Content Benchmark E.8.A.1
Students know seasons are caused by variations in the amounts of the Sun’s energy reaching
Earth’s surface due to the planet’s axial tilt. E/S
Students are likely aware that both the average daily temperature and the length of day light
hours vary with the seasons. Most students may even associate longer periods of day light with
warmer temperatures and shorter periods with cooler temperatures. Although this association is
partially correct it does not fully explain the reason why the Earth experiences seasons. There
are a number of common misconceptions held by students related to why there are seasons on the
Earth.
In order to help students develop an accurate conceptual understanding of the cause of the
seasons, students need to understand that the Sun is the major source of energy for the Earth.
Although the Earth generates some of its own heat, the vast majority of energy available to the
Earth is received from the Sun in the form a visible light.
Based on the fact that the Sun is the major source of energy for the Earth, it becomes imperative
for the students to understand that for seasons to exist, all parts of the Earth can not receive the
same amount of energy from the Sun. If the energy from the Sun were evenly distributed over
the surface of the Earth then variations in temperature would be limited to the variation in the
Earth’s surface. Albedo, or the reflectivity of the surface would be one of the sole factors in
determining the temperature of a given area. Areas that absorbed and then radiated the energy
(dark surfaces such as soil and vegitation) would be hotter than those that reflected large
amounts of the energy back into space (light colored surface such as snow). Although albedo
plays a role in weather, it is not a controlling factor in the seasons. What is the major cause of
the variation in the amount of solar energy reaching the Earth and thus the cause of the seasons?
In order to answer that question of what causes the seasons, we must first examine some
characteristics of the Earth and its orbit around the Sun. If the Earth were a flat disk that was
directly facing the incoming energy from the Sun (insolation), the Earth would receive
approximately 1300 watts/m2 of energy from the Sun. However, the Earth is not a flat disk, it is
a sphere and the surface is not always directly facing the sun, but rather is continually rotating,
transition from day to night and back again. As such, the majority of the Earth’s surface is not
perpendicular to the incoming rays of the Sun. With the curvature of the Earth’s surface factored
in, an equatorial region would receive and average of 250 watts/m2. The closer you get to the
poles the greater the surface angles away from the incoming rays. As figure #1 below shows, the
amount of insolation reaching a given area of the Earth’s surface is directly related to how close
to perpendicular that area is to the incoming rays of the Sun. The closer the angle is to a right
angle (90°), the greater the concentration of insolation striking the surface. As the surface is
tilted away from perpendicular the insolation spreads out over a larger and larger area. This
angling reduces the amount of energy each square meter receives.
Figure 1. Comparison of the amount of solar energy reaching the surface of the Earth. When the Sun is
directly overhead, its rays strike Earth perpendicular to the ground and so they deliver the maximum amount of
energy possible. When the Sun is lower in the sky, a sunbeam strikes the ground at an angle (in the example above,
45°) and so its energy is "spread out" over a larger area thus "diluting" its energy. In this example, the energy is
spread over an area of 1.41 square meters (instead of 1 square meter when the Sun is directly overhead), so the
energy per unit area is reduced from 342 W/m2 to 242 W/m2 (342 ÷ 1.41 = 242). Credit: Artwork by Randy Russell.
(From http://www.windows.ucar.edu/tour/link=/earth/climate/sun_radiation_at_earth.html)
The Earth is basically a sphere that orbits (revolves) around the Sun once a year. As the Earth
orbits the Sun it is spinning or rotating, on an imaginary axis. Each spin, or rotation, is referred to
as a day and take approximately 24 hours to complete. If Earth’s axis of spin was vertical,
straight up and down, in relation to its orbit (The Plane of the Ecliptic) there would be no day-today variation in amount of insolation a given area of Earth received throughout the year. The
variation would be based on the changes in the angle of the surface that occurs as you progress
from equator to the poles. However, the Earth’s spin is not vertical in relation to its orbit, it is
tilted 23.5°. This tilt is constant, in that if you were to draw an imaginary line through the poles
of the Earth, it would “point” at the North Star, also known as Polaris, at all times of the year.
