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John Aubert ( American River College Reasons for the Seasons
Physical Geography Lecture Notes Page 1 of 4
aubertj@arc.losrios.edu
Reasons for the Seasons
I. The Earth in Space
A. Milky Way Galaxy (see text)
B. Solar System: There are eight planets orbiting around the sun, forming our solar system.
C. Earth - Basic Facts:
(Earth is the third planet from the sun
(It averages 150 million km (or 93 million miles).
(Light from the sun actually takes about 8 minutes to reach Earth
( The shape is a geoid , literally meaning that it is “Earth shaped”. This means that it is a generally round sphere which is slightly bulging at the equator and flattened at the poles. This is caused by the outward force of Earth's rotation. Additionally, it has very large and very subtle “dents” and “bumps”.
II. Latitude and Sun Angles (Why are the Poles so cold? Why is the Equator so warm?):
Sun angle refers to the angle of the sun in the sky when measured above the horizon. If the sun were directly overhead the sun angle would measure 90 degrees.
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Sun Angles |
Strength of Beam: |
Found at: |
TWO IMPORTANT REASONS! |
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Reason #1 Beam Concentration |
Reason #2 Atmospheric Interference |
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HIGH (high in the sky) |
Strong |
Low latitudes |
Very concentrated! |
Low |
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LOW (down low close to the horizon) |
Weak |
High latitudes |
Spread out, diffused |
High |
III. Why do we have Seasons??
A. The Earth Revolves: The earth takes an elliptical path around the sun, which it completes once every 3651/4 days. We account for this quarter day by adding an extra day once every four years. We add Feb. 29 every four years (leap year). Note: Look up perihelion and aphelion.
1. Perihelion (see text)
2. Aphelion (see text)
B. The Earth Is Tilted (Inclined): Earth's axis is tilted 231/2 ° from a right angle with the plane of the ecliptic; the axis is always directed to a fixed point in space (Polaris, also called the North Star).
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The Tilted Earth
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C. Combine Revolution and Tilt to Understand Seasons
(The vertical ray is not always at the Equator.
(As the Earth revolves around the sun, the latitude of the vertical ray migrates.
(It migrates north in our summer, and then south in our winter.
(In other words, the Northern Hemisphere tilts towards the sun in June and away in Dec.
(It moves as far north as the Tropic of Cancer (June Solstice)
(It moves as far south as the Tropic of Capricorn (December Solstice)
(It crosses the Equator twice during the year on the March and September Equinoxes!
Summary of Seasonal Changes in Earth / Sun Relationships
Event |
Approx. Date |
Declination(latitude of vertical or direct ray) |
Northern Hemisphere |
Southern Hemisphere |
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Season Starting |
Angles of Sunlight
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Hours of Sunlight
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Season Starting |
Angles of Sunlight
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Hours of Sunlight
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December Solstice |
Dec. 21 |
23.5( South “Tropic of Capricorn” |
Winter |
Lowest |
Lowest |
Summer |
Highest |
Highest |
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March Equinox |
March 21 |
Equator (0(Lat) |
Spring |
Increas- ing |
Exactly 12 |
Fall |
Decreas- ing |
Exactly 12 |
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June Solstice |
June 21 |
23.5( North “Tropic of Cancer” |
Summer |
Highest |
Highest |
Winter |
Lowest |
Lowest |
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September Equinox |
Sept. 21 |
Equator (0(Lat) |
Fall |
Decreas- ing |
Exactly 12 |
Spring |
Increas- ing |
Exactly 12 |
Problem: It is valuable to know what the maximum angle of the sun will be on a given day at any location. Higher maximum sun angles are higher energy; conversely, lower maximum sun angles are lower energy. This angle can be calculated for any location by first calculating the total difference (in degrees) between the latitude of your chosen location and the declination for that day. Then, subtract this number from 90(. Calculate the maximum angle and horizon of the sun in Sacramento on the following dates:
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Sacramento's Latitude |
Declination (see chart on previous page) |
Difference* |
Maximum Sun Angle** |
Horizon*** |
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December Solstice? |
39( North |
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March Equinox? |
39( North |
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June Solstice? |
39( North |
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September Equinox? |
39( North |
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*This is the difference, in degrees, between Sacramento’s latitude and the latitude of the vertical or direct ray (called the declination). It is not a latitude, so you should not indicate North or South!!! Also, you will need to be careful when the declination is in the Southern Hemisphere. Under this circumstance, you may NOT subtract to find the difference. THINK!!!
**Subtract the difference from 90( (ex. if the difference is 40(, then your equation should read: 90( - 40( = 50 () Again, the answer here is in degrees only; it is not a latitude!!!!!! Do not indicate North or South!!!
