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-to-day 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 EarthTopic / 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.