ASD Planetarium: The Reasons for the Seasons

ASD Planetarium: The Reasons for the Seasons The Reasons for the Seasons

Moravian College Astronomy
©Gary A. Becker

THE SEASONS THROUGH PICTURES | A SEASONS LESSON FOR TEACHERS

PA Standards addressed

3.4.D-2: Explain and illustrate the causes of seasonal changes. 3.4.C-4: Describe solar system motions and use them to explain time (e.g. days, seasons). 3.4.C-3: Describe various types of motions.

Lesson Goal

Most students and adults do not understand the causes and effects of the seasons. This lesson will attempt to clarify the misconceptions which prevent pupils from comprehending the reasons why seasons occur.

UNDERSTANDING THE SEASONS THROUGH PICTURES


EARTH’S ORBIT: A lot of people believe that the seasons are caused by the Earth’s changing distance from the sun. But it just isn’t so. Look at Earth’s orbit. It is almost a perfect circle. On average the Earth’s distance from the sun is just a little less than 93 million miles. The closest the Earth can get to the sun is 91-1/2 million miles while the farthest distance of the Earth to the sun is 94-1/2 million miles.

EARTH’S ORBIT: A lot of people believe that the seasons are caused by the Earth’s changing distance from the sun. But it just isn’t so. Look at Earth’s orbit. It is almost a perfect circle. On average the Earth’s distance from the sun is just a little less than 93 million miles. The closest the Earth can get to the sun is 91-1/2 million miles while the farthest distance of the Earth to the sun is 94-1/2 million miles.


EARTH’S CHANGING DISTANCE FROM THE SUN: Look more closely at the picture. You should be able to tell that the sun is not quite at the center of Earth’s orbit. Is the Earth closer to the sun to the right or left of its orbit? When the Earth is closest to the sun, what season are we in? Your answer, at first, may not make any sense. But it is true. Earth’s changing distance from the sun has nothing to do with the seasons. When Earth is closest to the sun we are in the middle of winter. When Earth is farthest from the sun, Allentown is in the midst of summer.

EARTH’S CHANGING DISTANCE FROM THE SUN: Look more closely at the picture. You should be able to tell that the sun is not quite at the center of Earth’s orbit. Is the Earth closer to the sun to the right or left of its orbit? When the Earth is closest to the sun, what season are we in? Your answer, at first, may not make any sense. But it is true. Earth’s changing distance from the sun has nothing to do with the seasons. When Earth is closest to the sun we are in the middle of winter. When Earth is farthest from the sun, Allentown is in the midst of summer.


EFFECTS OF THE SEASONS: Using this picture you can discover by clicking on the day arrows what the sun is doing at 12 noon on the first day of each season, as well as where we are looking at night. Remember to take the seasons in their correct order starting with spring, summer, autumn, and winter. During the summer the sun is high in the sky? During the winter the sun is low in the sky? In the spring and autumn, the sun is right in the middle. You will also notice that our view of the constellations change as Earth orbits the sun over the time of one year.

EFFECTS OF THE SEASONS: Using this picture you can discover by clicking on the day arrows what the sun is doing at 12 noon on the first day of each season, as well as where we are looking at night. Remember to take the seasons in their correct order starting with spring, summer, autumn, and winter. During the summer the sun is high in the sky? During the winter the sun is low in the sky? In the spring and autumn, the sun is right in the middle. You will also notice that our view of the constellations change as Earth orbits the sun over the time of one year.

If you think about it, something about the Earth must cause the seasons. Something about the Earth must allow us to receive more energy in the summer and less energy in the winter. We know it is NOT the changing distance of the Earth from the sun, because we are closest to the sun in winter when it is coldest. Somehow we get more energy in the summer heating us up, and less energy in the winter, so our part of the world cools down. Please continue to the next picture.


CHANGING SEASONS, CHANGING SUN: We really do get more energy from the sun in the summer. This heats us up. We receive less energy from the sun in the winter which cools us down. The sun is higher in the sky in the summer and shines down on us for a longer part of the day. You can count the number of hours in Allentown that the sun is up in the spring, summer, winter, and autumn. In the picture above, each sun is spaced one hour apart. Count the spaces and don’t forget to add the half spaces that are below the sun on the first day of summer and the first day of winter. Click here on the words spring, summer, autumn, and winter to see if you are correct.

