and so far away (93 million miles) that its
rays striking the Earth are parallel. Therefore, the angle that the sun appears above the
horizon from any given location is entirely a matter of the curvature of the Earth.
Pursue the same aspects of modeling with the Earth at the opposite side of its orbit,
the December-January portion of the orbit when the Earth’s North Pole is tilted away
from the sun. Here, assuming that Joe is at the same location in the northern hemisphere,
students will find that Joe should experience days considerably shorter than 12 hours and
a noon sun much lower in the sky.
Note: I have assumed Joe to be located in the northern hemisphere for descriptive
Bernard J. Nebel, Ph.D.
purposes only. In actuality, you will want to place Joe at your location. Wherever that
is, students should find that their modeling closely parallels their real-life
observations and experience. If there is a marked discrepancy they should look for
errors.
After finding a correspondence between your modeling and real-life observations,
move on to placing “Joe” in other locations on the globe as students may choose, and
conduct the same analysis. They should find a correspondence between their modeling
and any data obtained from other sources. For example, if they place Joe in New Zealand
they will note that he will experience long days, higher mid-day sun, and summer
conditions in the December-January period and the winter season in the June-July period.
The modeling of Joe at the North Pole will show why there are six months of continuous
darkness and six months of continuous light there.
There are four points in the Earth’s orbit, hence dates on the calendar, that are
particularly noteworthy: (Note the perspective is from the northern hemisphere.)
The Summer Solstice, June 21-22
The Fall Equinox, September 21-22
The Winter Solstice, December 21-22
The Spring Equinox, March 21-22
The official change from one season to the next is designated according to these dates.
Ask students, “What do they mean in terms of the Earth’s orbit around the sun?”
With a few hints and doing the same sort of modeling, they will be able to derive the
answer. The Winter Solstice is the point at which the Earth’s tilt lines up exactly away
from the sun. It is the point (date) at which days stop getting shorter and start getting
longer, and the mid-day sun stops getting lower and starts getting higher again. The
Summer Solstice is the opposite point. The equinoxes are the exact midway points. At the
equinoxes, the tilt of the Earth (the North Pole) is neither toward nor away from the sun;
it is exactly to the side with respect to the sun. At the point of the equinoxes, every
location on Earth experiences day and night as equal (12 hour day; 12 hour night) hence
the term. Standing on the North or South Pole might be considered an exception. Here,
Joe would experience the sun apparently traveling (because of the Earth’s rotation) 360
degrees around on the horizon.
In addition to the North and South Poles and the equator, there are four particularly
significant latitudes, “circles” that are commonly mentioned. They are the:
Arctic Circle
(latitude 66.5 degrees north)
Antarctic Circle (latitude 66.5 degrees south)
Tropic of Cancer (latitude 23.5 degrees north)
Tropic of Capricorn (latitude 23.5 degrees south)
Ask students, “What is the significance of these latitudes? What makes them special?”
Lesson D-9. Cause of Seasonal Changes
Challenge students to determine the answers by using the same modeling activity, but by
focusing on where the vertical rays of the mid-day sun strike the Earth at various times of
the year.
Students will discover that at the Summer Solstice the sun’s rays will be
perpendicular to the Earth at the Tropic of Cancer. Said another way, if Joe is standing on
the Tropic of Cancer at the Summer Solstice, he will experience the sun passing exactly
overhead so that, at mid-day (solar noon) he would cast no shadow at all. The Tropic of
Cancer marks the northernmost latitude where this will happen. The same holds for the
Tropic of Capricorn at the Winter Solstice. The Tropic of Capricorn marks the
southernmost latitude where vertical rays of the sun may be experienced.
Note: A common but erroneous belief is that if one is standing on the equator, the sun
passes exactly overhead every day. FALSE! Standing on the equator, the sun will go
exactly overhead on just two days, the Spring Equinox and the Fall Equinox. From
the Spring Equinox to the Fall Equinox, the mid-day sun to will be to north of
overhead; from the Fall Equinox to the Spring Equinox, it will be to the south of
exactly overhead. Students may show this to be the case by their modeling.
The Arctic and Antarctic Circles are also designated by sun’s rays at the Summer and
Winter Solstices respectively. Challenge students to determine the significant aspect.
Their modeling should reveal that at the Summer Solstice (the North Pole tilted toward
the sun) the sun’s rays extend exactly to the far side of the Arctic Circle. At the Winter
Solstice they extend just to the near side.
Have students note in their modeling that the Arctic Circle is the southern most
latitude where Joe can experience a 24-hour day (on the Summer Solstice) and a 24-hour
night (on the Winter Solstice). If he is further north, he will experience more summer
days in which the sun does not go below the horizon and more winter days in which the
sun does not get above the horizon. Only at the North Pole, however, will he experience a
continuous six months of light followed by six months of darkness. On the Spring
Equinox, at the North Pole, he will experience a 24-hour “sunrise” as he sees the sun
move all the way around the horizon with Earth’s rotation.
