Why Earth Has Seasons


Knott's Scary Farm is one of the benefits of living on a planet with seasons!

Everyone loves the seasons. They bring different holidays, reasons to celebrate and a change in wardrobe (especially Halloween). The seasons are defined as the yearly change in temperature on our planet. These changes in temperature on our planet are a direct result of the amount of solar radiation that hits our planet at a particular time of year. But what causes the amount of solar radiation that hits our planet to change? Do you think you know what causes the seasons? You better keep reading to make sure...

The concepts you are about to learn may require that you unlearn what you think you know. Misconceptions concerning the reasons for the seasons abound. READ THIS NEXT SENTENCE VERY CAREFULLY: The Earth does not get any closer or any further away from the sun as it orbits the sun. The Earth maintains a near-circular orbit around the sun; thus it remains about the same distance from the sun all the time (about 93 million miles). There are very good reasons for the seasons on our planet, but the distance between the Earth and the sun has nothing to do with it.

Cement that fact in your head. Write it on a piece of paper and tape it to your bathroom mirror. Then, go to the web link below and answer all the questions. This link will ask you some questions about the seasons and the moon. The moon information will help you with tides at a later date. Concentrate on the seasons information. When you are finished, return to this page.

Seasons and Moon Phase Tutorial. This is a must!!!
http://www.learner.org/teacherslab/pup/yourideas.html

Now that you know a little bit more than when you started, let me provide a full explanation. All of use are aware that the Earth orbits the sun once in 365 days, 5 hours and 48 minutes. We are also aware that in the winter our weather gets colder and in the summer our weather gets hotter. Spring and fall are transition periods between winter and summer. How can this be true if the Earth is NOT CLOSER to the sun in summer and NOT FURTHER from the sun in winter? (Truth is, the Earth is actually a little bit closer to the sun in winter.)

The biggest reason that the Earth experiences seasons has to do with the TILT of the Earth on its axis, relative to its plane of orbit. (Recall that a plane is an imaginary flat surface containing some object or objects) A piece of paper defines a plane; the plane of the Earth's orbit is the imaginary flat surface that contains the Earth as it orbits around the sun. If we place the Earth in its plane, we find that the North Pole does not stick straight up and down; it's not perpendicular (at a 90 degree angle) to the plane. The Earth tilts at a 23.5 degree angle relative to the plane. The Earth's axis (the imaginary line around which it rotates) is tilted towards the North Star, Polaris, and this angle measures 23.5 degrees. Got that?

The figure at left illustrates the tilt of the Earth on its axis relative to its plane of orbit. If the Earth was not tilted, its axis (the solid line running through the middle of the Earth around which the Earth rotates) would not be tilted; it would be perpendicular to the plane of orbit. But as we can see, the Earth is tilted and as the Earth orbits around the sun, this tilt will have a significant effect on where and how much sunlight a particular portion of the Earth receives.

 

Let's look at the Earth as it rotates around the sun and see how the tilt of the Earth affects the way sunlight reaches different parts of our planet.

What do you notice from this figure? First of all, you should observe the direction of the Earth's tilt (away from the sun or towards the sun) at different times of the year? Which way is the Earth tilted in relation to the sun on the Winter Solstice (Dec. 21)? Which way is the Earth tilted in relation to the sun on the Summer Solstice (June 21)? You should be able to see that the Northern Hemisphere is facing away from the sun in winter and towards the sun in summer. But what effect does this have on the amount of sunlight reaching our planet?

One effect of the tilt of the Earth in relation to the sun is the angle at which the sun's rays hit the Earth, called the angle of incidence. The figure at left illustrates the angle of incidence for the sun's rays for an Earth with no tilt. It should be pretty apparent to you that the sun hits more directly at the equator and spreads out at the poles. Another way to express this is to say that the amount of solar radiation per unit area is large at the equator and small at the poles. Think of it this way: the same amount of sunlight heats a smaller area at the equator and a larger area at the poles because the sunlight is spread over a greater area at the poles. As you might imagine this effect is exaggerated when we add the Earth's tilt into the equation.

