Tuesday, Oct. 9, 2007

Optional Assignment #3 was collected today and answers to the questions were handed out in class.  The hidden optional assignment was also collected.  Answers will appear online sometime today.

The Experiment #2 reports and the revised Expt. #1 reports were also collected today.  It will take about one week to grade the Expt. #2 reports.  It will probably be more than a week before the Expt. #1 revised reports are returned.

A copy of the Quiz #2 Study Guide was handed out in class.

The Experiment #3 materials and the Scientific Paper should be available in class on Thursday.
Today we will cover a lot of material on the seasons.  The first part, causes of the seasons, will be on this week's quiz.  The second part, a more detailed look at the equinoxes and the summer and winter solstices will not be on this week's quiz.

First some very basic information (that every college graduate should know) found on p. 73 in the photocopied Class Notes.

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, furthest on the apehelion.

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 instead of standing at Point A you moved to the other side of the scene and looked back toward the sun from Point B.   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.

These area differences 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.  These aren't too much different from the highest angle of the sun in the sky in Tucson in the summer, on the equinoxes, and on the winter solstice, respectively.

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 (daylight hours) 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.

The remainder of the material covered today won't be on this week's quiz.  The material will be added to today's online notes sometime after the quiz.