The Reasons for the Seasons
Steve Carson
sc@gfdl.noaa.gov, 609.452.65956The existence of seasons on earth is a consequence of a number of factors:
1) The earth receives most of its heat energy from light from the sun.
2) The earth is almost a sphere.
The most observable causes of the seasons are that the length of the period of daylight changes and the position of the sun in the sky changes. In the summer there are more daylight hours and the sun is higher in the sky than in winter. These causes of the seasons are consequences of the factors listed above as will be seen.
The fact that the earth is closest to the sun at one part of its orbit and farthest at the opposite side of its orbit is not the cause of seasons. As a matter of fact the earth is farthest from the sun during northern hemisphere summer and closest during northern hemisphere winter.
1) The earth receives most of its heat energy from light from the sun.It is essential that the importance of the sun is clear. The basic concept can be illustrated through demonstration and/or discussion of how it feels warmer in the sun versus the shade. Thermometers can be placed in the sun versus the shade to show that the one exposed to sunlight reads a higher temperature than the one in the shade. This can also be done by exposing one thermometer to light from a light bulb and keeping another in just normal room light to show that the one lighted by the bulb shows a higher temperature.
The amount of heating depends on the time of exposure as can be demonstrated by watching the change of temperature with time as it is exposed to light or by exposing thermometers to light for varying amounts of time. The amount of heating is also dependent on the angle with which the light strikes a surface as will be explored in the next section. Other factors that affect heating such as land versus water will not be covered in detail on this page. Experiments with heating soil or water with sunlight or lamplight and measuring temperature changes can also be done to relate to observations more directly to the earth.
2) The earth is almost a sphere.The fact that the earth is almost a sphere (except for a slight bulge at the equator and flattening at the poles and surface features) is important to many related features of temperatures on the earth; the seasons, the fact that the polar regions are colder than near the equator, the changes in temperature and sun position throughout a day.
Light Patch on a Globe
Consequences of the earth's spericity can be demonstrated using a globe and a light source that produces a small beam. A large ball can be used instead of a globe, but a globe is more effective for relating the observations to the earth. A very effective light source is an overhead projector with the glass covered with paper with a square hole cut out to block out all of the light except a square patch. One can also use a flashlight if the beam is sufficiently focused. If using a flashlight, be sure to hold it so that the beam is always parallel to the table.
Align the globe and light so that the patch of light is centered on the globe. Move the globe or light source so that the light patch moves up and down on the globe and side to side on the globe. Note that the farther the light patch gets from the central portion of the globe the more elongated it gets. Near the central portion, the shape is more square (or circular if using a flashlight).
What does this mean? A certain amount of light is coming through the square hole (or from the flashlight) and reaching the globe. When the light patch is hitting the central part of the globe and is the smallest (most square or most circular in the case of a flashlight) that amount of light is concentrated in a small area. The light is striking the globe perpendicularly, i.e. a line from the light source to the patch would be perpendicular to the globe surface at the location of the light patch. If you imagined yourself standing on the globe at that point, you would have to look directly overhead to see the light source (sun).
As you move the light away from the central portion of the globe the same amount of light from the light source is now spread over a greater area. This means that now each location within the lighted patch receives a smaller portion of the light in the total patch. As an analogy think about spreading a certain amount of jelly on one slice of toast versus two slices of toast. Imaging taking one of the small containers of jelly that restaurants provide and spreading the entire contents on one slice of toast. Now think about the same amount of jelly on two slices of toast. In the second case each slice of toast will get less jelly than the single piece of toast in the first case. In this analogy the jelly represents the light coming through the hole (or from the flashlight). The toast represents the amount of surface over which the light is spread. The case with one slice of toast represents the case in which the light is hitting the central portion of the globe perpendicularly and the light hits a relatively small area. The case with two slices of toast represents a case when the light hits a part of the globe away from the central portion and hence is more spread out. In the second case each piece of toast now receives a smaller portion of the jelly than did the single piece just as each location in the spread out light patch receives a smaller portion of the total light in the whole patch. (Note: This analogy was adapted from a suggestion by a QUEST 2000 participant as an improvement on a previous analogy I had used.)
