Discussion #3: The Reason for Seasons
 

"Beautiful is your shining forth at heaven's edge, o living Aten, beginning of life!
When you arise on the eastern horizon, you fill every land with your beauty.
You are lovely, great and glittering, and go high above every land." - Amenhotep IV of Egypt


25 points - Due: Tuesday, 9/6 in class at 10:30 AM

You are encouraged to post this in our Blackboard Discussion Forum, or bring it to class on paper.

Late work receives reduced credit.

First, answer these questions yourself, record your own answers, and read the actual answers.  Look at the Questions to Ponder as well, and record your answers to those. Then, ask 2-3 family members to complete the same "Reasons for the Seasons" questionnaire, and explain the correct answers to them. Record in a spell-checked essay of at least 250 words:
 

(a) Who you gave the survey to,

(b) What level of school they have completed,

(c) Their answers, verbatim (Try to resist telling them whether they are right or wrong - instead just state that you need their opinions for your research. Did any of your respondents mention distance from the Earth to the Sun as the reason we have seasons - that we are closer to the sun in summer, and farther in winter?)

(d) Their reactions when you explained the correct answers.  Did they believe you? 

(e) What you had to do to explain the correct answers.   This is the most important part of the assignment, and worth the most credit, so spend some time here.    Did you have to build a model? An example of a model is shown at the end of the exercise.  Did you have to show them the answers in the table of distances?  Did you find a YouTube video like Ignite! Learning's "What Causes Earth's Seasons" (2005, Nov. 2) to help them "see" the right answer?  (If so, cite it in your essay!)  Did you use the Mastering Astronomy tutorials?  Explain what happened!  

If you have to miss class, and can only post this in Blackboard but not attend when it is due, remember to read and respond to the posts of at least two other students.

Questions to Ponder: You don't have to go through the following with your friends or family, but you might find them very useful in explaining what is going on!

1) If DISTANCE to the sun was the most important factor, when "should" summer occur during the year (based on the table of distances)?  Does it?  So can distance be the most important factor in creating seasons?

2) Note the difference in distance of Earth from the Sun in January versus July.  And remember the Earth as a planet is about 8,000 miles in diameter, tilted on its axis about 23.5 degrees.  Now consider if DISTANCE to the sun was the most important factor, would the seasons for the southern hemisphere occur at a different time of year, compared with the northern hemisphere? Why or why not?

The Official Chabot College Reasons for Seasons Survey!

 

1. The longest day of the year for us (in the northern hemisphere) occurs on:

a) July 4

b) June 21

c) September 22

d) December 21

e) April 14 (before taxes are due!)

2. The hottest month(s) of the year for us in the northern hemisphere are usually:

a) June-July

b) July-August

c) August-September

d) December-January

e) Whenever your car's air conditioning breaks!

3. Planet Earth is closest to the Sun during the month of:

a) June

b) January

c) August

d) July

e) Hah! You can't fool me! We are always the same distance!

4. Seasons in the southern hemisphere are:

a) Much more severe (hotter in summer, colder in winter)

b) Much milder

c) Exactly the same times and duration as ours

d) Opposite of those in the northern hemisphere

5. Summer for us in the northern hemisphere occurs because

a) The Earth is closest to the Sun during this season

b) The Earth is tilted so that the Northern Hemisphere is closer to the Sun than the Southern Hemisphere

c) The Earth is tilted so that the Sun stays up higher in the sky, for a longer time.

d) There are fewer clouds and more sunny days.

 

Answers to the Reasons for Seasons Quiz:

1. (b) The longest day in the northern hemisphere is June 21st, the summer solstice. The word solstice comes from sol (the Sun) and stice (meaning stop). On this day, the Sun "stops", reaching its highest point in the noon sky, and on the succeeding days, is lower in the sky.

2. (b) The hottest months are usually July and August, after the oceans have warmed up. Note that the days are longer in June and July, but the Northern Hemisphere and its oceans are still "chilled" from winter and spring. Also note that local temperature variations may make some areas warmer in August and September ("Indian Summer").

3. (b) The Earth is actually closest to the Sun in JANUARY! Check the chart of distances if you don't believe me!

4. (d) The Southern Hemisphere has summer in December - February, and winter in June - August, exactly opposite to us in the Northern Hemisphere. And when it is spring in the northern hemisphere, it is autumn in the southern. And when it is autumn in the northern hemisphere, it is spring in the southern.