CHANGING SEASONS, CHANGING SUN: We really do get more energy from the sun in the summer. This heats us up. We receive less energy from the sun in the winter which cools us down. The sun is higher in the sky in the summer and shines down on us for a longer part of the day. You can count the number of hours in Allentown that the sun is up in the spring, summer, winter, and autumn. In the picture above, each sun is spaced one hour apart. Count the spaces and don’t forget to add the half spaces that are below the sun on the first day of summer and the first day of winter. Click here on the words spring, summer, autumn, and winter to see if you are correct.


DIRECT AND INDIRECT ENERGY: Imagine the flashlights to be the sun. The energy coming from each flashlight is the same, but the way the light is striking the ground is different. The two flashlights on the left are allowing their energy to strike the ground DIRECTLY in a concentrated manner. The flashlight on the right is tilted so that when its energy strikes the ground, the energy is spread over a much larger area. The energy from the tilted flashlight is striking the ground INDIRECTLY, and its energy is less concentrated.

DIRECT AND INDIRECT ENERGY: Imagine the flashlights to be the sun. The energy coming from each flashlight is the same, but the way the light is striking the ground is different. The two flashlights on the left are allowing their energy to strike the ground DIRECTLY in a concentrated manner. The flashlight on the right is tilted so that when its energy strikes the ground, the energy is spread over a much larger area. The energy from the tilted flashlight is striking the ground INDIRECTLY, and its energy is less concentrated.

If you were cold and wanted to get warm, would you want DIRECT or INDIRECT energy to heat you? Click on the word you think is correct.


LONG SHADOWS--SHORT SHADOWS: Here is another way that you can tell whether the energy from the sun or a flashlight is direct or indirect. Just look at the shadows which the light is making. Direct energy always produces short shadows while indirect energy creates long shadows. The energy isn't any different, but the way it is striking you or the Earth makes all the difference in the amount of energy we receive and how warm we feel.

LONG SHADOWS--SHORT SHADOWS: Here is another way that you can tell whether the energy from the sun or a flashlight is direct or indirect. Just look at the shadows which the light is making. Direct energy always produces short shadows while indirect energy creates long shadows. The energy isn't any different, but the way it is striking you or the Earth makes all the difference in the amount of energy we receive and how warm we feel.


THE POWER OF THE SUN: At sunrise the energy from the sun reaches us in a very indirect way. The sun is very low in the sky, near the horizon where the sky seems to touch the Earth. Shadows are very long. In the winter, the sun is always low in the sky. The winter sun’s indirect light has very little heating effect upon us. Also the sun can be in the sky for as little as nine hours. In the summer the sun climbs to a very high position, so that for many hours its light is striking us very directly. In the summer the sun is also in the sky for as much as 15 hours.

THE POWER OF THE SUN: At sunrise the energy from the sun reaches us in a very indirect way. The sun is very low in the sky, near the horizon where the sky seems to touch the Earth. Shadows are very long. In the winter, the sun is always low in the sky. The winter sun’s indirect light has very little heating effect upon us. Also the sun can be in the sky for as little as nine hours. In the summer the sun climbs to a very high position, so that for many hours its light is striking us very directly. In the summer the sun is also in the sky for as much as 15 hours.

ENERGY AND THE EARTH’S SHAPE: Because the Earth is in the shape of a ball, there will be parts of the Earth that receive direct energy from the sun and other regions of the world that receive indirect energy. Notice the Earth’s axis in this drawing. The axis is the imaginary line about which the Earth spins. It is straight up and down. If this is how the Earth went around the sun each year, the seasons would always remain the same.


EARTH'S AXIS IS TILTED: Here is the most important fact about why we have seasons in Allentown. The Earth’s axis is tilted or tipped. Because the Earth’s axis is tilted, we lean back from the sun in winter getting only INDIRECT energy from the sun. Shadows are long and the sun is only up for nine hours. Temperatures must go down. In the summer we lean in towards the sun, causing the sun’s energy to strike us more DIRECTLY. The sun’s energy is more concentrated and temperatures must get warmer. Shadows are short around noon and the sun is up for 15 hours.

