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Seasons; A Demonstration

by
Kenneth Fuller
(copyright information 2004)


Teacher's preparation:

Teacher introduction:
The  causes of seasons on a planet are complex.  There is no single sentence explanation.  year after year, when I ask my eighth grade, "Why is our weather usually warmer in July than in January?"  The first answer is always, "Because the Sun is closer in July."  (I prefer to present this lesson on or near January 4, perihelion, so I can respond, "But, you are 5 million kilometers closer to Sun now than you were in July.")  The second answer is, "It's tilted."

Warning: There a number of misunderstandings of this subject are deeply embedded in the collective memory of our society.  As a result many science book explanations, though "technically" correct, are misleading and incomplete.  Whenever I shorten an explanation to save time or ink, I must assume the student knows what I left out and is able to put it back in.  

Reminder:  A concept is rarely mastered with the first exposure.  If the student cannot explain the concept using different words and different examples, The concept has not been mastered.  (When asked, "Why does Mars have seasons?"  They complain, "You never told us about Mars.")  Repeating the same statement verbatim seldom clarifies the meaning.

Preparing to teach:  Read "Causes of Seasons. "  
                             Read "Modern Myths, Seasons are caused by... "
                             Read through the demonstration, so you will know where you are going.

Materials needed:  A model of Earth, preferably a globe mounted with an inclined axle.
                           A model of Sun, a floodlight (In a large room, I use a 200 watt bulb in an 18 inch bell reflector.)
                           The light should be mounted on a stand which can be rotated as "Earth" revolves around it.


Student Introduction:

"Here we have a model of Earth.  Remember, in science a model can be anything which is like the thing being modeled in at least one significant  way."
"In what ways is this globe of Earth similar to the real Earth?"   [shape - round, patterns on the surface - the print on the globe show the relationships of surface features, "It's tilted"]

"The axle of this globe represents the axis of Earth, the line around which Earth rotates once every 23 hr.. 56 min.  The horizontal plane through the center of the globe represents the plane of Earth's orbit around Sun.  (This is a convention, just as we put north at the "top" of a map unless it is marked otherwise.  This "convention"  is just for our convenience, and has no connection whatever with the real Earth.)  The angle which the axle of our model makes with the horizontal is the same as the angle which Earth's axis makes with the plane of Earth's orbit around Sun."

"Remember, the north end of Earth's axis always points toward the star Polaris (during our lifetimes). [Here I always point in the direction of Polaris.]  Even though we can't see Polaris in daylight, or through the roof, it is there, all the time."

1. Set the globe in  position for the winter solstice, with its center on a horizontal line with the center of the light.

2. Turn on the light.  "This is our model of Sun.  In what way is it like the real Sun?  It produces light, and for this demonstration that is all we need."

 
3.  While rotating the globe from west to east (counterclockwise), "Notice that at all times half of Earth is lighted by Sun, and half is dark.  The dark half we call night, the light half we call day."

4.  Placing finger on "Our City", follow it around with the rotation of the globe.  "As Earth rotates Our City goes from the light side to the dark side to the light side.  From day to night to day and so on."
Winter Solstice JPG
5.  "As the model is set now, Polaris is on the side of Earth away from Sun.  This represents the winter solstice, a few days before Christmas.  As we follow Our City around, do we spend more time on the light half or on the dark half?"   (Dark half.)
"Yes, in fall and winter we spend more time on the dark half than on the light half.  We say the nights are long and the days short.  Since light absorbed by Earth's surface is converted into heat, the fewer hours spent on the light half, the less light absorbed each day, the cooler the weather."

6. Continuing to rotate the globe, place finger on the North Pole.  "At this time of year, how many hours a day would an observer at the North Pole see Sun?"  (None.)
"That's right.  The North Pole spends 24 hours a day on the dark half of Earth."

7.
Continuing to rotate the globe, place finger on Argentina.  "At this time of year does Argentina spend more time on the light half or on the dark half?"  (Light half.)

8.
Continuing to rotate the globe, place finger on the South Pole.  "How  hours a day will an observer at the South Pole see Sun?"  (24 hours a day.)Vernal Equinox JPG
"That's right, at this time of year at the South Pole Sun is above the horizon 24 hours a day."

9. Carefully reiterate; "So, when we in the northern hemisphere have long nights and short days, those in the southern hemisphere have short nights and long days.  When the North Pole is on the dark half 24 hr.. a day, the South Pole is on the light half 24 hr. a day."

10. As one student rotates the light, another (remembering to keep the North Pole always pointing at Polaris) moves the globe around its orbit until the line between light and dark touches both poles.

11. "This is called the vernal equinox.  Notice, on this day the observer at the North Pole will see the one and only sunrise of the year.  The one at the South Pole will see the one and only sunset of the year.  Every place else on Earth will spend 12 hr.. on the light half and 12 hr.. on the dark half.  (equi = equal  nox = night, day and night are equal in length.)

12. Students continue to rotate the light and move the globe in its orbit until they reach the position for summer solstice.

13. "This is called the summer solstice, a couple weeks before the 4th of July.  The north end of Earth's axis is still pointed toward Polaris, but now Polaris is on the same side of Earth as Sun."
Summer Solstice JPG
14.
Placing finger on "Our City", follow it around with the rotation of the globe.  As we follow Our City around, do we spend more time on the light half or on the dark half?"
(Light half.)
"Yes, in spring and summer we spend less time on the dark half than on the light half.  We say the nights are short and the days long.  Since light absorbed by Earth's surface is converted into heat, the more hours spent on the light half, the more light absorbed each day, the warmer the weather."

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6. Continuing to rotate the globe, place finger on the North Pole.  "At this time of year, how many hours a day would an observer at the North Pole see Sun?"  (24)
"That's right.  The North Pole spends 24 hours a day on the light half of Earth."


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7. Continuing to rotate the globe, place finger on Argentina.  "At this time of year does Argentina spend more time on the light half or on the dark half?"  (dark half.)

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8. Continuing to rotate the globe, place finger on the South Pole.  "How  hours a day will an observer at the South Pole see Sun?"  (None.)
"That's right, at this time of year at the South Pole Sun is below the horizon 24 hours a day."Autumnal Equinox JPG

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9. Carefully reiterate; "So, when we in the northern hemisphere have short nights and long days, those in the southern hemisphere have long nights and short days.  When the North Pole is on the light half 24 hr.. a day, the South Pole is on the dark half 24 hr. a day."

20. 
Students continue to rotate the light and move the globe in its orbit until they reach the position for the autumnal equinox.

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1. "This is called the autumnal equinox.  Notice, on this day the observer at the North Pole will see the one and only sunset of the year.  The one at the South Pole will see the one and only sunrise of the year.  Every place else on Earth will spend 12 hr.. on the light half and 12 hr.. on the dark half.

22. 
Students continue to rotate the light and move the globe in its orbit until they reach the position for winter solstice.

Seasons JPG




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