Following the Spring Equinox, Joe’s perception from his North Pole position, is that
the sun continues its movement all the way around the sky but gradually gets higher until
it reaches its maximum height, 23.5 degrees above the horizon, on the Summer Solstice.
Then Joe will see the sun circling gradually lower and lower to a 24-hour “sunset” at the
Fall Equinox to be followed by the six-month night. Have students verify all of this by
their modeling and the same thing, except for a reverse of seasons, for the Antarctic
Circle and South Pole.
Temperature Lag
Bernard J. Nebel, Ph.D.
Students’ records and experience reveal that the hottest period of summer (from the
perspective of the northern hemisphere again) generally occurs during July and August,
the two to eight weeks following the Summer Solstice. Similarly, the coldest period of
winter occurs during January and February, two to eight weeks following the Winter
Solstice. If students don’t recognize this discrepancy themselves, point it out and inquire:
Strictly from the amount of solar radiation received, one would logically expect the high
and low temperatures to center around the solstices. Why is this not the case?
Let students ponder; they may come up with the answer themselves. If they don’t
provide hints: Does the temperature of a pot of water change immediately with the
intensity of the burner? No! With the burner on high, it takes time for the water to heat
up; after the burner is turned off, the water gives off heat slowly, cooling gradually. This
is known as a TEMPERATURE LAG effect. The temperature change of the water lags
behind the heat from the burner. The same is true for the Earth. Two thirds of the Earth’s
surface is covered by water. Like the pot of water on the stove, water on Earth (oceans,
etc.) cause the temperature on Earth to lag behind the intensity of the sun’s heating.
In conclusion, students should understand that the basic cycle of seasons from winter
to summer and back is directly related to and dependent upon the amount (duration) and
intensity (angle of the sun above the horizontal) of solar radiation received. Both duration
and intensity for any given region of the Earth change throughout the year, because of the
way that the Earth maintains its degree and direction of tilt as it orbits the sun.
Hasten to point out, though, this explains only the general seasonal cycle of average
temperatures. It does not explain why some regions regularly receive large amounts of
precipitation while other regions receive very little and are consequently deserts. Nor
does it explain day-to-day weather fluctuations. In other words there are many additional
factors that are involved, some of which we will turn to in Lesson D-13 and following
lessons concerning climate and weather.
History
We have approached this entire lesson starting with the given that the Earth maintains
its degree and direction of tilt as it orbits the sun. Did the ancients have such knowledge
to start with? Obviously not! They started with the concept, based on casual experience,
that the Earth was flat and the sun revolved around the Earth. This only began to change
after Copernicus (1473-1543) proposed the sun-centered solar system. There were
countless sailors and explorers sighting the sun at different times from different locations
that led to the understanding we have today. In other words, the early explorers had no
knowledge of the existence of the Tropic of Cancer or what it represented. It was their
observations, that this latitude was the farthest north that vertical rays from the sun could
be experienced that led them to demark this latitude (23.5 degrees north) as the Tropic of
Cancer, and the same for the other lines/circles we have discussed. In turn, such
observations led to the conclusion that the Earth’s axis of rotation is tilted as it is (23.5
degrees) to the plane of its orbit around the sun and continually aimed at the North Star.
Lesson D-9. Cause of Seasonal Changes
You may pursue this history at your discretion.
Questions/Discussion/Activities To Review, Reinforce, Expand, and Assess Learning:
Students should record in their science notebooks (see Appendix 3):
a) a list of the major factors that distinguish winter and summer with supporting
description of how they gradually change from one to the other
b) diagrams with supporting statements explaining how solar heating of the Earth’s
surface changes with the length of the day and height of the sun at its zenith
c) a diagram showing how the Earth maintains its direction and degree of tilt as it
orbits the sun and how this, in turn, causes the shift in day length and height of the
sun at its zenith
e) an explanation as to why seasons in the southern hemisphere are reversed from
those in northern hemisphere
f) diagrams showing how the sun’s rays strike the Earth at the summer solstice, the
winter solstice, and at the spring and fall equinoxes, and in this regard, point out
the significance of the Topics of Cancer and Capricorn and the arctic circles
Set up an activity center where students can continue modeling how “Joe” perceives the
sun from different locations on the globe at different seasons.
Make a poster depicting the angle above the horizontal that people would see the midday
sun on the Summer Solstice. … the Winter Solstice.
What causes the average change of temperatures that result in changing seasons? List all
the factors involved.
Why are seasons in the southern hemisphere reversed from those in the northern
hemisphere?