EARTH'S AXIS IS TILTED: Here is the most important fact about why we have seasons in Allentown. The Earth’s axis is tilted or tipped. Because the Earth’s axis is tilted, we lean back from the sun in winter getting only INDIRECT energy from the sun. Shadows are long and the sun is only up for nine hours. Temperatures must go down. In the summer we lean in towards the sun, causing the sun’s energy to strike us more DIRECTLY. The sun’s energy is more concentrated and temperatures must get warmer. Shadows are short around noon and the sun is up for 15 hours.


SUMMER: This is how the sun’s energy would be striking Allentown on the first day of summer at noon. Direct energy, high sun, short shadows, and long days occur as the tilt of the Earth’s axis causes us to lean in towards the sun.


WINTER: This is how the sun’s energy strikes Allentown on the first day of winter at noon. Indirect energy from a sun which is low in the sky is the result of the tilt of the Earth’s axis causing us to lean back at this time of year.


OPPOSITE HEMISPHERES, OPPOSITE SEASONS: Here is a really interesting fact about the seasons. They are opposite from ours south of the equator. If it is summer in Allentown, PA which is north of the equator, then it will be winter, in the city of Buenos Aires, Argentina, which is south of the equator. Because the Earth’s axis is tilted, and the axis points in the same direction, if Buenos Aires is leaning back, than Allentown must be leaning forward.

OPPOSITE HEMISPHERES, OPPOSITE SEASONS: Here is a really interesting fact about the seasons. They are opposite from ours south of the equator. If it is summer in Allentown, PA which is north of the equator, then it will be winter, in the city of Buenos Aires, Argentina, which is south of the equator. Because the Earth’s axis is tilted, and the axis points in the same direction, if Buenos Aires is leaning back, than Allentown must be leaning forward.


FLIPPED AXIS, SAME SEASON: Some people think that the seasons are caused by the Earth’s axis flipping over or wobbling around as our planet orbits the sun. That is exactly what has happen in this drawing. If this really did occur, we would have no change in the seasons. You can prove this to yourself by looking at the length of the shadows which are being made by the red pole, which is suppose to be where Allentown is located. The shadows on both sides of our orbit would be long, and we would be forever stuck in the same season. In this case it would be winter--yuck!

[Flipping Axis Produces the Same Season]

FLIPPED AXIS, SAME SEASON: Some people think that the seasons are caused by the Earth’s axis flipping over or wobbling around as our planet orbits the sun. That is exactly what has happen in this drawing. If this really did occur, we would have no change in the seasons. You can prove this to yourself by looking at the length of the shadows which are being made by the red pole, which is suppose to be where Allentown is located. The shadows on both sides of our orbit would be long, and we would be forever stuck in the same season. In this case it would be winter--yuck!

Many people know that the axis of the Earth points to a very famous star in the nighttime sky called the North Star or Polaris. As the Earth rotates, the axis point near enough to Polaris so that it appears as if the entire sky is spinning or rotating around this star. Since you can always count upon the North Star to be in the same position, day or night, the Earth’s axis cannot be flipping back and forth. If the Earth’s axis did flip, we would have many different "North Stars" over the duration of one year.


NORTH STAR IS FAMOUS FOR HARDLY MOVING: The bright star in the upper left is the North Star. Astronomers usually call it Polaris. Earth’s axis nearly points in the direction of the North Star, but because it is off just slightly, even the North Star makes a tiny circle in the sky. Directly below the North Star is the direction north. Keep in mind that the North Star is famous NOT for its brightness, but for that fact that it hardly moves. This 3-hour photo was taken in New Mexico by Gary A. Becker.

NORTH STAR IS FAMOUS FOR HARDLY MOVING: The bright star in the upper left is the North Star. Astronomers usually call it Polaris. Earth’s axis nearly points in the direction of the North Star, but because it is off just slightly, even the North Star makes a tiny circle in the sky. Directly below the North Star is the direction north. Keep in mind that the North Star is famous NOT for its brightness, but for that fact that it hardly moves. This 3-hour photo was taken in New Mexico by Gary A. Becker.

