Part I: The Reasons for the Seasons
Below are three facts about the seasons that we wish to explain. Below the facts are
four hypotheses about what might cause the seasons to occur. Your job is to think
about each one and figure out which of the four hypotheses are most right and which
ones are wrong. Be sure to keep these three facts in mind while going through the lab.
Key facts about the seasons:
I. In northern latitudes, it’s warm in June/July and cool in Dec/Jan, on average.
II. In southern latitudes, the seasons are reversed: it’s warm in Dec/Jan and cool in
June/July.
III. It’s warmer at latitudes close to the equator than at latitudes close to the poles
(on average).
Hypothesis #1: The Sun-Earth distance changing due to
Earth’s elliptical orbit causes the seasons.
If its orbit were a circle, the Earth would always be the same distance from the Sun.
But it’s not. The orbit is an ellipse. As compared to the average Earth/Sun distance, the
Earth is sometimes 1.7% closer and at other times 1.7% farther away from the Sun than
the average.
Is this difference significant? To answer this question, it helps to be able to refer to
a scale model of the Sun/Earth system. Recall the scale model of the solar system we
made in Lab 1. We made the size of the Sun and Earth and the distance between them
all smaller by the same factor: 1010. Let’s repeat these calculations for the Earth-Sun
system.
Sun diameter: 1.4 × 106 km → scale model Sun diameter = 14 cm
To get the above, remember that we divide by the scale factor to shrink the normal
size:
1.4 × 106 km
1010 = 1.4 × 10−4
km
Then we convert to units of cm:
1.4 × 10−4
✟km✟×
103m
1✟km✟ = 1.4 × 10−1✚m✚×
102
cm
1✚m✚
= 1.4 × 101
cm = 14cm
(c) 2020 SFSU. DO NOT DISTRIBUTE 1
Now try to get the values for the Earth diameter and the Earth-Sun distance, you can
also look up the values from Lab 1, but make sure they are right:
Earth diameter: 1.3 × 104 km → scale model Earth diameter = cm
→ scale model Earth diameter = mm
Earth-Sun distance: 1.5 × 108 km → scale model distance = m
A 1.7% change in the Earth-Sun distance is thus:
scale model Earth-Sun distance = m ×0.017 = m =
cm
To make sure we all have the same model, you may remember, the Sun was represented
by a small watermelon, the Earth would be about the size of a candy sprinkle and the
distance between them would be the size of an apartment, small house or a big great room
inside a medium/large house (15 meters).
Now that we have the Earth-Sun model in our heads, imagine standing in one corner
of a big room holding the tiny Earth sprinkle and you see the watermelon Sun in the
other corner of the room. Also pretend that the watermelon is sitting in a bon fire, this
will represent the heat we feel from the Sun.
1. If you moved closer to the fire by the amount above (1.7%), do you think you would
get significantly warmer? Why or why not?
2. The Earth is closest to the Sun in early January and farthest from it in early July.
Can both facts I and II above be explained by the changing Earth-Sun distance? Give
your reasoning.
(c) 2020 SFSU. DO NOT DISTRIBUTE 2
Hypothesis #2: The change in the Sun-Earth distance due to
the tilt of the Earth causes the seasons.
In the summer months, the northern hemisphere of Earth tilts 23.5 degrees toward
the Sun, while in the winter months, it tilts away from the Sun. Another hypothesis we
could make is that the hemisphere that is tilted toward the sun is warmer because it’s
closer to the Sun than the hemisphere that is tilted away from the Sun. (See diagram
from Prather, Slater, Adams, and Brissenden below)
To test this hypothesis, consider the scale model you constructed. Try tilting the
North Pole of your model Earth toward or away from the model Sun on the other side of
the room (without changing the distance from the center of the Earth to the Sun).
1. Is there a significant difference in distance from our model Sun (15 meters away) to the
northern hemisphere of the sprinkle then from our model Sun to the southern hemisphere
of the sprinkle?
2. Is this a plausible explanation for the seasons? Explain.
(c) 2020 SFSU. DO NOT DISTRIBUTE 3
Hypothesis #3: The change in the length of the day due to the
tilt of the Earth’s axis causes the seasons.
Before going into how the length of the day changes due to the tilt, let’s focus on the
tilt itself. The image below shows how over the course of a year (orbit) the tilt stays the
same. Why is that? Recall that the Earth’s spin axis points at the North Celestial Pole,
a point on the Celestial Sphere that is very close to Polaris, also known as the North Star.
It keeps pointing steadily at the same position as the Earth goes around the Sun. (Recall
that precession occurs so slowly that even over your whole lifetime, the effect will be very
small and can be ignored for most purposes.)
Figure 1: source: https://www.timeanddate.com/astronomy/equinox-not-equal.html
Notice how when it is the Summer in the Northern hemisphere and Winter in the
Southern Hemisphere (June), the North Pole is tilted towards the Sun while the South
Pole is tilted away. Also notice that when it is the Winter in the Northern Hemisphere
and Summer in the Southern Hemisphere (December), the North Pole is tilted away from
the Sun and the South Pole is tilted towards the Sun. The two Earth’s in the middle of
the diagram are neither tilted towards or away from the Sun. This is when it is Spring
and Autumn in the Northern and Southern Hemispheres.
1. How does the Earth’s tilt change with respect to the Sun over the course of one
year?
(c) 2020 SFSU. DO NOT DISTRIBUTE 4
Now let’s see what effect the tilt has on the length of days. Watch the youtube video
linked below and answer the following questions. Remember, you can always pause and
rewind the video if you missed something! (Note: If you are in a place that can’t access
YouTube, you can still answer the questions below by using Figure 1 above.)
Video break down:
2. Pay attention to the Earth at 1:09 OR look at the March and September Earths in
Figure 1. With the Earth’s spin axis vertical, does it look like the whole Earth gets the
same amount of daylight no matter your latitude?
3. Now pay attention to the Earth at 1:48 OR look at the June Earth in Figure 1. With
the North Pole pointed towards the Sun, does it look like both the Northern Hemisphere
and the Southern Hemisphere get the same amount of day light? If not, which gets longer
days?
a. Does it look like the North Pole gets both day and night? If not, what does it get?
b. What is happening in Antarctica (South Pole)? Does it get day and night? If not,
what does it get?
(c) 2020 SFSU. DO NOT DISTRIBUTE 5
4. Finally, pay attention to the Earth at 3:16 OR take a look at the December Earth in
Figure 1. With the North Pole pointed away from the Sun, does it look like both the
Northern and Southern Hemispheres get the same amount of day light? If not, which gets
longer days?
a. Has anything changed about the amount of daylight at the North or South Poles?
If so, describe the changes.
5. Which do you think corresponds to summer in the northern hemisphere: the Earth’s
axis pointing toward the Sun or away from the Sun? Explain.
6. Based on your results above, can the change in the length of the day potentially help
to explain the seasons? Discuss.