Explain how Earth’s seasons would be if the Earth did not tilt on its axis.

Explain how Earth’s seasons would be if the Earth did not tilt

on its axis.

Because of the tilt of the Earth, the amount of energy Earth receives from the Sun

is dependent on location and time of year. On the equinoxes (March 20 and

September 22 or 23), the Sun is directly overhead (the sub-solar point) and all

areas on Earth receive the same 12 hours of solar energy (sunlight). On the

solstices (June 20 or 21 and December 21 or 22), the subsolar point is on the tropic

of cancer (23.5 degrees North) or the tropic of capricorn (23.5 degrees South),

resulting in the most unequal distribution of solar energy on Earth.

Expand and select the REASONS FOR SEASONS folder. Double-click

Overview and then read the text and watch the animation.

Question 20: What is the relationship between the seasons and the position

of the sub-solar point?

A. The sub-solar point is furthest north during the spring equinox

B. The sub-solar point is furthest north during the autumn equinox

C. The sub-solar point is furthest north in summer (June) solstice

D. The sub-solar point is furthest north in winter (December) solstice

Question 21: Explain how Earth’s seasons would be if the Earth did not tilt

on its axis.

A. Annually, there would be more than four seasons

B. Annually, there would be no more seasons

C. Annually, there would be one dry season and one wet season

D. Annually, there would one “hot” season on Earth

II. Revolution

It takes 365.24 days for the Earth to complete one revolution around the Sun. And

although the Earth’s orbit is elliptical , the variation in distance between the Earth’s

orbit nearest to the Sum (perihelion) or farthest from the Sun (alphelion) is not

great enough to account for the seasons.

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Question 22: Assume the Earth was tilted and rotated, but did not revolve

around the Sun. How would this influence the location of sub-solar point over

a given year?

A. The sub-solar point would not move

B. The sub-solar point would move daily instead of annually

C. The sub-solar point would move between the tropics just like it does

today

D. There would be no sub-solar point

III. Rotation

Earth completes one rotation approximately every 24 hours. This rotation is what

gives us days and nights.

Double-click Circle of Illumination. This figure shows the circle of illumination,

or the day-night line, for June 21.

At 9:00pm EST in New York, South America is in darkness, while North America is

still in day light. If we fast forward 2 hours to 11pm EDT in New York, the circle of

illumination has moved westward. Indeed, the Earth’s rotation helps ensure the

Sun’s energy is spread over the Earth’s surface.

Question 23: Assume the Earth was tilted and revolved, but did not rotate.

What would the seasons be like if the Earth did not rotate?

A. No change to the current seasons/seasonality on Earth

B. There would be one season on Earth

C. There would a constant summer-type season on one side of Earth and a

constant winter-type season on the other side of Earth.

D. Earth would experience a summer-type season (with sunlight) for about 6

months and a winter-type season (with no light) for about 6 months

Click Back to Google Earth, which is located in the top-left corner in the

Google 3D viewer.

We are now going to go through one rotation on Earth.

Zoom out as far as you can until the Earth is as small as Google Earth allows.

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Click Show sunlight acrosss the landscape ( ). A time stamp displays at

the top of the slide bar. (Note: Verify that the Historical Imagery is off

because it can hide the Show sunlight acrosss the landscape slide bar).

Using your mouse, place the cursor on the Earth and move it around until the

Sun is behind the Earth. Then, set North in the default position (press N).

Question 24: How does the circle of illumination look to the portion of the

Earth currently facing you?

A. The portion of the globe facing me is illuminated

B. The portion of the globe facing me is not illuminated (shadowed)

C. The western portion of the globe facing me is illuminated

D. The eastern portion of the globe facing me is illuminated

Move the slide bar slowly over the next 24 hours.

Question 25: What is the direction of Earth’s circle of illumination?

A. Predominately westward (right to left)

B. Predominately eastward (left to right)

C. Predominately northward (bottom to top)

D. Predominately southward (top to bottom)

Turn off Show sunlight acrosss the landscape ( ).

Collapse and uncheck the REASON FOR SEASONS folder.

NDVI

Expand the NDVI folder.

This folder contains a series of images showing Normalized Difference Vegetation

Index (NDVI) for the year 2011. NDVI is a relatively simple way of displaying where

vegetation is most green, which means that the vegetation is alive and producing

greenness from its leaves and other plant parts. In general, the darker the green is

for a given area, the more vegetation cover and/or growth exists for that area.

In this section you will be looking at three locations – Africa, North America, and

Southeast Asia. To start, let’s go to North America in January.

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Double-click North America.

Remember that in the Northern hemisphere, the Sun is lower in the sky in January,

and thereby receives less direct sunlight (solar energy). As a result, the green

appears absent at higher latitudes.