You can create your own "North Star" by standing up and bending your head back. Your head is now the Earth and your eyes represent the axis of the Earth extended into space. Find a mark on the ceiling which is directly over your head and then begin to spin slowly. Notice how all of the other marks on the ceiling seem to go around the point which is directly over your head. This is the location to which the axis of your Earth is pointing. Likewise, the positions directly over the North and South Poles of Earth also act as circling points and do not move.


THERE IS NO SOUTH STAR: This picture shows the rotating Earth from Australia. The fuzzy area on the left is the Milky Way, while the two fuzzy blobs on the right are the Large and Small Magellanic Clouds. The LMC and SMC are two small galaxies that orbit our Milky Way. Photo by Gary A. Becker

-- USING THE SUN AND ANGLES INSTEAD OF FLASHLIGHTS --


FLIPPED AXIS, SAME SEASON: In this drawing we have stopped using flashlights and have substituted rays of sunlight instead. Imagine that you are standing next to the flagpole which is pointing straight up. The two straight red lines going out from either side represent the north and south horizons. The northern horizon looks in the direction of the North Star, while the southern horizon faces towards the equator. The two rays of sunlight that strike the bottom of the flagpole make the same angle to the southern horizon, meaning that the sun is just as high in the sky on either side of its orbit. It doesn’t take a rocket scientist to realized that the season would always remain the same if the Earth’s axis wobbled around during the time of one year. Based upon the picture, what season is Allentown experiencing?

FLIPPED AXIS, SAME SEASON: In this drawing we have stopped using flashlights and have substituted rays of sunlight instead. Imagine that you are standing next to the flagpole which is pointing straight up. The two straight red lines going out from either side represent the north and south horizons. The northern horizon looks in the direction of the North Star, while the southern horizon faces towards the equator. The two rays of sunlight that strike the bottom of the flagpole make the same angle to the southern horizon, meaning that the sun is just as high in the sky on either side of its orbit. It doesn’t take a rocket scientist to realized that the season would always remain the same if the Earth’s axis wobbled around during the time of one year. Based upon the picture, what season is Allentown experiencing?


THE SEASONS EXPLAINED FOR OLDER STUDENTS: It is exactly the same as any of the other explanations, but now we are using the sun instead of a flashlight. Allentown, PA is represented by the flagpole which is at an angle of 40 degrees north of the equator. The northern and southern horizons are at 90 degrees to it. The sun reaches its highest position in the south at noon each day. The direction to the sun at this time is indicated by the ray of sunlight which continues to the bottom of the flagpole. It should be easy to see that the angle from the southern horizon to the sun on the left is smaller than the angle from the southern horizon to the sun on the right. The sun at noon is lower in the sky to the left, and higher in the sky on the right.

THE SEASONS EXPLAINED FOR OLDER STUDENTS: It is exactly the same as any of the other explanations, but now we are using the sun instead of a flashlight. Allentown, PA is represented by the flagpole which is at an angle of 40 degrees north of the equator. The northern and southern horizons are at 90 degrees to it. The sun reaches its highest position in the south at noon each day. The direction to the sun at this time is indicated by the ray of sunlight which continues to the bottom of the flagpole. It should be easy to see that the angle from the southern horizon to the sun on the left is smaller than the angle from the southern horizon to the sun on the right. The sun at noon is lower in the sky to the left, and higher in the sky on the right.

The seasons are the result of the 23-1/2 degree tilt of the Earth’s axis from the perpendicular to the Earth’s orbital plane. The Earth’s orbital plane is called the ecliptic. The season are also the result of the Earth’s axis pointing in the same direction.


SUMMER: This drawing of summer shows the Northern Hemisphere leaning into the sun. It has been constructed accurately enough for you to measure the altitude or angle of the sun above the horizon on the first day of summer. The center measuring point of your protractor should be placed at the intersection of the base of the flagpole and the southern horizon. The protractor’s zero angle position should fall along the southern horizon.

SUMMER: This drawing of summer shows the Northern Hemisphere leaning into the sun. It has been constructed accurately enough for you to measure the altitude or angle of the sun above the horizon on the first day of summer. The center measuring point of your protractor should be placed at the intersection of the base of the flagpole and the southern horizon. The protractor’s zero angle position should fall along the southern horizon.