Why is the patch more spread out? Notice that the angle between the globe surface and the light beam changes as you move the patch away from the central portion of the globe due to the curvature of the globe. If you imagine standing on the globe away from the central portion, you would not have to look strait up to see the light source (sun). The light source (sun) would look lower in your "sky". The light is now striking the surface at a lower angle than perpendicular. The lower the angle, the more the light is spread out.
To illustrate this further use a long piece of white cardboard in front of the globe. First hold the cardboard so that it is perpendicular to the light beam. This represents the surface of the globe in the central portion. The light should form a square patch (or roughly circular if using a flashlight). Now gradually tilt the cardboard (i.e. hold the card in the middle of one of the vertical edges and tilt the card so the top is tilted away from the light source and the bottom is tilted toward the light source). Notice that the light patch becomes more elongated just as it did on the globe when you moved the light patch away from the central portion that you started with using the light. Both on the globe and on the cardboard the light patch becomes more elongated as the light hits the surface at a lower and lower angle.
Shadows
Another observable consequence of the angle with which light strikes the surface is in the lengths of shadows produced. By using a toy figure or small sticks with globe or cardboard and the light source you can see that lower angles of light produce longer shadows.
Measuring Temperature
The temperature affects of the angle with which light strikes a surface can be demonstrated using a lamp that can direct light and two thermometers with the bulbs covered in dark cardboard, paper or cloth to create a relatively flat surface covering the bulb. Direct the lamp downward. Place one thermometer flat on top of a white piece of paper on the table below the lamp. The dark surface of this thermometer should receive light from the lamp perpendicularly. Placing the second thermometer next to the first, support the second so that the flat surface covering the bulb received light at a very low angle. The two black surfaced should be not quite perpendicular to each other. Turn on the lamp and periodically record the temperatures on the two thermometers. The horizontal thermometer should show a higher temperature than the tilted thermometer since the light striking the surface covering the tilted thermometer bulb should be more spread out than light striking the surface covering the horizontal thermometer bulb.
As stated earlier the spherical shape of the earth (and the effects on how light hits and is spread over surfaces on different parts of the earth) helps explain a number of things. The poles are colder than the equator because the poles tend to receive light at a lower angle (and sometimes not at all as seen in the next section) than the equator. Mid-day tends to be warmer than dawn or dusk because the latter times receive sunlight at a lower angle than at mid-day. Remember that as a point on the earth rotates from west to east, the sun is low in the sky in the morning, moves higher toward mid-day and then becomes lower in the sky toward sunset. You will also see longer shadows near dawn or dusk than at mid-day. The spherical shape of the earth is also important to understanding seasons, but now we must include the next concept.
3) The earth rotates on an axis that is tilted relative to the path the earth takes around the sun. The earth's rotation axis maintains a constant orientation relative to the distant stars and hence its orientation relative to the sun in an annual cycle.This is the key to the seasons. To understand why use a globe with the light source that makes a small patch of light. For students the globes can be modified with markers or pieces of tape to highlight the equator and 45-degrees (or 40-degrees) North and South latitudes. The choice of latitudes might depend on the latitude lines that are shown. Some globes use 15-degree intervals while others use 10-degrees. 40-degrees is near to our latitude in New Jersey.
Moving Globes (Very Important)
First it is important to understand and physically demonstrate the earth's rotation and how the rotating earth travels around the sun. The globes used and most globes that are designed to rotate have the rotation axis tilted by 23.5 degrees relative to a vertical line. Pick an imaginary sun to move the globe around. Move the globe in a roughly circular path around the "sun" so that the rotation axis always points in the same direction relative to the sides of the room. The north pole will always be pointing toward same general area of the ceiling. This will result in the north pole being inclined toward the sun on one side of its orbit and away from the sun on the opposite side. Between the two extremes the north pole is inclined neither toward nor away from the sun. On the time scales important to the seasons, the earth's rotation axis maintains a constant orientation relative to distant stars. The north pole of the real earth always points toward Polaris (the North Star) at the present time. The orientation of the earth's axis does vary with periods of 10's of thousands of years which is important to ice ages, but not to the seasons that we experience.
