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Lesson 1 - Introduction to Physical Geography, Solar Energy and Seasons

• Learning Outcomes
• Overview of the Geography Continuum
• The Scientific Method
• The Four Earth Systems
• Solar Energy
• Seasons

To do: Check the schedule for this week's reading & upcoming assignments Read the lecture and assigned reading in the text Participate in discussions Take the Week 1 Quiz By the end of this lesson you should be able to: Describe how the scientific method can be used to find an explanation for a problem Define the four earth systems Describe the forms of sun energy and relate them to what we call 'light' Explain the cause of seasons on earth

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Overview of the Geography Continuum

Welcome to Physical Geography!  Geography is an exciting subject that focuses on the spatial distribution of natural phenomena and the interaction of humans with the natural world.  Geography is much, much more than simply memorizing the names of state capitals and major rivers.  Geography looks to explain and understand the natural world that we live in.  The four major questions that every geographer asks when trying to understand a problem are:  Where is it?  Why is it there? How did it get there?  How does it interact with other things? This course in physical geography will provide you with the tools to better understand the spatial distribution of natural phenomena.  By the end of this course, you will be able to explain why San Francisco is always cold and foggy, why we have earthquakes and what causes seasons. 

 

The Scientific Method:
An Orderly Way to Think about Problems

There is no one agreed upon description of the scientific method.  Imagine for a moment what the world would be like if there were:  there would be a totalitarian set of procedures, a recipe, which if obeyed would yield fact!  By now, most of you have probably had enough exposure to science to realize that this would not work for long.  The scientific method is an umbrella term for logical reasoning by which people explain unknown circumstances.  One generalization of the scientific method is:  Problem -> Idea of a possible solution -> Action.  The ‘action’ in  the scientific method is usually thought of as some sort of experiment.  One methodology presented by Popper (1972) is P1 -> TT -> EE -> P2, where P1 is the initial problem, TT is the tentative theory (working hypothesis or trial application of a theory), EE is the attempts at error elimination with the tentative theory, leading to P2, additional errors or problems.  Note that this methodology is not cyclic.  P2 is the result of problems from TT (the tentative theory). P2 is then incorporated into TT, resulting in a different (or reduced) set of errors, EE, and P3, or a third set of residual problems.  This can continue until the practitioner is satisfied with the explanation for the original problem. 

One way in which you might apply the scientific method is to try to figure out why it rains. Your P1 would be 'Why does it rain?'. You might observe that the sun is not visible when it rains, so your TT might be 'It rains when the sun goes away'. Now you need to apply error elimination. Can you think of times when the sun is not visible, and it is not raining? Nighttime might come to mind. It sometimes rains at night (no sun), but sometimes it does not. So our EE would lead us to P2, thinking of what other things might block the sun. Our second round TT may lead us to think of clouds, which block the sun, and also are present when it rains. How might you continue this line of reasoning?

Applying the scientific method does not always yield the correct answer or explanation.  For example, Nicholas Copernicus (1473-1543) proved the heliostatic model of the universe (that the sun, not the earth, was the center of the universe).  Copernicus's heliostatic cosmology involved giving several distinct motions to the Earth, which he backed up with painstaking observations of the motions of the stars, sun and planets.  While the Copernican theory was almost correct, it was later improved upon by Johannes Keppler. 

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The Four Earth Systems

Your text defines a system as any ordered, interrelated set of things and their attributes, linked by flows of energy and matter, as distance from the surrounding environment outside of the system.  Read the definitions of open system  and closed system  carefully, then list three examples of open and closed systems below, including lists of the inputs and outputs for an open system, and the cycle of material for a closed system.

Open System
Closed System

Feedback loops help to regulate natural systems.  Most systems maintain a structure of steady state equilibrium over time. That is not to say that they are always steady, but rather that the average state is that of equilibrium.  This is a phenomenon known as dynamic equilibrium.  An example of dynamic equilibrium might be the average daytime September temperature in the San Francisco Bay Area.  The ‘average’ daytime temperature might be 78 degrees (F), but the temperature on any given day is somewhat higher or lower than that. 

This topic is centered around the four earth systems, and the interaction of each system with the others.  The graphic below shows a Venn diagram as a way to visualize the interrelationship between the four earth 'spheres'. We will revisit this diagram throughout this course as we focus on the different ways that the regions of Physical Geography interact with each other.

Read the definitions of each earth system carefully, then write down definitions for the following four earth systems in your own words. Atmosphere Lithosphere Hydrosphere Biosphere

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Solar Energy

The sun is a mass of incandescent gas, where hydrogen is turned into helium at temperatures of millions of degrees.  The fusion of hydrogen to helium (two hydrogen equals one helium) releases a lot of energy in the form of waves which are sent out in all directions.  Some of the waves (one 2-billionth, or 1/2,000,000,000) intersect the earth.  These are called insolation or INncoming SOLAr radiaTION.  The waves range from very short to very long (see the figure below and Figure 2.6 in your text), but each wave length (distance from trough to trough) carries approximately the same amount of energy.  Therefore, as you can see in Figure 2.5 of your text, short-wave radiation (the purple lines) have much more energy per unit time than long wave radiation (red lines).

The figure above shows the approximate real-world wavelength of each type of wave. Gamma rays are very, very, very small, about the size of an atomic nuclei.  X-rays are about the size of an atom.  The waves that cook your popcorn in the microwave and bring you music over the radio in your car are fundamentally the same as gamma rays and x-rays; they just have much less energy per unit area. 