WINTER: Measure the altitude of the winter sun by placing the center measuring point of your protractor at the base of the flagpole where the flagpole and horizon intersect. Place the southern horizon along the protractor’s zero degree position. It is easy to see that the sun is much lower in the sky during the winter months.

For Teachers and Older Students

I N T R O D U C T I O N

Most people believe that the seasonal variations that we experience are the result of our changing distance from the sun. Nothing could be farther from the truth. Although the Earth’s distance from the sun varies by about three million miles, we are closest to the sun in winter and farthest from our daystar during the summer months, just the opposite of what would be expected. The seasons are the result of changes in the amount of solar energy which is received at the Earth's surface, but the Earth-sun distance plays a minor role. Instead, these energy changes come about because the Earth’s axis it tilted in relation to its orbital plane which is called the ecliptic. The Earth's axis is the imaginary line about which our planet rotates. The measure of Earth’s axial tilt is referenced from the perpendicular to the ecliptic. It is generally stated in the following manner. The Earth’s axis is tilted 23-½° from the perpendicular to the ecliptic. Another factor affecting the seasons is that Earth’s axis points in the same direction. Currently the axis is pointing in the direction of the North Star, also called Polaris. This is why Polaris represents the hub of the wheel about which the sky pivots as Earth rotates. Expressed in another way, the ecliptic is tilted to the plane of the Earth’s equator by 23-½°. Our orbital motion makes the sun move eastward among the stars. Our axial tilt also causes the sun to move northward or southward with respect to the equator. This change in the position on Earth over which the sun is shining directly overhead results in three yearly cycles which can be readily monitored as the seasons progress:

The altitude of the sun changes: The sun reaches its highest position above the horizon at local noon each day. In Allentown, PA when the sun is at its most northerly position, with respect to the equator, its altitude at noon is at an extreme of 73o. This occurs on the first day of summer. In Allentown, the sun is never directly overhead. On the first day of winter, the sun is as far south of the equator as it can be positioned, and it achieves its minimum altitude of 26o at noon as seen from the city.

The duration of daylight changes: The longest day of the year occurs for residents of the Northern Hemisphere when the sun is at its most northerly position with respect to the equator. This marks the first day of summer. In Allentown, the sun rises in the northeast taking approximately 15 hours to cross the sky before setting in the northwest. The path that the sun takes from rising to setting is longest at summer solstice. Therefore, the day must also be at its longest because the earth rotates at a uniform rate. When the sun is at its greatest deviation south of the equator, the day is the shortest. This marks the beginning of winter. In Allentown the sun rises in the southeast, and about nine hours later, it sets in the southwest.

The positions of sunrise and sunset change: For observers in Allentown on the first day of summer, the sun rises as far to the north of east and sets as far to the north of west. The sun is positioned at its maximum extreme north of the equator. When the sun is positioned as far to the south of the equator as it can move on the first day of winter, it rises as far to the south of east and sets as far to the south of west as it can for that location. The sun, therefore, changes its daily rise and set positions with respect to the horizon.

In winter, the northern hemisphere leans back from the sun. The daily duration of sunshine is restricted, and the sun is lower at noon. The sun's energy strikes the ground at a shallower angle, and thus less energy is received per unit area. The temperatures generally become colder. In summer, the Northern Hemisphere is tilted toward the sun. Not only is the daily duration of sunshine longer, but the sun also climbs to a higher altitude in the sky, so that its energy strikes our position more directly, and we receive more energy per unit area. All of these effects result from the tilt of Earth’s axis and its consistent pointing direction.

Lesson for Students

Students will be able to define and then demonstrate the two basic motions of the Earth which are called rotation and revolution. They will know the meaning of the other italicized words noted below. Students will prepare flash cards with these words written on one side and their definitions on the other side.

Rotation: The spinning of a body on its axis. Rotation equals spinning. The Earth rotates on its axis, or the Earth spins on its axis. Both ideas are correct.

Basic concepts of Earth's rotation: It takes the Earth one day to complete one rotation. Rotation is responsible for day and night.