Position the globe so that the north pole is inclined most directly toward the "sun". This represents the summer solstice for the northern hemisphere. Note that the south pole is inclined away from the sun, so this position represents the winter solstice for the southern hemisphere. This occurs on June 21 or 22.
Move the globe in a counter clockwise direction (when viewed from above) one quarter of the way around its roughly circular path, keeping the north pole oriented in the same direction relative to the features of the room. Now you have reached a position in which neither pole is inclined toward or away from the sun. This represents the orientation of the earth at the autumnal equinox in the northern hemisphere and the spring (vernal) equinox in the southern hemisphere on September 22 or 23.
Continue the globe another quarter of the way around its path keeping the north pole oriented in the same direction relative to the features of the room. Now the north pole is inclined the most away from the sun. This position represents the winter solstice for the northern hemisphere and the summer solstice for the southern hemisphere on December 21 or 22.
One more quarter of the way around brings us to another position in which neither pole is inclined toward or away from the sun. This represents the orientation of the earth at the spring (vernal) equinox in the northern hemisphere and the autumnal equinox in the southern hemisphere on March 21 or 22.
The solstices and equinoxes are generally described as the first days of each of the seasons: summer solstice called the first day of summer, autumnal equinox the first day of fall, winter solstice the first day of winter, and vernal equinox the first day of spring. In terms of climate it is probably more accurate to describe summer in the northern hemisphere as the months of June, July and August (JJA_, fall as September, October, November (SON), winter as December, January, February (DJF), and spring as March, April, May (MAM). In the southern hemisphere the seasons are offset by six months from the northern hemisphere.
Tilted Globes and Light Patch
How does this tilt of the rotation axis result in the seasons? Position the globe relative to the light source with the small patch so that the globe is in the northern summer solstice position, i.e. the north pole inclined most toward the light source. Move the globe or the light source up and down through the central portion of the globe to show how the light patch changes to the north and south. You should see that through much of the northern hemisphere the light patch stays fairly small, whereas in the southern hemisphere the light patch is more spread out and does not even reach the regions around the south pole. Compare for example the light patch size and shape at 45-degrees north, the equator and 45-degrees south. You should see that at both 45-degrees north and at the equator the light patch is fairly small, while at 14-degrees south, the light patch is quite elongated. At this time of year the sun is higher in the sky in the northern hemisphere and lower in the southern hemisphere resulting in more heating in the northern hemisphere than in the southern hemisphere where the light is more spread out. This results in summer in the northern hemisphere and winter in the southern hemisphere.
Now position the globe to the light source to represent the northern winter solstice and southern solstice, i.e. the north pole inclined most away from the light source and the south pole inclined most toward the light source. Moving the position of the light patch you should now see the opposite of the previous case. The light patch remains small through much of the southern hemisphere while in the northern hemisphere the light patch is elongated and does not even reach the north polar regions. The patch is now similar at 45-degrees south and the equator, but quite elongated at 45-degrees north. At this time of year the sun is higher in the sky in the southern hemisphere and lower in the northern hemisphere where the light is more spread out. This results in summer in the southern hemisphere and winter in the northern hemisphere.
Finally, position the globe relative to the light source to represent on of the equinoxes, i.e. neither pole is inclined toward or away from the light source. Now the light patch should spread the same way both north and south of the equator as you move the patch away from the equator. Both hemispheres should receive the same distribution of light and this is characteristic of both equinoxes.
Remember that the importance of the tilt of the earth's rotation axis is in how it affects the spread of light in different hemispheres and not in how near or far each hemisphere is from the sun. The earth has a radius of about 4000 miles compared to an average distance from the sun of 93,000,000 miles. If you represent the distance between the earth and the sun as 93 yards of a football field, the radius of the earth would only be 1/7 of an inch. Thus the winter and summer hemispheres are not appreciably different in distance from the sun.
