The following material was covered in class on Tue. Oct. 9 but, because it is fairly confusing and takes some time to sort through and understand, it wasn't on Quiz #2.  We might have a vote in class to determine whether it should be covered on Quiz #3 or not.

Sun path diagrams are a way of showing the path that you would see the sun follow during the day.  We'll look at how the sun's path changes during the course of the year at Tucson, and we'll also see how the sun's path is different at different locations on the globe.

We'll come to understand better why the largest seasonal changes (biggest differences between summer and winter) are found at high latitude and why there is very little seasonal change at the equator.

The poles (the South Pole in particular because it is land) are the coldest locations on earth.  The hottest locations on earth aren't found at the equator as you might expect rather near 30 latitude.

The situation is probably the simplest on the equinoxes, we'll start there.  This is covered on p. 77a in the photocopied Class Notes.

There is a lot of information on this figure.  We  worked through this figure numbered point by numbered point.  You'll find written explanations of each point below and on p. 77b in the photocopied notes.

Notice first of all that the line separating day from night passes through the poles.  That is how you know this is one of the equinoxes and not the summer or winter solstice.

1.     The line separating day from night passes through the north and south poles.  As the earth spins on its axis, a person standing anywhere on the globe will spend exactly half the day on the nighttime side of the picture and half the day on the daytime side of the picture.  Thus the day and night are both 12 hours long.  This is true everywhere except at the poles.  We'll see what happens at the poles later.

2.     Imagine standing at the equator.  At point A you are positioned in the middle of the nighttime side of the globe; it is midnight at Point A.  6 hours later you will be standing at Point B where you will move from night to day; this is sunrise.  To see the sun you must look exactly back along one of the rays of light coming from the sun.  You must turn and look straight east to do this.  One the equinoxes, the sun will rise in the east (not just somewhere in the east but exactly due east).  This only happens on the spring and fall equinox.  The rest of the year the sun will rise south or north of east.

3.     Six hours later you arrive at Point C; it is noon.  Now to see the sun you must tilt your head and look straight overhead.  The sun passes directly overhead at noon at the equator on the equinoxes.

THe picture above shows the earth viewed from outer space.  We will next look at the sun's path in the sky viewed from the ground where most of us will spend our entire lives.

3.     This shows the path of the sun at the equator.  The sun rises in the east at 6 am, passes directly overhead at noon, and sets in the west at 6 pm.

4.     The cloud shown next to Point 4 above refers to a band of clouds that circles the globe at the latitude where the sun passes overhead at noon.  This marks the position of the "intertropical convergence zone (ITCZ)"; we'll learn more about the ITCZ later in the semester.  You can usually make out this band of clouds on a global satellite picture.  At the present time, just a couple of weeks after the fall equinox the band of clouds is near the equator.  It will move south of the equator as we get closer to the winter solstice.  Then it will move back to the equator by next March and move into the northern hemisphere next summer.

5.     A list of few cities that are located on or very close to the equator.

6a.     This sun path diagram shows the path that the sun follows in the sky on the equinoxes in Tucson (or another city located at 32o N latitude).  The sun rises in the east (just like it does elsewhere on the globe) at around 6:30 local time (the precise time depends on your location within a time zone), reaches its highest point in the sky (58o above the southern horizon) just after noon and sets in the west at about 6:30 pm.

Because the sun rises in the east and sets in the west, crossing an east-west oriented street near sunrise or sunrise can be dangerous around the times of the equinoxes.

In this case the car driver would have had the sun shining directly in his/her eyes and might really not have seen the pedestrian crossing the street.

6b.     Sydney Australia is located at  32o S latitude.  In the southern hemisphere the sun rises in the east, travels into the northern sky and then sets in the west.

7.     At Minneapolis the sun rises in the east, doesn't get quite as high in the sky at noon (only 45o above the southern horizon) and sets in the wet.  Even though the sun shines for the same amount of time in Minneapolis as it does in Tucson (12 hours), Minneapolis will receive less energy during the day because of the lower elevation angle.  Remember that when the sun is low in the sky the sun's rays must pass through a longer path of atmosphere.  A larger percentage of the sunlight is absorbed and reflected.  Once this attenuated sunlight reaches the ground it illuminates a larger area on the ground.