Advanced concepts of rotation: Rotation is responsible for giving us day and night, and helping us to determine the length of the day. One complete rotation of the Earth is not exactly equal to one day. One Earth rotation takes 23 hours, 56 minutes, 4 seconds on average. The 24-hour day, according to the clocks on our walls, is created by allowing the sun to return to the same position in the sky. This requires an additional four minutes more time than it takes the Earth to complete one rotation on its axis. As the Earth spins, it is also going around the sun. The motion of the Earth around the sun causes the sun to change its position eastward in the sky by about one degree each day. In order to correct for this change and return the sun to its same due south position in the sky, it takes an extra four minutes of rotation. Four minutes are added to the 23 hours, 56 minute rotational period to give us the 24-hour day that we use on our clocks.

Axis: The imaginary line about which a body spins or rotates. Any object which is rotating must have an axis.

Revolution: The orbiting of one object around another. Orbiting equals revolution. The Earth revolves around the sun, or the Earth orbits the sun.

Basic concepts of Earth's revolution: It takes the Earth one year to complete one revolution. Revolution is responsible for determining the period of one year.

Advanced concepts of Earth's revolution: Revolution is responsible for helping us to define the period of time which we call a year. Although one year contains an average of 365 days, the actual time it takes for the Earth to complete one orbit around the sun is 365-1/4 days (actually 365.24 days). This means that we must, on average, correct our calendars once every four years by adding an extra day. The year in which this occurs is called a Leap Year.

Here is how a leap year works: We give the Earth 365 days to complete one orbit around the sun. We call this a year, but it really takes the Earth 365-1/4 days to complete one revolution around the sun. So at the end of the first year, the Earth is ¼ day behind schedule. After the second year the Earth is ½ day behind where it should be in its orbit around the sun. At the end of the third year, Earth is now ¾ of a day behind. By the end of the fourth year, Earth is now one full day behind schedule. The Earth needs a full day to catch up, and our calendars give it the necessary time by adding an extra day to the year at the end of February. Every four years, there is a February 29th.

Which of the following years is a leap year? Divide the number four into the year in question. If there is no remaining fraction, it is a leap year. The year 2005/4=501-1/4, 2006/4=501-1/2, 2007/4=501-3/4, 2008/4=502. The year 2008 is the leap year. The only exception to this rule is with century years which can be found in the next section.

The leap year and century years: Keep in mind that the actual period of revolution of the Earth is 365.24 days. Because of this, we actually overcorrect for the Earth’s position when we allow years which are solely divisible by four to become leap years. To correct for this, a century year such as 1900 or 2000, etc., must be divisible by 400 without any remainder in order for it to be a leap year. If you divide 400 into the century years of 1700, 1800, and 1900, a fraction will remain. Therefore, these century years were not leap years. The year 2000 was a leap year. This keeps the calendar accurate to about one day in 2500 years.

Remember this: Rotation is to spinning as revolution is to orbiting.

Tilt: To lean, incline, slope, or slant…

Orbit: The path in space of one object as it goes around another body.

Four Seasons: spring, summer, fall, and winter… These seasons start respectively on or about the 21st of March, June, September, and December.

See the Earth rotate:

During the day: The easiest way to witness the spinning Earth during the day is to observe the movements of shadows which are being created by the sun. Choose your shadow maker carefully. Trees won’t work very well because they may be swaying in a breeze and create a confusing pattern on the ground too complicated for easy measurement. The corner of a high building is much better, but a tall, thin flagpole is probably the best. Tape a sheet of white paper or graph paper to a board or use a piece of white cardboard with a grid pattern drawn upon it to follow the shadow near the top of the flagpole as it moves across the paper lying on the ground. Regardless of your method, it should only take a few minutes for the shadow to cross the paper, proof indeed, that the sun is changing its position in the sky due to the Earth’s rotation. If your shadow maker casts its shadow in an open area, you could even design a clock by marking the location of the shadow at various times of the day, such as lunch, recess, or the end of the school day. This is the basis for any sundial.

During the night: The Earth's rotation is responsible for the daily paths that the sun, moon, planets, and stars take across the sky. It is possible to observe our planet rotating by lining up a bright star or planet with the wall of a more distant building. Depending upon the location of the star in the sky, it might be approaching a wall or moving away from it. You'll have to decide how to position your head to perform the experiment, but the trick is to bring the star very close to the obstruction. Hold your head very still, and watch the star blink out as the spinning Earth carries it behind the wall. You are witnessing the moment-by-moment rotation of the Earth. It's even more fun to attempt this observation with a planet. Rather than a point of light, the planet presents a tiny, bright disk. If your head is held very still, the planet will appear to emerge or disappear, becoming brighter or dimmer over a period of several seconds. Positioning the star or planet near a telephone wire will produce a brief occultation of the object as it passes behind the wire, only to emerge rapidly from the other side.