5. (c) Seasons result from the Earth's tilt of 23.5°, which means that each hemisphere will alternately be tilted towards the Sun, exposing the ground to more direct sunlight for longer periods. Just because the planet is tilted with the northern hemisphere tilted toward the Sun does NOT mean that the northern hemisphere is appreciably closer to the Sun. Recall that in July, the entire planet is much farther from the Sun than it is in January. So if distance was the key, the entire planet would still have winter, regardless of which hemisphere was tilted towards or away. The size of the earth is inconsequential compared with the difference in distance (millions of miles) between January and July.

Discussion to Reasons for the Seasons!

Often we create explanations for phenomena based on our prior experiences, only to find out that those explanations are not entirely correct. But these incorrect misconceptions are very hard to dispel, to forget, to replace with the truth. Our prior ideas return again and again; we have to work hard to eliminate them altogether.  Explaining seasons is one area where we most of us have an internal "guess" or "feeling" for an explanation, created from prior schooling, information from parents or friends, and what we have heard or read. For many people who haven't studied astronomy, that personal explanation is often incorrect!

From your reading of the textbook (and in-class discussion) you already know that distance from the sun does NOT account correctly for the seasons. But many folks aren't convinced unless they see "numbers". Examine the following data about the distance from the Earth to the Sun during the year:

Sources:

NASA (2006). Astronomical Unit. Retrieved from http://neo.jpl.nasa.gov/glossary/au.html

Espanak, F. (1999).  NASA Reference Publication 1349. Twelve Year Planetary Ephemeris: 1995-2006. Goddard Space Flight Center. NASA. Retrieved from: http://sunearth.gsfc.nasa.gov/eclipse/TYPE/sun2.html#su2006

Answers to the Questions to Ponder:

1) If DISTANCE to the sun was the most important factor, when "should" summer occur during the year (based on the table of distances)?  Does it?  So can distance be the most important factor in creating seasons?

Summer should occur during the period from December through February, when the Earth is closest to the sun, if distance was the most important factor in creating seasons. Clearly summer does not occur then for us, so distance cannot be the most important factor!

2) Note the difference in distance of Earth from the Sun in January versus July.  And remember the Earth as a planet is about 8,000 miles in diameter, tilted on its axis about 23.5 degrees.  Now consider if DISTANCE to the sun was the most important factor, would the seasons for the southern hemisphere occur at a different time of year, compared with the northern hemisphere? Why or why not?

The difference in distance from the Earth to the Sun between January and July is over 3 million miles!  If distance was the most important factor, whether we were tilted or not wouldn't matter - tilting toward the sun would bring one hemisphere a *bit* closer - perhaps a few hundred miles.  But given the entire planet is millions of miles farther in July, tilting doesn't make that hemisphere closer to the sun than it was in January!  Again, if distance was the most important factor, tilted or not, the entire planet Earth would have summer in January!  It doesn't - and distance cannot be the most important factor in creating seasons.

Building a Model!

For some folks, no amount of numbers or words will convince them. So build a model! (We do this in science all the time.)

1) Grab an apple, pear, or similar-sized object that is handy. Be sure it has an identifiable "top" - like the stem, or put a toothpick into the object so that you can create an "axis of rotation". Pick out a light elsewhere in the room - a lamp with a light bulb is best, in the middle of the room, but anything will do. This light will represent the sun. Since the earth is inclined to the plane of our orbit around the sun (the ecliptic), in the summer months we (in the northern hemisphere) are tilted towards the Sun, and in winter we are tilted away. Suppose it is summer.

2a) First, spin the object in your hand around that stem or toothpick. What does this motion represent?
(Answer: one day!)

2b) Second, tilt the object in your hand so that the top - the stem part - is inclined a bit towards the light bulb. What does this orientation represent?
(Answer: Summer, for those on the "top" half of the object - i.e., observers in the northern hemisphere.)

2c) Third, move the object in a big circle AROUND the light bulb. What does this represent?
(Answer: The Earth's Orbit!)

You'll have to move to do this correctly! You can't do this sitting down. But don't keep the top tilted towards the light - that would mean that the Northern Hemisphere is ALWAYS in summer! Instead, keep the object tilted the same direction (relative to your house or apartment or room) the entire time. Half of the time around the light, the stem-end will be tilted towards the light; the other half of the time, the other end (the "bottom") will be - if you are doing this step correctly.

3. When the northern hemisphere is tilted toward the sun, the north pole will be nearer to the sun, and the southern hemisphere will be farther away. (Based on the angle our planet is tilted, the northern hemisphere on June 21st is about 2,500 km closer to the sun than is the center of the planet.) Could this fact - that the northern hemisphere is closer to sun than the southern hemisphere, account for our summer and their winter? Before you answer, consider how far the northern hemisphere is from the Sun in July vs. January, regardless of whether it is tilted toward the sun.