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.
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."
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.)
"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."
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."
16. 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."
17. 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.)
18. 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."
19. 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.
21. "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.