(Although this is a bit of an oversimplification in that the Earth does have a slight wobble or
gyroscopic progression, much like a top, the wobble takes an excessive amount of time to occur approximately 26,000 years).
Due to the tilt of the Earth’s axis as it spins, and the fact that the tilt does not change through out
the course of the year, the amount of insolation an area receives changes throughout the course
of the year. The sun will appear directly overhead at least one day of the year for all points that
fall between Tropic of Cancer (23.5° North) through the Tropic of Capricorn (23.5° South). This
region or band of the Earth is referred to as the Tropics. Students may also be familiar with the
general climate of the tropics, as being warm year round. Twice a year the Sun is directly over
the Equator, this day all areas of the Earth experience 12 hours of daylight and 12 hours of night.
These days are referred to as equinox and occur on March 20th or 21st (Vernal Equinox) and on
September 21st or 22nd (Autumnal Equinox). The Summer Solstice is the day the Sun reaches its
highest point in the northern sky, and is directly overhead the Tropic of Cancer. On this day all
points north of the Arctic Circle (66.5° north latitude) experience 24 hours of daylight. On this
day, the amount of daylight hours experienced gradually decreases the further south you go from
the Arctic Circle. On this day, once you pass south of the Antarctic Circle (66.5° South latitude)
you will experience 24 hours of darkness.
The fact that the Earth’s axis is tilted, and is always pointed in same direction is key to
understanding why the Earth experience different seasons. See Figure #2 for an illustration of
how the axial tilt remains constant in relation to the Earth’s orbit.
Figure 2. Earth’s Orbital Motion. Illustrates the axial causes on hemisphere to point toward the Sun while the
other points away from the Sun. The Earth is illustrated in 4 positions in its orbit around the Sun. Each location is
illustrating an important point in the Earth’s orbit. The far left and far right position illustrate when either the
Northern Hemisphere (Summer Solstice) or the Southern Hemisphere (Winter Solstices) is pointed most directly at
the Sun. The two middle positions illustrate when neither hemisphere is pointed directly at the Sun, thus they are
both receiving near identical amounts of solar energy. At these middle locations the length of day and of night are
equal, they are called equinoxes, meaning equal day and equal night.
(From http://calgary.rasc.ca/images/radec_earth_orbit.gif )
At this point the major pieces of the seasonal puzzle are in place. The Earth’s tilt and spherical
shape both contribute to the differential heating of its surface. For basically a quarter of the year
during winter in the Northern Hemisphere, the Northern Hemisphere is pointed away from the
Sun. This tilt angles this hemisphere more steeply from the perpendicular and so this hemisphere
receives a less concentrated dose of sunlight. During that same time it is summer in the Southern
Hemisphere, its surface is angled more closely to perpendicular to the Sun’s rays and thus it gets
a more concentrated dose of the Sun’s energy. This cycle is reversed approximately six months
later, at which time the Earth has moved halfway around the Sun. As the Earth moved, the tilt
remained constant both in amount (23.5°) and direction. With the Northern Hemisphere now
tilted toward the Sun it is receiving the more concentrated energy, while the Southern
Hemisphere receives less. Spring and fall are the transitional seasons where the Earth is neither
pointed toward nor away from the Sun, its only the Earth’s spherical shape that impacts how
much energy a given area receives.
Distance from the Sun has Little Effect on the Seasons
A common misconception held by many is that the distance the Earth is from the Sun is a cause
of the seasons. The amount of energy the Earth receives is almost solely dependant on the
Earth’s spherical shape and the axial tilt. Although the Earth is in an elliptical orbit and gets
closer and further from the Sun over the course of its orbit the variation is very minimal, less
than 4%. This limited variation in distance makes very little difference to the amount of energy
reaching the Earth’s surface. In fact the Earth is closer to the Sun, and thus receiving more
energy from the Sun, when it is winter in the Northern Hemisphere. At perihelion, or its closest
point in its orbit around the Sun, the Earth is approximately 147 million km from the Sun. At its
furthest point the Earth is approximately 152 million km from the sun. Earth is further from the
Sun and receives less energy when it is summer in the Northern Hemisphere.
