GEO200 Northern Virginia Earth Sun Relationship Paper The questions pertaining to the assignment are pretty straightforward. There’s a body of text illustr

GEO200 Northern Virginia Earth Sun Relationship Paper The questions pertaining to the assignment are pretty straightforward. There’s a body of text illustrating what was covered in class giving an intro on the topic as well as providing a little insight on the subject content. GEO 200 In-Class Activity
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In-class activity 4: Earth – Sun Relationships
Introduction
Solar radiation that enters the Earth-Atmosphere system is the primary source of
energy for nearly every atmospheric process on Earth. The unique relationship between
the Earth and Sun is what causes the seasons, controls the length of days, and organizes the
basis for keeping track of time. An understanding of this relationship is essential when
learning about atmospheric processes on Earth.
Basic Earth-Sun Geometric Relationships
The earth’s orbit around the sun is elliptical, varying the distance
between the earth and sun throughout the year. While the
average distance between the earth and the sun is approximately
150 million kilometers (93 million miles), the actual distance at
any given time fluctuates by as much as 5 million kilometers (3
million miles). The earth is nearest the sun (perihelion) during
the Northern Hemisphere’s winter (January) and is farthest from
the sun (aphelion) during the Northern Hemisphere’s summer
(July).
The sun’s rays are close to parallel to each other as they
stream toward earth, so if the earth’s axis of rotation
was perpendicular to the plane of the ecliptic, the sun’s
most direct rays would always be received at the
Equator. In this case, there would be no seasons.
Seasons occur due to the tilt of Earth’s axis of rotation.
The axis is an imaginary line that connects both poles, and
it is tilted at an angle of 23.5? relative to the plane of the
ecliptic, the plane on which the Earth revolves around the
Sun. Since the axis of rotation is always oriented in the
same direction (pointing toward the North Star), different
latitudes receive direct solar radiation at different times
throughout the year.
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GEO 200 In-Class Activity
Due to its rotation, half of the Earth is always receiving some portion of Sunlight, known as
the circle of illumination. However, the tilt of the Earth’s axis also controls daylength.
During June, the Northern Hemisphere is tilted toward the Sun and experiences longer
daylengths. During December, the Northern Hemisphere is tilted away from the Sun and
experiences shorter daylengths.
The Arctic Circle (66.5?N) and the
Antarctic Circle (66.5?S) outline the polar
regions of our planet. The area within
each circle experiences 24 hours of
daylight on its June Solstice (Summer in
the Northern Hemisphere; Winter in the
Southern Hemisphere); likewise, the
December Solstice (Winter in the
Northern Hemisphere; Summer in the
Southern Hemisphere) brings 24 hours of
darkness. During both Equinoxes (Vernal
in March and Autumnal in September),
daylength is 12 hours at all latitudes
across the globe.
Solar Declination
The seasonal temperature changes are controlled by the amount of direct radiation
received at the surface. As a result of the tilt of the axis and the curvature of the Earth, some
latitudes receive direct radiation while other latitudes receive radiation at an oblique angle.
When radiation strikes an object at an oblique angle, the energy is distributed over a larger
area and is less intense.
The latitude at which the Sun is directly overhead at noon is the solar declination.
The solar declination for the June Solstice is 23.5?N (Tropic of Cancer), and 23.5?S (Tropic
of Capricorn) for the December Solstice. During both Equinoxes, the solar declination is at
the Equator (0?). The solar declination changes every day as the Earth revolves around the
Sun, but is constrained between the Tropics.
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GEO 200 In-Class Activity
1. List the date and the solar declination for each position.
Date
Solar Declination
Summer Solstice
Autumnal Equinox
Winter Solstice
Vernal Equinox
2. Label the diagram below with the appropriate date for each position.
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GEO 200 In-Class Activity
3. In the diagram below, what is the date?
4. Using the diagram above, describe the day or night length from the Arctic Circle to
the North Pole.
5. What percentage of the Earth is illuminated at noon December 21 (or at any
time)?
6. How many hours of daylight does the South Pole receive on March 21?
7. How many hours of daylight does the South Pole receive on June 21?
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GEO 200 In-Class Activity
8. Which latitude(s) experience the GREATEST seasonal change in daylight hours?
(In other words, do any areas on the globe change from completely dark to
completely lit over the year? Where does this happen?)
9. What would happen if the earth’s axis of rotation was NOT tilted at a 23.5° angle?
10. Give the numerical latitude and cardinal direction for the 5 major lines of
latitude.
Arctic Circle
Tropic of Cancer
Equator
Tropic of Capricorn
Antarctic Circle
Solar Angle
In addition to the solar declination, it is useful to understand some related
geometric terms: zenith angle: the angle between a point directly overhead and the Sun at
solar noon, and solar angle: the angle of the Sun above the horizon at solar noon. These
angles are important because they determine the amount of insolation (incoming solar
radiation) potentially received at the surface of the Earth.
To determine the zenith angle at a particular location, calculate the number of
degrees of latitude separating the solar declination and the location in question. If the
declination or latitude is in the southern hemisphere, it will be a negative value. The zenith
angle should always be positive; therefore, you should report the absolute value of the
zenith angle.
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GEO 200 In-Class Activity
Example: zenith angle = (location latitude) – (solar declination)
At Alexandria, VA (39?N) on January 20 (solar declination: 20?S)
Zenith angle = 39? – (-20?)
Zenith angle = 59?
At Sao Paulo, Brazil (23?S) on January 20 (solar declination: 20?S)
Zenith angle = -23? – (-20?)
Zenith angle = -3?
Absolute value zenith angle = 3?
The solar altitude angle is calculated by subtracting the absolute value of the zenith
angle from 90?. As the solar declination progresses, the zenith angle decreases and the
solar altitude increases. At solar noon at the latitude of the solar declination, the zenith
angle is 0? and the solar altitude angle is 90?. The zenith angle and the solar altitude angle
are significant because the Sun’s rays are much more intense where they strike the Earth
directly (zenith angle of 0? and a solar altitude of 90?) (Figure 3.3).
Figure 3.3: Zenith angle (A) and solar altitude angle (B) for 30?N on December 21.
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GEO 200 In-Class Activity
11. First, calculate the zenith angle for Alexandria, VA (39?N), St. Petersburg, Russia
(60?N), and Sydney, Australia (33?S) on the following dates. Show your work, and then
check your work before you proceed with the solar angle table.
Alexandria
St. Petersburg
Sydney
March 21
June 21
September 21
December 21
12. Now, using your answers from the table of zenith angles, calculate the solar angle for
Alexandria, VA (39?N), St. Petersburg, Russia (60?N), and Sydney, Australia (33?S) on the
following dates. Show your work.
Alexandria
St. Petersburg
Sydney
March 21
June 21
September 21
December 21
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GEO 200 In-Class Activity
13. Graph your solar altitude angle results for Alexandria, St. Petersburg, and Sydney on a
line graph. Your x-axis should be time of year, and your y-axis should be solar altitude
angle. A line graph requires that you connect the plotted data with a line, per each location,
so you will have 3 different lines. Make sure that you follow the rules of making graphs and
supply a name for the graph, and correct units and labels for each axis.
Answer the following questions using your graph.
14. Which location likely receives the most insolation during June?
15. Which location is probably the warmest during December?
16. In which month does Sydney likely receive the most insolation?
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