Lab 3 Earth Sun Relationship Lesson and Notes.docx
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Lab 3: Earth-Sun Relationships
Learning Objectives
●
Describe how Earth’s rotation, revolution, and tilt, influence the Annual March of the
Seasons.
●
Diagram the seven special lines of latitude and explain their relationship to solstices and
equinoxes.
●
Explain the relationship between the angle of incidence and radiation intensity.
●
Identify vertical, oblique, and tangent rays from the Sun.
●
Analyze daylength at different locations and times of the year.
Introduction
Earth’s annual revolution around the Sun influences the angle of incoming solar rays and the
length of day at different latitudes. The amount of incoming solar radiation, or
insolation
, along
with day length influence the Earth’s seasons. In this lab, we will study the relationships
between the Earth and the Sun and how those relationships influence the seasons we
experience every year. This relationship results in what is known as the
Annual Seasons
.
Part A. The Annual Seasons
Earth has five key characteristics that play a role in the Annual March of the Seasons: rotation,
revolution, tilt, polarity, and shape.
1. Rotation
Earth rotates on its axis every 24 hours, which we consider to be one day (Figure 3.1). Each
rotation can be seen in the daily change from day to night. The circle of illumination is the line
separating the part of the planet receiving sunlight and the part of the planet in darkness.
Why does the Earth rotate? It will help to understand how our solar system formed. Almost five
billion years ago, our solar system had its beginnings as a vast cloud of dust and gas. The cloud
began to collapse, flattening into a giant disk that rotated faster and faster, just as an ice skater
spins faster as she brings her arms in. The Sun formed at the center, and the swirling gas and
dust in the rest of the spinning disk clumped together to produce the planets, moons, asteroids,
and comets. The reason so many objects orbit the Sun in nearly the same plane (called the
ecliptic) and in the same direction is that they all formed from this same disk. While the planets
were forming, clumps of matter of all sizes often collided, and either stuck together or
side-swiped each other, knocking off pieces and sending each other spinning.
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1
Text
adapted from NASA is in the public domain
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Lab 3: Earth-Sun Relationships
Figure 3.1: Earth’s Rotation.
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2. Revolution
Earth revolves around the Sun every 365.25 days, which we consider to be one year. This orbit is
not a perfect circle as we might imagine; it is actually an elliptical orbit (Figure 3.2). In one
revolution, Earth travels approximately 940 million kilometers (584 million miles)! Because
Earth is traveling in an
elliptical orbit
, it is closer to the Sun on or around January 3 (known as
perihelion
) than it is on or around July 4 (known as
aphelion
). At perihelion, Earth is 147.5
million kilometers (approximately 91 million miles) from the Sun and at aphelion Earth is 152.6
million kilometers (approximately 95 million miles) from the Sun. Tip: to help you remember
that aphelion occurs when Earth is furthest away from the Sun, think of the “a” in aphelion as
further “away”.
Figure A-Aphelion is when the sun is farthest away from Earth, Perihelion is when the sun is
nearest the Earth.
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Figure by Waverly Ray is licensed under
CC BY-NC-SA 4.0
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Lab 3: Earth-Sun Relationships
Figure B.
Figure 3.2: Earth’s Revolution.
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Figure by Scott Crosier is licensed under
CC BY-NC-SA 4.0
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Lab 3: Earth-Sun Relationships
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3. Tilt
Currently, the axis of the Earth is tilted at 23.5°.
Earth revolves around the Sun on an imaginary
straight line known as the
plane of the ecliptic
(Figure 3.3).
While Earth’s axis is currently pointing toward Polaris, known as the North Star, the top of the
Earth or the North Pole is not always orientated toward the Sun. As you can see in Figure 3.3
sometimes the North Pole is orientated toward the Sun and other times it is orientated away
from the Sun. This phenomenon influences the amount of daylight received by the Earth at
various latitudes, known as
daylength
.
Figure 3.3: Earth’s Tilt and the Plane of the Ecliptic. Note: the diagram is not drawn to scale.
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Figure by Waverly Ray is licensed under
CC BY-NC-SA 4.0
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Lab 3: Earth-Sun Relationships
Figure C
Where did the idea come from to have 4 seasons?
Not everyone experiences 4 seasons!
The four-season year is typical only in the mid-latitudes. Like Europe! The mid-latitudes
are places that are neither near the poles nor near the Equator. The mid latitudes run
from 34 degrees North and South to 60 degrees North and South.
The farther north you
go, the bigger the differences in the seasons. Helsinki, Finland, sees 18.5 hours of
daylight in the middle of June. In mid-December, however, it is light for less than 6
hours.
This means that cities like New York and London have 4 seasons.
Cities and locations below 34 degrees North and South usually only have 1 or 2
seasons. Athens, Greece, in southern Europe, has a smaller variation. It has 14.5 hours
of daylight in June and 9.5 hours in December.
Places near the Equator experience little seasonal variation. They have about the same
amount of daylight and darkness throughout the year. These places remain warm
year-round. Near the Equator, regions typically have alternating rainy and dry seasons.
In fact, in India, they have more than 4 seasons! They have 6!