See the effects of Earth’s revolution and axial tilt: The Earth’s revolution and axial tilt affects the rising and setting positions of the sun, as well as the height or altitude of the sun in the sky at noon.

Shadow lengths: Using the shadow created by the corner of a high building or a tall flagpole, carefully measure its length and record the time at which the measurement was made. Try making your observations around noon because shadows are shortest at this time. Return at intervals of one week at the same time to repeat the measurement to see whether the shadow is longer or shorter. A longer shadow will indicate a lower sun, while a shorter shadow signifies a higher sun.

Spring and fall equinoxes: Shadow length measurements made within four weeks of either equinox, September 21 or March 21, should easily show changes in the sun’s altitude within a few days. The word equinox is from the Latin meaning “equal nights,” and therefore, equal days.

Winter and summer solstices: Shadow length measurements made within four weeks of the solstices, June 21 or December 21, will require several weeks of observations before a difference in the height of the sun is detected. Solstice is also from the Latin meaning “sun standstill.” The sun’s change in altitude at noon near the solstices is very small.

Rising or setting sun positions: Make a sketch of the eastern or western horizon from where you live and plot the location of the rising or the setting sun at weekly intervals for at least three weeks near the equinoxes and six weeks near the solstices. Avoid looking directly at the sun.

Seasonal changes of the constellations: We associate some constellations like Orion the Hunter and Taurus the Bull with winter, while other star patterns, such as Scorpius the Scorpion and Sagittarius the Archer, are only visible during the summer. There is a seasonal change of the constellations due to our revolution around the sun. To demonstrate this, place a light source in the center of a room. You will become the rotating and revolving Earth, and your eyes represent where you live and your view into space. As you spin, the sun passes in front of you, and it is day. What you see with your back towards the sun represents your view into space at night. Remember this view. Now complete a quarter of a revolution around the sun. Then complete another rotation. Again the sun passes in front of your eyes during the daytime, but as you rotate into night, your view of what you see should be entirely different. Likewise, as the Earth orbits the sun, our view of the stars that we see at night slowly changes. During the course of a year, we get the opportunity to view all of the sky that could be possibly seen from our location on the Earth.

Students will be able to describe two of the three effects of the seasons as witnessed from Allentown, PA. Keep in mind that these effects are not the causes for the seasons. Seasons result from the axial tilt of the Earth.

In the summer, the sun is high in the sky, “beating” down upon us. Shadows around noontime are very short. During winter the sun is always low in the sky, and shadows are always long.

How does the height of the sun in the sky affect the amount of energy that we receive? This can be dramatically demonstrated by varying the angle at which a flashlight beam strikes a blackboard. The flashlight represents the sun, while the board represents the Earth’s surface. Make sure that the distance of the flashlight to the board remains as constant as possible while the angle changes. Have a student sketch the boundary of the beam when the flashlight is shining directly onto the board. Repeat the procedure and sketch the energy distribution with the flashlight beam striking the board at a very oblique (low) angle. Compare the low sun energy distribution with that of the vertical beam. You should notice that the energy pattern broadens as the beam strikes the blackboard at lower and lower angles.

When the sun is low, the amount of energy per unit area is less. This results in a loss of energy at Earth’s surface and cooler temperatures.

When the sun is high, the amount of energy received per unit area is greater. This results in a gain of energy at Earth’s surface and warmer temperatures.

The amount of time that the sun is visible in the sky changes during the course of a year.

Summer: The days are longer, while the nights are shorter. Keep in mind that here the “day” means the number of hours that the sun is in the sky. The amount of solar energy absorbed by the ground is greater.

Winter: The days are shorter, while the nights are longer. The amount of energy received at the Earth’s surface is far less.

Spring and fall: Near the equinoxes, the days and nights are about the same length.