Playact early scientists giving evidence (from various observations) for the Earth being
spherical and orbiting the sun to those believing that the sun was circling a flat Earth.
List five circles on the globe, giving the latitude of each, that have particular significance.
Tell why each is significant.
Embark students on a project of contrasting the height of the noon sun (for your location
at your time of year) as determined by modeling with real-life observations of the
height of the noon sun above the horizon (level). The angle of the sun above the
horizon can be measured easily by tying one end of a two-to-three-foot string to the
center point of a protractor. Hold the protractor in a vertical position with its flat edge
level and aimed toward the sun. With the other end of the string tied around a finger,
adjust the position of the finger so that the shadow of the finger falls over the center
of the protractor. Where the string crosses the side of the protractor gives the angle of
the sun above the horizon. The determinations of the angle from modeling and reallife observations should correspond, allowing for some inaccuracy in measurements.
Bernard J. Nebel, Ph.D.
In small groups, pose and discuss questions such as:
Explain how/why the amount of solar heating varies with day-length and the height of
the sun above the horizon.
How could early navigators determine their latitude as they sailed across oceans?
(Sighting the height of the sun at its highest point and adjusting for the time of year
will give latitude.)
Ask questions such as: If Joe observes the sun passing exactly overhead on the Summer
Solstice, where (on what latitude) is he located?
Many people hold the misunderstanding that the change in seasons is caused by the
Earth’s orbit taking it closer or further from the sun. Have students give evidence
that refutes this idea.
To Parents and Others Providing Support:
Call attention to solstices and equinoxes as they occur and discuss their significance.
Call children’s attention to the greater intensity of solar heating as the sun gets higher in
the sky as they experience it. Discuss why this occurs.
Call children’s attention to lengthening days in the spring, shortening days in the fall, and
the height of the noon-day sun above the horizon. (This may be done by noting the
length of noon-day shadows.) Discuss how/why these changes are responsible for the
change of seasons.
Assist children in modeling how they would perceive the sun (day-length and height
above the horizon at mid-day) if they were to travel to a chosen location at a certain
time of year.
As children note changes (physical and/or biological) that herald the change of seasons,
discuss: What is causing these changes?
Connections to other topics and follow up to higher levels:
This lesson, along with Lesson A-18, Convection Currents, lead directly into Lesson
D-13, Climate and Weather I, which will explain why different biomes, e.g., tropical rain
forests and deserts occur where they do. Further considerations why various climate and
weather patterns occur in different regions of the world are addressed in Lesson D-14 and
D-15 [Vol. III].
Countless ways in which biological species are adapted to the changing physical
parameters of the seasons should not go unnoticed. Indeed, many species are adapted to
perceive change in day-length and this is the major trigger leading to their adaptive
response(s) to the coming season.
Lesson D-9. Cause of Seasonal Changes
Re: National Science Education (NSE) Standards
This lesson is a stepping-stone toward developing students’ understanding and abilities
aligned with NSE Standards. Standards applicable to K-4 are marked by a single asterisk
(*); those applicable to 5-8 marked by a double asterisk (**); those applicable to both, not
marked.
Unifying Concepts and Processes:
• Systems, order, and organization
• Evidence, models, and explanation
• Constancy, change, and measurement
Content Standard A, Science as Inquiry:
• Abilities necessary to do scientific inquiry
• Understanding about scientific inquiry
Content Standard B, Physical Science:
• Position and motion of objects *
• Light, heat, electricity, and magnetism *
Content Standard D, Earth and Space Science:
• Properties of earth materials *
• Structure of the earth system **
• Objects in the sky *
• Changes in earth and sky *
• Earth in the solar system **
Content Standard F, Science in Personal and Social Perspectives:
• Populations, resources, and environments **
• Changes in environments *
Content Standard G, History and Nature of Science:
• Science as a human endeavor
• Nature of science **
• History of science **
Books for Correlated Reading:
Anderson, Maxine. Explore Spring: 25 Great Ways to Learn About Winter (Explore Your
Worlds). Nomad Press, 2007.
__________. Explore Winter: 25 Great Ways to Learn About Winter (Explore Your
Worlds). Nomad Press, 2007.
Cooper, Jason. Day and Night (Nature’s Cycle). Rourke Publishing, 2007.
__________. Season to Season (Nature’s Cycle). Rourke Publishing, 2007.
Bernard J. Nebel, Ph.D.
Jones, Annie. The Four Seasons: Uncovering Nature. Firebly Books, 2006.
Sipiera, Paul and Diane M. Sipiera. Seasons (True Books-Nature). Children’s Press,
1999.
For younger children:
Branley, Franklyn M. Sunshine Makes the Seasons. HarperCollins, 2005.
Gail, Gibbons. The Reasons for Seasons. Holiday House, 1996.