***Does the sun pass through the Northern or Southern sky on this day? To determine this, determine whether the declination for that day is north or south of your latitude… then you have the answer…
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Summary of Global Cause and Effect
Sample Questions: Questions similar to these will be on your exam. As you study you should anticipate how I might use these questions to create new questions on the same concepts.
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1 |
Due to the rotation of the Earth, the “Direct Ray” or “Vertical Ray” of the sun seems to migrate (move) across the globe during the day . Which direction does it move? A. East to West B. West to East C. North to South D. South to North |
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Where might the direct ray of the sun be on about December 15th of any year? A. 21(North latitude and moving north B. 21(North latitude and moving south C. 21(South latitude and moving north D. 21(South latitude and moving south |
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Where might the direct ray of the sun be on about June 28th of any year? A. 21(North latitude and moving north B. 21(North latitude and moving south C. 21(South latitude and moving north D. 21(South latitude and moving south |
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4 |
Assume the declination (latitude of the vertical ray) is 23.5(South Latitude. Approximately what day is it? A. June 21 B. Sept 21 C. Dec 21 D. Feb 29 E. Mar 21 |
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On approximately what two dates during the year is the declination 0( Latitude (the Equator)? A. June 21 and July 21 B. March 21 and May 21 C. Dec 21 and July 21 D. Sept 21 and Mar 21 E. |
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What is the approximate noontime (maximum) sun angle in Sacramento on the December Solstice? (Hint: Sac's latitude is approx. 39.5(N) A. 74( B. 27( C. 50( D. 90( E. 8( |
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What is the noontime (maximum) sun angle at the Tropic of Capricorn (23.5(South Latitude) on the December Solstice? A. 23.5( B. 55( C. 90( D. 1( E. 45( |
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HYPOTHETICALLY, if the declination is at 5( North, but then a few days later it is at 3( North, what event will occur soon? A. June Solstice B. December Solstice C. March Equinox D. September Equinox E. Mardi Gras |
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HYPOTHETICALLY, if the declination is at 20( North, but then a few days later it is at 22( North, what event will occur soon? A. June Solstice B. December Solstice C. March Equinox D. September Equinox E. Mardi Gras |
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Which set of conditions will provide the highest solar energy to a given location? A. High sun angles and high hours of sunshine per day B. Low sun angles and low hours of sunshine per day C. High sun angles and low hours of sunshine per day D. Low sun angles and high hours of sunshine per day |
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Why does a low sun angle provide less solar energy than a high sun angle? A. Low sun angles are concentrated and minimally interfered with by the atmosphere B. Low sun angles are concentrated and highly interfered with by the atmosphere C. Low sun angles are diffused and minimally interfered with by the atmosphere D. Low sun angles are diffused and highly interfered with by the atmosphere |
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On the December Solstice Sacramento is receiving <__________> sun angles and <_________> hours of sunshine per day than at any other time during the year. (Which words complete the sentence accurately?) A. <higher> …… <higher> B. <higher> …… <lower> C. <lower> …… <higher> D. <lower> …… <lower> |
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True or False: Summer occurs in the Northern Hemisphere because Earth is passing closer to the Sun than at any other time during the year. |
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Possible Essay Question: What are the differences between Earth’s rotation and revolution? Include a simple diagram. |
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Possible Essay Question: Briefly explain perihelion and aphelion, including the dates and including a simple diagram. |
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Possible Essay Question: What is a leap day, and why do we have it? |
GOOD NEWS!!! The practice questions above - plus many, many more – can be found in the online practice quizzes discussed in your syllabus and in class. You can take each quiz multiple times, and each time you will get some new questions. Once submitted, the quizzes are graded automatically, with the correct answers provided immediately. This is a great way to prepare for the exams!!!
SERIOUSLY, TAKE THE ONLINE PRACTICE QUIZZES!!!
Low Angle – High Interference by Atmosphere – Diffused beam
Equator
N
S
(Weakest Ray)
(Weakest Ray)
(Strongest Ray – also called Vertical Ray)
High Angle – Low Interference by Atmosphere – Concentrated beam
Low Angle – High Interference by Atmosphere – Diffused beam
Plane of the
Ecliptic
23.5(
North Pole
South Pole
Earth / Sun Relationships
Sun angles are high at low latitudes and low at high latitudes.
Net Radiation Absorbtion
Low latitudes experience radiation surpluses, while high latitudes experience radiation deficits.
Temperature
Low latitudes become warm, while high latitudes become cold.
Atmospheric Pressure
Low latitudes develop low pressure, while high latitudes develop high pressure.
Atmospheric Circulation (Wind)
Air moves from high to low pressure, generating large scale, semi-permanent circulation patterns.
Oceanic
Circulation
Wind pushes (drags) water, causing large scale, semi-permanent ocean currents.
TIP: Review online notes before every lecture! TIP: Review your notes as soon as possible after every lecture!