Light is also the same kind of wave, but it occupies a very small portion of the electromagnetic spectrum (see figure 2.6).  However, our atmosphere filters out (by absorbing or reflecting) almost all of the other wavelengths of incoming solar radiation. As a result, the majority of the electromagnetic energy received by the surface of the earth is in the visible spectrum (see Figure 2.7).  Because of this, our eyes have evolved to respond to this tiny portion of the spectrum.

All exogenic systems (those that function above the earth’s surface) require energy to function, and all of that energy comes from the sun.  This is a very important point – the energy from the sun drives all systems on earth.  For example, people (open systems) require the input of food (energy) and water.  Food can come from plants (or animals who eat plants) which are themselves open systems requiring water and sun energy to drive photosynthesis, the process by which they produce the sugars that they require to grow and sustain themselves.  The water cycle (which we will learn about in more detail in week 5) gives us fresh water through rain or groundwater sources.  The driving force behind the water cycle is sun-energy.

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Seasons

What causes seasons? The distance between the Earth and the Sun does not account for the seasons. Look at page xii in Goodes World Atlas.  First, you will note that the earth’s orbit is indeed not quite circular.  The distance between the sun and the earth on the winter solstice (December 22) is 91.5 million miles, while the distance between the sun and the earth on the summer solstice is a whopping 94.5 million miles.  So the earth is actually closer to the sun in December than it is in June!  Click on the image thumbnail below to see a full-size image of Spreading Sunlight Over the Spherical Earth.

The earth is tilted 23.5 degrees off of vertical, and this tilt accounts for the seasons. Insolation is distributed over the (nearly) spherical earth.  The sunlight is most concentrated at the point where the ray of the sun is perpendicular to the tangent of the earth.  As the angle between the sun’s rays and the tangent becomes smaller, the effect is that the same amount of sun energy is spread over a larger area. Or, each area gets less energy. The graphic below is another way to visualize this concept:

The seasons are caused not by distance from the sun, but by the effect of the uneven distribution of sunlight on the tilted earth. The earth rotates around the sun on a flat plane. This is called the plane of the ecliptic. The earth is tilted 23.5 degrees off of this plane. The tilt does not change, a concept called axial parallelism. In other words, the axis of the earth is parallel to itself throughout the orbit.

You can see in the diagram below (Earth Sun relationships and Incoming Solar Energy) that at the December solstice, the majority of the incoming solar energy is in the Southern Hemisphere, and in June, the majority of the incoming solar energy is in the northern hemisphere.  That is because on the December solstice, the sun’s rays are perpendicular to the earth at 23.5 degrees South. This is the subsolar point, or the point of maximum insolation. On the June solstice, the subsolar point is at 23.5 degrees north (see the graphic below). The subsolar points at the solstices define the Tropics. The Tropic of Cancer is 23.5 degrees North, and the Tropic of Capricorn is 23.5 degrees South. At this point it is worth pointing out that while the northern hemisphere experiences summer in June, the southern hemisphere experiences summer in December.  Another way to think about it is that the southern hemisphere is ‘in reverse’ of our seasons in the northern hemisphere.

The next piece of the puzzle is the Circle of illumination. At any time, half of the earth is illuminated by our sun. If the earth had no tilt, this would mean that we could have 12 hours of daylight and 12 hours of darkness everywhere on earth all the time, and as there would be no shift in the subsolar point, we would not have seasons. As I am sure you are aware, this is not the case! If you have every traveled anyplace very far north in the summertime, for example, you may have noted the very long days. I vividly recall sitting in a garden pub while I was in graduate school in England (at about 53 degrees N) and enjoying the sunset at 10PM! The circle of illumination defines the Arctic Circle and the Antarctic circle (see below). How long are the days in the Arctic Circle in June? How long are the days in the Antarctic Circle in June? Why?

 The equinoxes occur in March and September. I like to think of them as 'equal equinoxes' -- in other words, at the equinox the subsolar point is at the equator and the circle of illumination is distributed equally over the earth.  In the Earth Sun Relationships diagram above you can see for yourself that the insolation is pretty much equally distributed over the northern and southern hemispheres. Visualize what the circle of illumination looks like at the equinoxes. How long are the days at various latitudes at the equinoxes?

To review key points:

• The Earth's tilted axis (23.5 degrees) causes some parts of the Earth to receive direct sunlight while other parts are receiving indirect sunlight.
• Seasons occur due to the tilt of the Earth's axis and the orbit of the Earth around the Sun.
• The seasons of the Northern and Southern Hemispheres are reversed: when the Northern Hemisphere is experiencing winter, the Southern Hemisphere is experiencing summer.
• The vernal equinox: the first day of spring in the Northern Hemisphere (March 21), when the Sun is perpendicular to the equator.
• The summer solstice: first day of summer in the Northern Hemisphere (June 21), when the Sun is perpendicular to the tropic of Cancer.
• The autumnal equinox: first day of autumn in the Northern Hemisphere (September 22), when the Sun is perpendicular to the equator.
• The winter solstice: first day of winter in the Northern Hemisphere (December 21), when the Sun is perpendicular to the Tropic of Capricorn.
• Key terms: plane of the ecliptic, axial parallelism, circle of illumination, subsolar point.

Try out the NASA Seasons Quiz.

Review learning outcomes.

Please complete the Assignments and Exams section for each lesson before proceeding to the next lesson.

 

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