The sun’s rising and setting positions change throughout the year. This concept follows logically from a higher and lower sun and the change in the amount of time that the sun is visible in the sky, but it is a difficult concept for students to understand. Although we generically say that the sun rises in the east and sets in the west, in reality, this only occurs on the first day of spring and the first day of fall.

Summer: (through spring and summer) The sun is north of the equator. The sun rises north of east and sets north of west.

Winter: (through fall and winter) The sun is south of the equator. The sun rises south of east and sets south of west.

First day of fall and spring: The sun is directly over the equator. The sun rises due east and sets due west.

Why do we have seasons? The first two reasons are common misconceptions. They are incorrect. The third explanation is the correct one.

WRONG! The changing distance of the Earth from the sun causes the seasons.: Most people believe that the seasons are a result of the Earth getting closer to the sun in the summer and farther from the sun in the winter. WRONG!
Most people believe that the seasons are a result of the Earth getting closer to the sun in the summer and farther from the sun in the winter. The reasoning behind this error usually results from an exaggerated concept of the shape of Earth’s elliptical (oval-shaped) path around the sun and the natural consequences of getting closer to or farther away from an energy source. Here are some true facts to counter this argument:

Earth is closest to the sun in very early Januaryof each year. At this time it is about 91-1/2 million miles from the sun. It is wintertime in the Northern Hemisphere.

Earth is farthest from the sun in very early July of each year. The sun is about 94-1/2 million miles from Earth. At this time of the year the Northern Hemisphere is experiencing summer.

The distance of the Earth from the sun is just the opposite of what someone would expect to find with respect to the seasons in the Northern Hemisphere. There must be another explanation for why we have seasons.

If the seasons resulted solely from the change in distance of the Earth from the sun, then both hemispheres would experience the same season at the same time. The seasons in the Southern Hemisphere are opposite in time to the seasons in the Northern Hemisphere.

WRONG! The seasons are caused by the Earth’s axis flipping back and forth.!
The following, however, is true. The Earth is spinning around its axis, and the axis of the Earth is tilted. The angle of Earth’s axial inclination is 23-1/2 degrees (from the perpendicular to Earth’s orbital plane). The direction of Earth’s axial tilt remains nearly constant.

The heavens seem to pivot around the North Star: Many people know that the axis of the Earth points near to a very famous star in the nighttime sky called the North Star or Polaris. As the Earth rotates, it appears as if all of the stars are going around Polaris. Polaris is the closest bright star to this pivot point, making it appear virtually stationary. Since Polaris remains in the same position every night of the year, the Earth’s axis cannot be flipping back and forth. If the flipping of Earth’s axis caused the seasons, we would have many different stars that would function as the North Star over the duration of one year.

To demonstrate why the stars in the Northern Hemisphere circle around the North Star, stand up and bend your head back. Find a mark on the ceiling which is directly over your head. Your head is now the Earth and your eyes represent the axis of the Earth extended into space. Rotate slowly. Notice how all of the other marks on the ceiling seem to circle around the point which is directly over your head. This is the location to which the axis of your Earth is pointing. Likewise, the positions directly over the North and South Poles of Earth also act as pivot points and do not move. In the Northern Hemisphere, this position is very close to the North Star. This is the reason why the North Star is basically stationary. At present there is no comparable star in the Southern Hemisphere which acts as a pole star, even though the sky pivots in a similar fashion.

CORRECT! The seasons are the result of Earth’s axial tilt and the condition that the axis always points in the same direction as Earth orbits the sun. This means that:

In the summer the axis leans us toward the sun even though we are at a greater than average distance from the sun.

The sun is higher in the sky and the energy that we receive from the sun is more direct. Sometimes, we even say that the sun is “beating” down upon us in the summer. In a way this is a true statement.

The sun remains in the sky for a longer period of time. For Allentown the time from sunrise to sunset is 15 hours on the first day of summer.

The higher sun and longer days result in a greater amount of energy being absorbed by the Earth’s surface. Temperatures must go up.

We lean back from the sun in the winter months, even though we are actually closer to the sun in distance.

The sun is lower in the sky and its energy distribution is more spread out.

The sun is visible for a much shorter time during the day. In Allentown on the first day of winter, the sun is above the horizon for only nine hours.

The result of a lower sun giving us indirect energy and a shorter day means that we are receiving much less energy, and temperatures must go down.