Figure 3: The position of the equinoxes, solstices, aphelion, and perihelion
relative to the Earth's orbit around the Sun.
(From http://www.physicalgeography.net/fundamentals/images/helions.jpg )
With a bit of care on the part of the teacher, common misconceptions can be addressed and
hopefully corrected. The understanding that the seasons are directly related to the tilt of the
Earth on its axis, and how concentrated the insolation is at a given latitude is not too difficult for
students to understand. If students are engaged with good examples and simple activities while
learning about season, misconceptions can be avoided. The table below contains many of the
facts and figures mentioned in this Bench Mark.
Quick Facts About the Earth
Topic
Data
Diameter
12,756.28 km
Density
5.515 g/cm3
Mass
5.976 x 1024 kg
Volume
1.087 x 1012 km3
Temperature Range
-69° C to 58° C
Atmosphere
Mostly Nitrogen and Oxygen
Winds
483 km/hr
Moons
One
Average Distance from Sun
149,597,870 km
Perihelion, closets distance to the Sun.
147,300,0000 km
Aphelion, furthest distance to the Sun.
152,100,000 km
Orbital Period
1 Year, 0 Days, 0 Hours
Rotation
23 Hours 56.1 Min
Tilt
23.45°
Rings
None
Composition
Iron Core, Silicate Surface
Magnetic Field
Up to 362000 km from Surface
Vernal Equinox
March 20th or 21st
Summer Solstice
June 21st or 22nd
Autumnal Equinox
September 21st or 22nd
Winter Solstice
December 21st or 22nd
Figure 4. Original chart of Quick Facts about the Earth has been modified to include seasonal information.
(From http://www.kidscosmos.org/kid-stuff/earth-facts.html)
Content Benchmark E.8.A.1
Students know seasons are caused by variations in the amounts of the Sun’s energy reaching
Earth’s surface due to the planet’s axial tilt. E/S
Common misconceptions associated with this benchmark
1. Students mistakenly believe that the Earth’s axis “flip-flops” its direction as it orbits
the Sun.
This is a common misconception that can be perpetuated by the teacher. One of the main
reasons for this misconception it due to the terminology used by the teacher when discussing
the cause of the seasons, especially if the term “tips” is used to refer to the axis angling
toward the Sun or away from the Sun. Great care should be taken to not only be consistent in
stating the direction of axial tilt, but to also accompany the discussion with a diagram.
Frequently referring back to the diagram and the fact that the axis maintains a consistent
angle and direction of tilt is essential to help prevent reinforcing this misconception.
Emphasis should be placed on the fact that it is the Earth that is orbiting the Sun and as it
orbits it is the consistency in the direction of the tilt that controls when a hemisphere is
angled toward or away from the Sun.
For further information concerning this fallacy see
http://csep10.phys.utk.edu/astr161/lect/time/seasons.html
“The Sun: Our Seasons and Their Temperatures” an excellent computer based presentation
with animations addressing the axial tilt, seasons and insolation changes caused by orbital
position.
This website can be accessed by visiting,
http://www.artescapesonline.com/movies/Module%20IV_V15.swf
2. Students mistakenly believe that it is the Earth’s distance from the Sun that controls the
seasons.