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Lab 3: Earth-Sun Relationships
Ritu means seasons in Hindi.Hindi is one language in India.
A ritu is a season in the traditional Hindu calendar, used in parts of India. There are six
ritu: vasanta (spring); grishma (summer); varsha (rainy or monsoon); sharat (autumn);
hemant (pre-winter); and shishira (winter).
4. Shape
Earth is an oblate sphere and like all spheres, its surface is curved. This means that the Sun's
rays strike the Earth at different angles for each latitude. As you can see in Figure 3.4, the Sun’s
rays strike the Earth at the center (equator) directly, almost at 90°, while they strike toward the
poles at a lower angle, more like 10° or 20°. Because the Earth is curved, the
angle of incidence
(the angle of the Sun’s rays)
varies by latitude. And, because the Earth is titled, the angle of
incidence also varies by season.
A high angle of incidence means very direct sun! Low angle of
incidence means less direct sun.
Figure D-This figure diagrams Solar radiation in the Northern Hemisphere in June and
December.
Notice that the sun's rays in December in the Northern Hemisphere are not as direct.
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Lab 3: Earth-Sun Relationships
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Figure 3.4: Angle of Incidence.
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The Sun’s vertical rays strike the Earth at the equator (0°) on the March equinox (March 20) and
the September equinox (September 21). The
March equinox
is known as spring in the northern
hemisphere and fall in the southern hemisphere, while the
September equinox
is known as fall
in the northern hemisphere and spring in the southern hemisphere. During the equinoxes, the
circle of illumination
creates an equal amount of daylight of exactly 12 hours for all latitudes.
The circle of illumination can be seen in Figure 3.5.
Equinox means equal! During the March
and Fall Equinox, all places on Earth receive an equal amount of sun!
Figure 3.5: The Plane of the Ecliptic and the Circle of Illumination.
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6
Figure by Scott Crosier and Taya Lazootin is licensed under
CC BY-NC-SA 4.0
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Figure by Jeremy Patrich is licensed under
CC BY-NC-SA 4.0
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Lab 3: Earth-Sun Relationships
Figure E-June and December Solstice
Figure F
Figure F shows the June Solstice-This is the longest day of the year for the Northern Hemisphere
and the shortest day of the year for the Southern Hemisphere.
The sun rays are directly overhead at the Tropic of Cancer.
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Lab 3: Earth-Sun Relationships
Figure G
Figure G shows the December Solstice. The suns rays are directly overhead at the Tropic of
Capricorn.
June Solstice- Figure F-The Sun’s vertical rays strike the Earth at the Tropic of Cancer (23.5°N),
located in the northern hemisphere, on the June solstice (June 22).
December Solstice-Figure G-The Sun’s vertical rays strike the Earth at the Tropic of Capricorn
(23.5°S), located in the southern hemisphere, on the December solstice (December 21).
The June solstice is known as summer in the northern hemisphere and winter in the southern
hemisphere, while the December solstice is known as winter in the northern hemisphere and
summer in the southern hemisphere. In addition, both hemispheres experience longer
daylength in the summer and shorter daylength in the winter. During the December Solstice the
Arctic Circle receives 24 hours of darkness, while the Antarctic Circle receives 24 hours of
daylight.
As you have learned, Earth’s surface is curved. Therefore, the Sun’s rays strike Earth at different
angles depending on latitude. Rays that strike Earth directly at a
90° angle are known as
vertical
rays (VR)
, rays that strike Earth at an
angle less than 90° are known as
oblique rays (OR)
, and
rays that strike Earth at
exactly 0° are known as
tangent rays (TR)
. Note that there is only one
location that experiences vertical rays on a given day, while there are multiple latitudes that
experience oblique and tangent rays on a given day.
1.
On Figure 3.8, label each of the remaining seven rays correctly with terms listed below,
you can see one has been done for you.
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Lab 3: Earth-Sun Relationships
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a.
vertical ray label as VR
b.
oblique rays label as OR
c.
tangent rays label as TR
Figure 3.8: Vertical, Oblique, and Tangent Rays of the Sun.
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2.
Refer to Figure 3.8. Study the position of the Earth in relation to the Sun in order to
answer the following questions:
As you have just learned, vertical rays are those that strike Earth at 90°. These are the most
direct rays. Therefore, the latitudes on Earth that receive vertical rays will receive the most
intense insolation compared to other latitudes. The latitude of the Sun's vertical rays is called
the
declination of the Sun
.
The declination of the Sun (where the suns rays are most direct) changes throughout the year.
Let us determine the declination of the Sun for random days of the year so you can see how it
changes.
The suns rays can only ever be directly overhead between the Tropics! I guess that’s why we
love going on vacation to these places!
This means that the sun can only ever be directly overhead between 23.5 degrees North (Tropic
of Cancer) and 23.5 Degrees South (Tropic of Capricorn).
a.
Which solstice or equinox occurs closest to your birthdate?
