A Quarterly Activity Bulletin of The South Carolina Department
of Natural Resources-Southeast Regional Climate Center
The sun is the center of our solar system, and the planets in our solar system revolve around the sun. Earth is the third planet from the sun.
The earthís elliptical orbit changes from an ellipse to a more circular shape and back again over a cycle of 90,000 to 100,000 years. Because earthís path is elliptical, there are times when the earth is closer to the sun and times when it is further from the sun. But this difference in distance is not the reason we experience seasons, as many people believe. Even though the earth is closest to the sun on January 5 (it is 147 million km away), and farthest from the sun on July 5 (it is 152 million km away), the distance does not alter effect of the seasons in a significant way. The time when the earth is closest to the sun is referred to as perihelion; when it is farthest away is called aphelion.
Reasons for the Seasons
It is not distance from the sun, but tilt of the earth that causes the seasons. Earth rotates counterclockwise (from an "above" view) on an imaginary axis that extends from the North Pole to the South Pole. This pole is not oriented straight up and down but is tilted about 23.5 degrees from straight up and down, as the diagram below illustrates.
The tilt causes the seasons because it allows the sunís rays to shine more directly and for longer periods of time on some locations certain times of the year than others. The sun shines in parallel rays towards the earth, and this directness of the sunís rays upon the surface of the earth is what warms the surface. Recall that the Northern hemisphere refers to all latitudes located north of the equator, and the Southern hemisphere refers to all latitudes south of the equator. Because different parts of the earth are tilted towards the sun as earth revolves around the sun, the Northern and Southern hemispheres experience direct sunlight at the surface at different times of the year. Please see diagram below.
You can see that during the June solstice, the Northern hemisphere is tilted towards the sun, while the Southern hemisphere is tilted away. The Northern Hemisphere is experiencing summer and the Southern is experiencing winter. As earth continues to revolve around the sun throughout the year, we see that at the December solstice, the Northern hemisphere is tilted away from the sun, and the Southern hemisphere is tilted toward it. The December solstice represents the beginning of winter for the Northern Hemisphere and the beginning of summer for the Southern. To see the actual historical dates for Perihelion, Aphelion, June and December solstices, check out the US Naval Observatory's website.
The amount of daylight we experience will be greater in the Northern hemisphere in June, but will be less in the Southern hemisphere. In December, the Southern hemisphere will have the longer days. The following diagram illustrates hours of daylight that correspond to each hemisphere and each solstice:
Between the June and December solstices, there is a time that the sunís rays shine directly at the equator, and the tilt of the earth does not expose the Northern or Southern hemisphere to more or less radiation. These times are called equinoxes, and at an equinox all locations on earth receive an equal amount of sunlight, 12 hours. The vernal Equinox occurs on March 21-22 marking the beginning of spring in the Northern hemisphere, and the autumnal Equinox occurs on September 22-23 marking the beginning of fall.
The following table illustrates the differing hours of sunlight latitudes experience over the course of the earthís revolution around the sun.
(table courtesy http://www.colby.edu/sci.tech/st215/3.2view/sld008.htm)
The Living Earthģ Inc./Earth Imaging site shows what part of earth is experiencing day and what part is in night at a specific time and a specific month. As you experiment by plugging in different seasons of the year, notice that places above the Arctic circle (66.5 degrees North) and below the Antarctic circle (66.5 degrees South) are sometimes completely light or dark over the course of the entire day.
Not only does the number of daylight hours a location receives change as earth revolves around the sun, but also the intensity of the sunlight received. Think about it: if you were to shine a flashlight straight at a piece of paper, a very intense circle of light would light up a small area. If you were to tilt the flashlight so the light shone on the paper at an angle, the "circle" of light would not be a circle, but an ellipse. The ellipse would be larger in area than the circle. Because the amount of light shining from the flashlight had not changed, but area lit up had increased, the ellipse would be less bright than the circle. The earthís surface is affected by the sunís rays in the same way. The sunís rays shine on earth with an intensity of about 1370 Watts per square meter. Even though the sunís rays hit the earth in parallel beams, the tilt of the earth towards the sun causes the beams to hit more directly in some places than others.
The following image helps explain the relationship between beam angle, area covered, and intensity:
Notice that it is the same amount of sunlight hitting the surface, but the angle of the sunlight is what changes the way that sunlight is spread out. Because the earth is round, we can see the different angles that sunlight makes as it hits the earth.
The angle of incidence is the angle formed between the sunís rays and the earthís surface. The further from the equator North or South one travels, the smaller the angle of incidence becomes, the more surface area is lit by the sun, and the less intense the sunlight is as it is spread over more area.
Notice in the above diagram that the Earth is receiving sunlight at a 90 degree angle at about 23.5 degrees North in latitude. What time of year is this diagram showing? What would this picture look like if it were showing the vernal or autumnal equinox?
Check out the following website and applet to experiment with different latitudes and sunlight incidence.
Questions to Answer
To be a part of a worldwide effort among students and teaachers to chart the
effects of latitude on temperature and sunlight, join The
Global Sun/Temperature Project.
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Southeast Regional Climate Center
SC Department of Natural Resources
1201 Main Street, Suite 1100
Columbia, South Carolina 29201
The South Carolina Department of Natural Resources prohibits discrimination on the basis of race, color, sex, national origin, disability, religion, or age. Direct all inquiries to the Office of Human Resources, P.O. Box 167, Columbia, SC 29202.