Systematically click through the months (January through December) and note

the green areas in North America. (Note: The images might take some time to

load; as a hint, cycle through the months individually rather than checking all of

them at one time).

Question 26: Which of the followings months is the majority of North

America dark green?

A. January

B. April

C. July

D. October

Question 27: How does this month (you selected in Question 25)

correspond to the sub-solar point of the Sun?

A. The sub-solar point near the equator

B. The sub-solar point near its most northern position

C. The sub-solar point near its most southern position

D. The position of the sub-solar point does not matter

Double-click and select Africa.

Systematically click through the NDVI months (January through December) and

note the green areas in Africa.

Question 28: In which of the following month is the large green

(vegetation) area reach furthest South?

E. January

F. April

G. July

H. October

Question 29: How does the northernmost point correspond to the sub-solar

point of the Sun?

A. The sub-solar point is over the equator

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B. The sub-solar point is at its most northern position

C. The sub-solar point is at its most southern position

D. The position of the sub-solar point does not matter

Double-click and select Borneo.

This is the island of Borneo (center) and the surrounding islands that make up the

Philippines (to the north) and Indonesia (to the south) in Southeast Asia. The island

of Borneo straddles the Equator.

Systematically click through the NDVI months (January through December) and

note the green areas in Borneo.

Question 30: What is the overall trend in NDVI for the year?

A. The NDVI is distinctively lower in March

B. The NDVI is distinctively higher in September

C. The NDVI varies little over the entire year

D. The NDVI is distinctively lower in December

Question 31: With respect to Sun angle, why do we see such an NDVI trend

for the island of Borneo? (Choose the one that is incorrect)

A. There is little variation in Sun angle because Borneo is at the equator

B. Borneo basically receives the same amount of solar radiation year round

C. Borneo receives rainfall throughout the year

D. Few, if any clouds, obscure the Sun from Borneo year round

Collapse and uncheck the NDVI folder.

ANALEMMA

An analemma is a chart that you use to track the Sun’s declination and to

determine the equation of time. The Sun’s declination is the latitude of the Sun’s

solar point for a given date. The Sun’s solar point is the where the Sun is directly

overhead (90°) at mean solar time.

The Earth’s orbit is elliptical and, as a result, revolves around the Sun at varying

speeds depending on the time of year. In June and July, the Earth revolves slower,

compared to December and January. Hence, as the speed of revolution varies, we

need the equation of time to determine the difference between observed solar time

(the time when the Sun is at its highest point in the sky for your location) and

actual time:

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● If the Sun is at its highest point before noon (12:00pm), then the time is

said to be Sun-fast.

● If the Sun is at its highest point after 12:00pm, time is said to be Sun-slow.

An analemma will tell us how fast (or slow) the Sun is relative to noon.

Expand the Analemma folder and then click Introduction to view the

introduction animation.

I. Sun Angle

Expand Sun Angle.

Assume we are in Atlanta, Georgia, USA (33.95°N, 83.32°W). This city is in the

Northern hemisphere. It also implements daylight saving time, so “noon” is

technically at 1pm. Using the example in the animation, we can read the graph to

determine the Sun’s declination on August 1 is 18°N. In other words, the Sun is

directly overhead (Sun’s solar point) at 18°N. However, we are not located at 18°N

but farther north at ~34°N. This means that the Sun is not directly overhead but at

an angle, known also as an altitude angle or solar elevation angle. So what is the

Sun’s altitude angle at its highest point in Atlanta, Georgia (~34°N) on August 1?

To answer this question we can use the following equation:

Altitude Angle = 90° – latitude ± declination

When our location and the Sun’s declination are in the same hemisphere (North or

South), we add the declination value in the equation. When they are in opposite

hemispheres, we subtract the declination value. In our example then, we are in the

same hemisphere, so we add. We know our latitude is 34 degrees and the

declination is 18 degrees, so answer is:

Altitude Angle = 90° – 34° +18° = 74°

Altitude Angle = 74°

So, on August 1 in Atlanta, Georgia, the Sun angle at its highest point would be

74°.

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Suppose we were in Cape Town, South Africa (33.92°S, 18.45°E) on August 1.

Rounding the latitude to 34°S, what is the Sun angle at noon?

Altitude Angle = 90° – 34° – 18°

Altitude Angle = 38°

As we can see, the Sun’s altitude angle on August 1 at noon is much lower in Cape

Town, South Africa than in Atlanta, USA.

Using this equation, answer the following questions.

Double-click and select Location A.

Question 32: What is the latitude (degrees only) for Location A?

A. 0°E

B. 0°S

C. 78°W

D. 78°N

Question 33: What is the Sun’s altitude angle for Location A on September

21?

Altitude Angle = 90° – latitude ± declination =

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