Although the Earth’s orbit is not circular, it is very close to being circular. The variation in
the distance the Earth is from the Sun during Perihelion (closest point to the sun) verses
aphelion (furthest distance from the sun) is only about 3.5 percent. This minimal variation is
not sufficient to cause the massive fluctuation in seasonal temperatures that Earth
experiences. One decisive factor in refuting this misconception is the fact that during the
Northern Hemisphere’s summer, Earth is at its furthest point from the Sun, and during the
Northern Hemisphere’s winter, at perihelion, or its closest point, in its orbit around the Sun,
the Earth is approximately 147 million kilometers from the Sun. At its furthest point,
aphelion, the Earth is 152 million kilometers from the Sun. Earth is closest to the Sun in
winter in the Northern Hemisphere. If the distance form the Earth to the Sun were the
deciding factor for the cause of seasons, the seasons would be reversed for the Northern
Hemisphere. Furthermore, both hemispheres would experience the same season at the same
time of year.
For more information concerning this topic visit
http://www.weatherimagery.com/blog/myths-seasons-cause/
“My Angle on Cooling: Effects of Distance and Inclination” is a lesson plan and activity that
demonstrates the effect of both distance and inclination on the amount of insolation an object
would receive.
http://www.sciencenetlinks.com/lessons.cfm?BenchmarkID=4&DocID=418
Content Benchmark E.8.A.1
Students know seasons are caused by variations in the amounts of the Sun’s energy reaching
Earth’s surface due to the planet’s axial tilt. E/S
Sample Test Questions
Questions and Answers to be inserted from a separate document
Content Benchmark E.8.A.1
Students know seasons are caused by variations in the amounts of the Sun’s energy reaching
Earth’s surface due to the planet’s axial tilt. E/S
Answers to Sample Test Questions
Questions and Answers to be inserted from a separate document
Content Benchmark E.8.A.1
Students know seasons are caused by variations in the amounts of the Sun’s energy reaching
Earth’s surface due to the planet’s axial tilt. E/S
Intervention Strategies and Resources
The following is a list of intervention strategies and resources that will facilitate student
understanding of this benchmark.
1. Exploring Earth Visualization
View a short movie clip taken from a geosynchronous satellite of the transition zone between
night and day of the same location on Earth throughout a one year cycle. The seasonal
change in the angle of the shadow/sun can be clearly seen. This is a great way to quickly
illustrate how the inclination of the Earth directly impacts how close to perpendicular the
solar rays enter the atmosphere.
This animation can be accessed by visiting,
http://www.classzone.com/books/earth_science/terc/content/visualizations/es1704/es1704pag
e01.cfm?chapter_no=visualization
2. DATA (Demos and Animations for Teaching Astronomy)
Computer animation that illustrates the change in size and shape of a patch of sunlight based
on the seasonal variations. The animation shows the relative position of the Earth in its orbit
around the Sun, the declination of the Sun above or below the celestial equator and the
relative area of ground a given cross section of solar energy would illuminate during each
season. As the animation progresses each of the 3 variables change according to the relative
change in the tilt or inclination of the Earth’s surface to the Sun.
This animation can be accessed by visiting,
http://www.astro.uiuc.edu/projects/data/Seasons/seasons.html
DATA Home page contains animation on Lunar Phases, Kepler Law, Retrograde motion,
Doppler effect, and Spectral lines.
This site can be access by visiting,
http://www.astro.uiuc.edu/projects/data/index.html
3. Animated movie showing the Earth moving around the Sun.
This animation clearly shows that the axial tilt of the Earth remains constant and for part of
the year the Northern Hemisphere is angled toward the Sun, and for part it is angled away.
The animation also contains a very brief explanation of the combined impact of the angle and
location of the Earth in its orbit around the Sun has on the length of day light for a given
hemisphere.
This animation can be accessed by visiting,
http://www.rkm.com.au/ANIMATIONS/animation-seasons.html
4. “Solar System Exploration”
This is a NASA sponsored website that has extensive internal links to activities, facts, images
and information to all things related out our solar system and the planets.
This website can be accessed by visiting,
http://solarsystem.nasa.gov/index.cfm
The portion of the website that is Earth specific, can be directly accessed at,
http://solarsystem.nasa.gov/planets/profile.cfm?Object=Earth&Display=Facts