Cardboard Model (adapted from Neuberger and Nicholas, 1962)
Reference: Neuberger, Hand and George Nicholas, 1962. Manual of Lecture Demonstrations, Laboratory Experiments, and Observational Equipment for Teaching Elementary Meteorology in Schools and Colleges, 182 pp., College of Mineral Industries, Pennsylvania State University, University Park, PA.These relationships can be further explored using the cardboard cross section of the earth that can be rotated relative to the lines representing the sun's rays. Note that the central heavy line with the arrow strikes the circle perpendicularly so at that point at noon the sun would be directly overhead and hence would receive the most concentrated sunlight from the sun (ignoring clouds). The amount that the light is spread out over the surface can be understood by looking at the distance along the circle that separates the points at which two neighboring lines (light rays) intercept the circle. The farther you go from the central heavy line the greater the distance between points where neighboring light rays reach the circle. This can be related to how the light patch lengthened as you moved the patch away from the central part of the globe.
Turn the circle representing the earth so that the right end of the Tropic of Cancer line touches the arrow point on the central ray of the sun. This represents the summer solstice in the northern hemisphere and the winter solstice in the southern hemisphere. The Tropic of Cancer is at 23.5-degrees North latitude. Note that on the Tropic of Cancer at noon light rays are perpendicular to the surface and the sun would be directly overhead. As was seen with the light patch on the globe, the sunlight strikes most of the northern hemisphere at a higher angle than in the southern hemisphere and hence is less spread out and causing greater warming in the northern hemisphere. Also note that all points north of the Arctic Circle (66.5-degrees North - 23.5-degrees) receive sunlight while all points south of the Antarctic Circle (66.5-degrees South) are in darkness. This will be considered further when we explore day length in the next activity.
If you turn the circle so that the Tropic of Capricorn (23.5-degrees South) is lined up with the heavy arrow the opposite situation is illustrated. We are now showing the winter solstice in the northern hemisphere and the summer solstice in the southern hemisphere. The sun is directly overhead at noon on the Tropic of Capricorn. The sun's rays strike the surface at a higher angle in the southern hemisphere that in the northern hemisphere so there the light is more concentrated and causes greater heating in the southern hemisphere. Now all points north of the Arctic Circle are in darkness and all points south of the Antarctic Circle receive light.
Now turn the circle so the equator is lined up with the heavy arrow. This represents the two equinoxes. Sunlight is symmetrically distributed between the two hemispheres.
The cardboard model also shows one other reason for differences in heating at different latitudes. A circle representing an exaggerated atmosphere is shown. On the scale of this model the atmosphere would mainly be contained within less than the thickness of the line that makes the circle. The atmosphere is shown in this way to demonstrate that light represented by the thick arrow passes through less atmosphere than the light represented by other rays. The farther away from that central point, the greater thickness of the atmosphere through which the light must pass. Light passing through the atmosphere can be reflected, scattered, or absorbed thus reducing the amount that actually reaches the ground. The more atmosphere through which light has to pass, the less will make it to the ground. That is why you can safely look at the sun at sunset and sunrise, but not in the middle of the day. You can see from the model that wherever the light strikes the surface at a low angle and hence spreads over a large surface it also has passed through a greater thickness of atmosphere. The passage through the atmosphere also contributes to the reduction in heating of the surface, but is a much smaller effect than the spreading of the light over a greater surface.
Shadows RevisitedAn observable consequence of the changing height of the sun through the seasons is the change in the lengths of the shadows at noon. If you keep track of the shadows produced by a particular object, you should see that it casts a much longer noontime shadow in winter than in summer (keeping account of the shift in time due to daylight savings time). This is an observation that could be made once a week during the school year and graphed.
Hours of DaylightThere is another important way in which the tilt of the earth's axis contributes to the seasons. That is the variation in hours of daylight with latitude at different times of the year. This can also be illustrated with the globes.
The globes used has a 15-degree interval between longitude lines. In this case each longitude line represents 1 hour of turning of the earth. A full circle is 360-degrees. 360-degrees divided by 24 hours gives 15-degrees of rotation per hour. This each longitude line represents 1 hour. For students these can be highlighted at certain latitudes (eg. 45-degrees North and South and the equator) using a marker or pieces of tape. If you use globes with a 10-degree interval between lines you would probably need to mark 24 dots equally spaced around each of 40-degree North latitude, the equator and 40-degrees South latitude. Drafting tape can be used for the dots so that they could be easily removed and not leave residue. The dots would be separated by 15-degrees of latitude so you can use the longitude lines on the globe to place the dots.