Here's a simple demonstration you can try at home that will make the angle-of-incidence concept perfectly clear. You will need a flashlight, a piece of notebook paper, a pen and a ruler. First, turn on the flashlight and hold the notebook paper a couple feet away from it. Hold the flashlight perfectly horizontal and hold the paper perfectly vertical. You should see a focused beam of light on the paper. Draw the outline of the beam of light. Now, tilt the paper backwards at a slight angle (23.5 degrees if you know how much that is!). You should now see that the same beam of light is spread over a greater area of the paper. Draw the outline of the beam hitting the tilted paper.

Now measure the area of the two beams (vertical and tilted). If the beams were rectangular, then use length times width. If the beams were circular then measure the radius of the circles, square them (multiply the radius by itself) and multiply that value times pi (3.14159). Now perform the following calculation: assume that the amount of light coming out of the flashlight equals 1000 photons per square inch (this number is totally fabricated and nowhere near the true intensity but it's a convenient number for our purposes here). Calculate the intensity of light per square inch hitting the vertical paper? Calculate the intensity of the light per square inch hitting the tilted paper?

It should be fairly obvious to you even without the calculations that the amount of light per unit area is smaller when the paper is tilted. The same thing is true for the Earth. Near the equator, sunlight hits more directly and the solar radiation is more intense (a greater number of photons per square inch or whatever area measurement you choose). At the poles, sunlight is spread over a greater area and solar radiation is diminished (a smaller number of photons per unit area).

Let's consider the Earth's orbit in another way. As the Earth orbits the sun, the place on the Earth where sunlight hits directly changes day to day. We perceive this change as a difference in sun angle from day to day. In the summer, we see the sun getting higher in the sky. In the winter, we see the sun getting lower in the sky. This "raising and lowering" of the angle of the sun is a direct result of the Earth's tilt. Besides a change in sun angle, what else is happening between summer and winter?

As the Northern Hemisphere moves from winter to summer, the sun "climbs" higher in the sky. You can perhaps visualize this better by stabbing a pencil through the center of an orange (or any piece of round fruit) and orbiting the orange around a lamp. Tilt the orange as you orbit the lamp. In Northern Hemisphere winter (the orange tilted away from the lamp), sunlight hits most directly south of the equator. In Northern Hemisphere summer, the sun hits most directly north of the equator. Remember the Tropic of Cancer and Tropic of Capricorn? Remember their latitudes? Lo and behold, those latitudes were 23.5 degrees North and 23.5 degrees South, respectively. On June 21 in the Northern Hemisphere, our Summer Solstice, the sun reaches its highest angle in the sky and sunlight hits most directly on the Tropic of Cancer. On December 21 in the Northern Hemisphere, our Winter Solstice, the sun reaches its lowest angle in the sky and sunlight hits most directly on the Tropic of Capricorn.

A paragraph ago I asked what else was happening and of course, you answered that the length of the days was changing. Not only does the sun hit more directly in our summer but the days are longer as well. Thus, we get more intense solar radiation per unit area of Earth for a longer period of time. What about the Southern Hemisphere? What is happening in the Southern Hemisphere on June 21 and December 21? And what about Spring and Fall, where is the sun during those times of years?

To summarize, the tilt of the Earth in relation to its plane of orbit around the sun has the following effects:

The net result of these effects is (drum roll, here): differential heating of our planet. Repeat that word three times: differential heating, differential heating, differential heating. Then repeat it right before you close your eyes to go to sleep at night. Differential heating is the disparity or difference in the amount of heat received by one part of our planet as compared to another. In other words, some regions of our planet receive more heat and some regions of our planet receive less heat. That means that some parts of the planet heat more quickly while others heat more slowly.

We will return to this concept of differential heating of the planet when we examine how our planet responds to solar radiation and how heat is distributed in the oceans and the atmosphere. What is important at this point is that you understand WHY our planet heats differently in different parts. And now, you should have a much better understanding of the reasons for the seasons. Prove that you understand it by explaining it to a family member or friend.

Highly suggested reading:

Go to these links and read them!
They are way short but way informative.

Reasons for the Seasons, UWM
http://www.uwm.edu/~kahl/CoVis/Seasons/

NASA's Observatorium: Why Earth has Seasons
(requires Shockwave which you can download for free at their site)
http://observe.ivv.nasa.gov/nasa/earth/seasons/frames.html