Tuesday Sept. 26, 2006

Quiz #1 was returned in class today.  The average was -48 (77%), the class homepage shows you how to calculate your percentage grade.  Be sure to check you quiz carefully to be sure that it was graded correctly and that the points missed were added up correctly.

The Experiment #1 reports have been graded.  You may revise your report, revisions are due on Tues. Oct. 10.  Please return your original report with your revised report.  You only need to redo sections where you want to earn additional credit.

1S1P Topic #3 has been graded.

A new optional assignment will be handed out on Wednesday.  Expect a new 1S1P Assignment in the next week or so.
Today we will cover material on the causes of the seasons(found in the second part of Chapter 2).  First a little review.




Many people would have missed the 3rd question.  Many people think the moon orbits the earth in about a day.  This is because they see it in about the same position in the sky on successive nights.  We can see what actually happens in the next figure (not shown in class)

On the first night in Fig. A the person looks up and sees the moon.  One day later on night B,  the earth has completed one rotation on its axis and the person is looking up at the same point in space.  The person doesn't see the moon in the same position as the night before; the moon has moved a little bit in its orbit.  In Fig. C, a little more than 24 hours after Fig. A, the person again sees the moon overhead.  If you were to make a note of the time the moon rises you would notice it rises a little later each successive night.


Many people know that the earth's orbit around the sun is not circular and that the distance between the earth and sun changes during the year.  Many people think this is the main cause of the seasons.  The earth is closest to the sun on the perihelion (not perhelion as shown above)

The earth is closer to the sun in January than in July.  If this were the main cause of the seasons, summer in Tucson would be in January and winter would be in July.  Summer and winter would both occur at the same times in both hemispheres.  Neither of these is true.  The changing distance between the earth and the sun has an effect but is not the main cause of seasonal changes.

The main cause of the seasons is the fact that the earth is tilted with respect to its orbit around the sun.  This is shown in the next figure.


This figure shows the tilted earth at four locations in its orbit around the sun.  You should be able to start with a blank sheet of paper and draw a picture like this.  Note how the N. Pole tilts away from the sun on Dec. 21st, the winter solstice.  The N. Pole is tilted toward the sun on June 21.  Those are good places for you to start your sketch.  You should also be able to name and attach a date to each of the four locations.

Before going on, try to imagine what this picture would like if you moved to the other side of the scene and looked back toward the sun.  Click here for a sketch.

Seasons on the earth are caused by the changing orientation of the earth relative to the sun.  The figure above doesn't really explain why this is true.

In the summer when the sun reaches a high elevation angle above the horizon, an incoming beam of sunlight will shine on a small area of ground.  The ground will get hot.  The two people sharing the shaft of summer sunlight will get a sunburn.

In the winter the sun is lower in the sky.  The same beam of sunlight gets spread out over a larger area.  The energy is being used to try heat a larger amount of ground.  The result is the the ground won't get as hot.  4 people are able to share the winter sunlight and won't get burned as quickly.

This difference can be illustrated using three pieces of PVC pipe.  The end of one piece is cut perpendicularly.  The ends of the 2nd and 3rd pieces are cut at 30o and 60o angles.

The end of the 30o pipe covers twice the area of the 90o pipe.  The end of the 60o pipe is a little larger but not a lot larger than area of the end of the 90o pipe.

As sunlight passes through the atmosphere it can be absorbed or reflected.  On average (over the globe) only about 50% of the sunlight arriving at the top of the atmosphere actually makes it to the ground.  A beam of sunlight that travels through the atmosphere at a low angle (right picture above) is less intense than beam that passes through the atmosphere more directly (left picture).

The sun shines for more time in the summer than in the winter.  In Tucson the days are around 14 hours long near the time of the summer solstice.  In the winter the sun only shines for 10 hours on the winter solstice.  Days are 12 hours long on the equinoxes.
Next we will look at what happens on the earth on the equinoxes and the solstices.  The situation on the equinoxes is simplest, we will 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 you don't see 24 hours of day or 24 hours of night at the north pole.  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 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 few days 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, reaches its highest point in the sky (58o above the southern horizon) at about 12:30 and sets in the west at about 6:30 pm.



6b.     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.

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 move south.  The sun would be visible at midnight in the north.

8b.     The sun circles the sky at the horizon but in the oppostie direction at the south pole.


Looking west on Speedway Blvd. (east of Tucson Blvd.) at sunset.  On the equinoxes you will see the sun exactly in the west (a multiple exposure photograph was shown in class).  If you are driving west at this time the sun will be in your eyes and will make it hard to see in front of you.  In the winter the sun sets in the southwest (further west than shown here).  In the summer the sun sets in the NW.



An actual newspaper report on or near the fall equinox. 
Next we took a short break from sun path diagrams and the seasons and went back to something we have already covered.  People will float or sink in a pool of water depending on whether they are slightly less dense or slightly more dense than water.



Everyone floats in the Great Salt Lake because the density of the salty water is higher, around 1.1 g/cm3


If you place a can of regular Pepsi and a can of Diet Pepsi in water, you will find that the can of diet cola floats while the regular cola sinks.

The can of diet cola is mostly water will a little bubble of air inside the top of the can.  The density of this combination of cola, air, and aluminum (in the can) is less than the density of water.

The regular cola is similar except that instead of water there is a mixture of water and corn syrup (sweetener).  The corn syrup makes this can more dense than water.



We'll go through some of the numbered points on the winter solstice diagram on p. 78a in the photocopied class notes.  The numbered points are discussed in detail on p. 78b in the class notes.


1a     There are 0 hours of daylight north of the Arctic circle.

1b     There are 24 hours of day south of the Antarctic circle

1c     Halfway in between you would find 12 hours days at the equator.  Days are always 12 hours long at the equator.

As you move from the equator toward the north pole the days get shorter.  The days get longer as you move from the equator toward the south pole.

2     The sun is now overhead at 23.5o S latitude, the Tropic of Capricorn, at noon

3     On the winter solstice the sun rises in the SE and sets in the SW.


On Dec. 21 at the equator, the sun doesn't pass overhead.  The sun is still high in the sky at noon.  That coupled with the fact that the days are 12 hours long year round you can understand why seasonal changes are minimal at the equator.


5.     In Tucson on Dec. 21, the sun only gets 34.5 degrees above the southern horizon at noon.  The days are only 10 hours long.  Quite a bit less energy arrives at the ground during the winter than at other times of the year.  That's is why it is the winter season.

6.   Even less energy reaches the ground in Minneapolis because the days are 2 hours shorter and the sun is signficantly lower in the sky at noon.

7.   Remember the very small seasonal changes at the equator.  As you move away from the equator toward higher latitude the seasonal changes become larger and larger.  We'll see how small of a seasonal change there can be in class on Thursday.