Part B. Angle of Incidence
Angle of incidence is the angle at which Sun’s rays strike Earth’s surface. One way to understand
angle of incidence is to think of someone shining a flashlight in your direction. If the flashlight is
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Figure by Scott Crosier is licensed under
CC BY-NC-SA 4.0
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Lab 3: Earth-Sun Relationships
shining directly in your eyes, the light is intense and you look away. If the flashlight is shining on
the ground below you, the light is less intense and you can see clearly. Figure 3.10 shows two
angles of incidence represented by flashlights shining on two books.
Figure 3.10: Flashlight at Two Different Angles of Incidence.
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3.
Refer to Figure 3.10.
a.
Does the flashlight on the left or right represent a 90° angle of incidence? (Hint: a
90° angle of incidence is also known as a direct ray or a perpendicular ray.)
b.
Does the higher or lower angle of incidence result in the flashlight’s energy being
distributed over a wider area?
There is a direct relationship between the angle of incidence and the intensity of solar radiation
that actually reaches the ground. Higher angles of incidence result in higher percentages of
radiation reaching the surface of the Earth. When the angle of incidence is at a 90° angle to the
Earth (called direct or perpendicular rays), approximately 75% of the radiation emitted from the
Sun reaches the surface of the Earth.
Throughout the year, the angle of the Sun’s rays are low (less than 25°) at the poles, compared
to those closer to the equator. As a result, insolation at the poles is spread out over a much
wider area as represented by the yellow color on Figure 3.4 (shown previously in this lab). The
more spread out the yellow color, the more diffused the radiation will be. The opposite is true
at the equator. Throughout the year, the angle of the Sun’s rays at the equator is high (more
than 65°) and at the equinoxes sunlight strikes the equator directly, which results in a 90° angle
of incidence. Insolation at the equator is not as spread out as it is at the poles; therefore, the
radiation is more intense.
Table 3.1 provides angle of incidence data for the special lines of latitude at the solstices and
equinoxes. The data represent the angle of incidence at solar noon.
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Figure by Jeremy Patrich is licensed under
CC BY-NC-SA 4.0
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Lab 3: Earth-Sun Relationships
Table 3.1: Angles of Incidence for the Special Lines of Latitude
at the Solstice and Equinoxes
Latitude
March
Equinox
June
Solstice
September
Equinox
December
Solstice
90°N
0°
23.5°
0°
0°
66.5°N
23.5°
47°
23.5°
0°
23.5°N
66.5°
90°
66.5°
43°
0°
90°
66.5°
90°
66.5°
23.5°S
66.5°
43°
66.5°
90°
66.5°S
23.5°
0°
23.5°
47°
90°S
0°
0°
0°
23.5°
The amount of sunlight that strikes the Earth's surface in an hour and a half is enough to handle
the entire world's energy consumption for a full year. Solar technologies convert sunlight into
electrical energy either through photovoltaic (PV) panels or through mirrors that concentrate
solar radiation. This energy can be used to generate electricity or be stored in batteries or
thermal storage.
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Solar panel installations require an understanding of the angle of incidence.
The Solar Electricity Handbook (2019, n.p.) says “to get the best out of your photovoltaic panels,
you need to angle them towards the Sun. The optimum angle varies throughout the year,
depending on the seasons and your location”. If you lived at the Equator, you would want to lay
your solar panel flat on the ground (at a 0° angle) during the equinoxes so that the maximum
radiation is received.
Part C. Daylength Analysis
Table 3.3 shows the daylength for various locations on Earth on selected days of the year. The
dates February 16th, May 16th, August 16th, and November 16th were chosen at random and
do not align with the solstices and equinoxes to give you an idea of daylength during other
times of the year. You found the declination of the Sun for these dates earlier in this lab.
4.
Apply What You Learned: Add the daylength information for the North Pole and South
Pole into Table 3.3.
Table 3.3: Daylength in Hours for 2020 for Selected Locations and Dates
Location
(approximate latitude)
February
16th
May
16th
August
16th
November
16th
North Pole (90°N)
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Text
by U.S. Department of Energy is in the public domain
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Lab 3: Earth-Sun Relationships
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White Horse, Canada (60°N)
8.78
16.81
15.36
7.09
Tokyo, Japan (35°N)
10.73
13.84
13.32
10.13
Addis Ababa, Ethiopia (10°N)
11.68
12.46
12.33
11.53
Quito, Ecuador (0°)
12.00
12.00
12.00
12.00
Palmas, Brazil (10°S)
12.32
11.54
11.67
12.47
Canberra, Australia (35°S)
13.27
10.16
10.68
13.87
South Pole (90°S)
5.
Refer to Table 3.3.
a.
Use Your Critical Thinking Skills: Why does Table 3.3 have a specific location for
60°N but no location for 60°S? Hint: look at a globe, an atlas, or Google Earth!
b.
Other than the poles, which of the eight locations has the greatest variation in
daylength throughout the year? Explain why in one to two sentences.
c.
Other than the equator, which one of the eight locations has the least variation
in daylength throughout the year? Explain why in one to two sentences.
d.
Canberra and Tokyo are both located at a general latitude of 35°, however, their
daylengths are different from each other on each of the four dates. Explain why
in one to two sentences.
e.
Does the equator have seasons? Why or why not?
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Lab 3: Earth-Sun Relationships