Using a slide projector, an overhead projector or a lamp shine the full beam on the globe so that all of one side of the globe is illuminates. Orient the globe in the position of the northern hemisphere summer solstice (north pole inclined toward the light source, south pole away). Count the number of longitude lines (or dots) on the lighted side at each latitude. Ideally, you should get about 15 at 45-degrees North, 12 at the equator and 9 at 45-degrees South (give or take one). You should see that more longitude lines (or dots) are lighted at 45-degrees North than the equator which has more lighted than 45-degrees South. This means that there are more hours of daylight at 45-degrees North than at the equator which has more than 45-degrees South on the northern summer solstice.
Also note that the north pole is lighted and the south pole is in darkness. Although it may be hard to see, all points north of the Arctic Circle would be lighted throughout the day (rotate the globe to see that) and all points south of the Antarctic Circle would be in darkness throughout the 24 hours of rotation on that date. Note how this related to the cardboard model. On this date there are 24 hours of daylight north of the Arctic Circle and 24 of darkness south of the Antarctic Circle.
Move the globe to the position of the northern hemisphere winter solstice (north pole inclined most away from the light, south pole toward the light). Now you should count roughly 9 lighted longitude lines (or dots) at 45-degrees north, 12 at the equator, and 15 at 45-degrees South. Day lengths are longer in the southern hemisphere and shorter in the northern hemisphere. On this date there are 24 hours of daylight south of the Antarctic Circle and 24 hours of darkness north of the Arctic Circle. Note that in both of these cases there are 12 hours of daylight and 12 hours of darkness at the equator.
Now move the globe to the position of one of the equinoxes (neither pole inclined toward or away from the light). Now you should count 12 hours of daylight at all three latitudes. As a matter of fact every location on the globe except right at the poles should have 12 hours of daylight and 12 hours of darkness on the days of the equinoxes. Day length changes continuously between the extremes of the solstices and the intermediate equinoxes. The solstices represent the shortest and longest periods of daylight.
Length of daylight and its changes also varies with latitude. Within the Arctic and Antarctic Circles long periods of daylight and darkness can exist. At the poles daylight and darkness both last for six months. At 70-degrees latitude a two month period of continuous daylight exists beginning about one month before the summer solstice and ending about one month after the solstice when daylight periods shorten (with sunrises and sunsets each calendar day) to 12 hours by the autumn equinox and continue to shorten until two months of darkness occur beginning about one month before the winter solstice. Between the Arctic and Antarctic Circles the daily sunrise and sunset times vary, with the most dramatic variations occurring farther from the equator.
Sunrise and sunset times can be obtained from newspapers, almanacs and various web sites including http://aa.usno.navy.mil/AA/data/ where you can get sunrise/sunset (among many other things) for any location. You can find the sunrise and sunset times for Princeton at http://mach.usno.navy.mil/cgi-bin/aa_rstablew.pl. Such data can provide good graphing activities such as tracking sunrise, sunset and day length for one day each week.
But wait a minute! I told you that there are 12 hours of daylight all the time at the equator and at all locations on the equinoxes, but if you look up sunrise and sunset times you will find a dew minutes more than 12 hours difference between sunrise and sunset at the equinoxes. Sunrise and sunset are expressed as the times when the sun is first and last seen at the horizon. There would be 12 hours between the time that the center of sun's disk crossed the horizon before the middle at sunrise and after the middle at sunset there is a bit more time than 12 hours between sunrise and sunset. What is more, twilight before sunrise and after sunset due to the scattering of light by the atmosphere extend apparent daylight somewhat. So when I was talking about daylight and darkness above I was really referring to the time differences between the center of the sun passing the horizon.
The important point is here that the changes of the length of time that a location is exposed to the sun contributes to the seasons. Longer exposure leads to greater warming.
ConclusionSeasons exist on earth because the earth is nearly a sphere and the tilt of the earth's axis results in longer periods of daylight and a higher angle of the sun in the summer hemisphere and shorter periods of daylight and lower sun angles in the winter hemisphere. Longer daylight and higher sun angle result in greater heating. The fact that the earth's orbit is not perfectly circular has a much smaller effect than the factors listed above and does not produce the seasons, though it is important on the long time scale of ice ages (100,000 year time periods).