8a.     At the north pole the sun really doesn't rise or set.  At 6 am you would find the sun right on the horizon in the east.  At noon it would be positioned in the south.  The sun would be visible at midnight in the north.

8b.     The sun also circles the sky at the horizon at the south pole.  It just travels in the opposite direction than at the north pole.

The winter solstice is just over 2 months from now, we'll cover that next.

You'll find a detailed discussion of all the numbered points above on p. 78b in the photocopied Class Notes.
Rather than go over all of that again we'll review the main points.  The figures below differ from those shown in class.

The North Pole is tilted away from the sun.  There are 0 hours of daylight, 24 hours of night everywhere north of 66.5 N latitude (the Arctic Circle).  There are 24 hours of daylight south 66.5 S latitude (the Antarctic Circle).  The equator is halfway between the poles, the days are 12 hours long at the equator.  Days are less than 12 hours long in the Northern Hemisphere.  Days get shorter and shorter as you move from the equator to higher northern latitude.

On the winter solstice you need to turn to the southeast to see the sun rise.  The sun sets in the southwest.

The sun will pass overhead at noon at 23.5 S latitude, at the Tropic of Capricorn.

Next we will compare the sun's path in the sky in Tucson on the Winter Solstice (left figure below) and the Equinox (right figure).

On the equnoxes the days are 12 hours long and the sun rises to about 60 degrees above the southern horizon at noon.  On the winter solstice the days are shorter, 10 hours long, and the sun only manages to get about 35 degrees above the horizon at noon.  The two main factors (angle of the sun and number of daylight hours) that control the amount of sunlight energy arriving at the ground are working together to reduce the energy arriving at the ground.  This reduction in incoming sunlight energy is what causes winter in Tucson.

The situation on Dec. 21 in Minneapolis is even "worse" (see p. 78a in the photocopied Class Notes).  The days are only 8 hours long and the sun only gets just over 20 degrees above the horizon at noon.

This brings up another important point:

The sun is always pretty high in the sky (at noon) at the equator; not always overhead but always pretty high (66 degrees or more).  That coupled with the fact that days are 12 hours long throughout the year means that there is very little seasonal change in the amount of sunlight energy arriving at the equator.  Seasonal variability increases as you move away from the equator toward higher latitude.

We'll finish with the summer solstice.

You'll find all the numbered points discussed on p. 80.
Here we'll just review the main points.

The North Pole is tilted toward the sun.  There are 24 hours of daylight north of the Arctic Circle.  There are 0 hours of daylight (24 hours of night) south of the Antarctic Circle.  At the equator the days are always 12 hours long.

If you look east from position X in the picture you won't see the sun because you won't be looking back along the ray of light coming from the sun.  You must turn to the north.  The sun rises in the NE on the summer solstice and sets in the NW.

Rays of sunlight strike the globe perpendicularly at 23.5 N latitude, the Tropic of Cancer, at noon on the summer solstice.

Now the situation in Tucson on the summer solstice compared to the equinoxes.

On June 21 the sun rises in the NE and sets in the NW.  The sun is 81.5 degrees above the southern horizon at noon, almost overhead.  The days are 14 hours long.  There is a lot more sunlight energy arriving at the ground in summer than on the spring and fall equinoxes.

On the winter solstice we found that the two factors controlling the amount of sunlight energy arriving at the ground worked together.  As you moved toward higher latitude there was less and less energy reaching the ground because the days became shorter and the sun was lower in the sky.

On the summer solstice the two factors don't work together.  If you move from Tucson to Minneapolis, the days become longer but the sun is lower in the sky.  This is shown schematically in the next figure.

At high latitude the days are long but the sun is low in the sky.  At low latitude (23.5 degrees) the sun is overhead but the days are shorter (just a little over 12 hours long).  At some point in between these two extremes there must be an optimal combination of sun angle and number of daylight hours, a combination that will result in the maximum amount of sunlight energy arriving at the ground.  This "optimum" location is near 30 degrees latitude.  The hottest locations on earth are found near 30 degrees latitude.

Click here to review this material on sun path diagrams and the seasons.