Lesson 211: EARTH'S SEASONS Students learn the complex geometry and planetary motions that cause Earth to have four distinct seasons. Fundamental Questions Attempting to give thorough and reasonable answers to the following questions will help you gauge your level of understanding this lesson. Students that can confidently answer these questions have mastered the concepts of this lesson. 1. When is Earth happier, at aphelion or at perihelion? 2. To what extent does gravity affect our seasons? 3. For each season, describe how Earth would appear to an observer living on the sun. 4. How are constellations and seasons related? 5. How would changing the tilt of Earth s axis to 0 degrees impact our seasons? 6. How would changing the tilt of Earth s axis to 90 degrees impact our seasons? 7. How would seasons work if the solar system were geocentric? 8. How would seasons be different if the Earth was shaped like cube? 9. How would seasons be different if the Earth was shaped like cylinder? 10. How would seasons be different if the Earth did not rotate? 11. How are the seasons different from the weather? 12. How are the seasons different from climate? Lesson Objectives At the end of this lesson, students should have mastered the objectives listed below. 1. Students understand and can describe how Earth s orientation and motions cause day/night and seasons. 2. Students understand how the constellations, especially the Zodiac, relate to the seasons. 3. Students appreciate the complexity of seasons and the subtleties of earth's motions. 4. Students can describe the causes of seasons on Earth and other planets. 5. Students can draw, label, and describe a diagram that shows the important geographical features of the Earth. 6. Students can define all important lines of latitude on a globe of the Earth. 7. Students can name the angle that Earth's axis is tilted and can describe how this angle causes unequal days and nights. 8. Students can draw, label, and describe the important features of Earth's orbit around the Sun. 9. Students can describe how Foucault's Pendulum "proves" that the Earth rotates. 10. Students understand and can explain why all celestial objects rise in the East and set in the West. 11. Students recognize that Earth's revolution and rotation are counter-clockwise when viewed from above the North Pole. 12. Students can explain how insolation and our atmosphere cause parts of Earth to receive varying degrees of solar radiation. 13. Students can use basic geometry to identify where sunlight is most intense based on the latitudes of locations. 14. Students can state names, characteristics, and dates of the first days of each season for any location on Earth. 15. Students can explain why summer is colder than winter and why the equinoxes are neutral. 16. Students can explain why the poles and the equator experience seasons differently than all other locations. 17. Students can state the names, periods, and descriptions of the Milankovitch Cycles. 18. Students can model seasons on hypothetical Earth's that have various degrees of tilt. Important Terms The following terms are some of the vocabulary that students should be familiar with in order to fully master this lesson. 1. Analemma 2. Antarctic Circle 3. Aphelion 4. Apparent Solar Time 5. Apsidal precession 6. Arctic Circle 7. Astronomical Unit 8. Autumnal Equinox 9. Axial precession 10. Celestial Sphere 11. Circle of Illumination 12. Climate 13. Constellations 14. Day vs. Night 15. Eccentricity 16. Ecliptic 17. Ellipse 18. Elliptical orbits 19. Equator 20. Foci 21. Focus 22. Foucault s Pendulum 23. Geocentric 24. Gravity 25. Heliocentric 26. Inclination 27. Insolation 28. Latitude 29. Leap year 30. Longitude 31. Major Axis 32. Mean Solar Time 33. Milankovitch cycles 34. North Pole 35. Obliquity 36. Orbital Period 37. Orbital Plane 38. Perihelion 39. Plane of the ecliptic 40. Precession 41. Revolution 42. Rotation 43. Rotational Axis 44. Seasons 45. South Pole 46. Summer Solstice 47. Tropic of Cancer 48. Tropic of Capricorn 49. Vernal Equinox 50. Weather 51. Winter Solstice 52. Zenith 53. Zodiac Printed on 3/27/2014 1
Assessment Questions The following are examples of questions that students should be able to answer. These or similar questions are likely to appear on the exam. 1. What are the three main causes of seasons? 2. Describe the significance of the Tropic of Cancer, the Tropic of Capricorn, the Equator, the Arctic Circle, and the Antarctic Circle and state their latitudes. 3. What is the tilt of Earth's axis? 4. How is the angle of Earth's tilt measured? 5. Draw a model of a Foucault's Pendulum and describe how it works. 6. What is the difference between aphelion and perihelion? 7. What direction does the rotate and revolve if viewed from above the South Pole? 8. Draw an ellipse and label all of its features. 9. How are revolution and rotation different? 10. How would changing the tilt of Earth s axis impact our seasons? 11. For each season, describe how Earth would appear to an observer living on the sun. 12. Name and describe 5 ways that the Earth moves. 13. * Explain why the position of the Sun at 8:27 AM will vary throughout the year. 14. Draw an illustration of what the Earth looks like on the Vernal Equinox and Autumnal Equinox if you are observing it from the vantage point of the sun. What is the main difference between them? 15. How would seasons be different if the tilt of the axis increased by another 26? 16. How would seasons be different if the tilt of the axis increased to 90? 17. How would seasons be different if the the Earth s axis was not tilted? 18. What are Milankovitch Cycles and why are they important to seasons? 19. Compare and contrast weather, climate, and seasons. 20. Propose a mechanism that could cause seasons on Earth if Earth were flat and the solar system was geocentric. 21. Why doesn't the Earth's aphelion coincide with the Summer Solstice? 22. At what latitude is the Tropic of Cancer? 23. Describe what the Earth and Sun relationship is like on about December 21 each year. 24. At what point in Earth's orbit is Earth moving fastest? 25. What is the average speed that Earth moves around the Sun? 26. What is the average distance between Earth and the Sun? 27. How fast does Earth rotate at the equator? 28. How many atmosphere thicknesses must sunlight travel through to reach the surface of the South Pole on the Vernal Equinox? 29. About how many atmosphere thicknesses must sunlight travel through to reach the surface of the North Pole on the Summer Solstice? 30. About how many atmosphere thicknesses must sunlight travel through to reach the surface of Stoneham on the Summer Solstice? Related Web Sites The following are some web sites that are related to this lesson. You are encouraged to check out these sites to obtain additional information. 1. http://www.physicalgeography.net/fundamentals/6h.html 2. http://weather.about.com/od/climatechange/tp/earths-seasons.htm 3. http://people.cas.sc.edu/carbone/modules/mods4car/earthsun/index.html 4. http://www.crh.noaa.gov/lmk/?n=seasons 5. http://www.eoearth.org/view/article/151843/ 6. http://en.wikipedia.org/wiki/season 7. http://aa.usno.navy.mil/faq/docs/seasons_orbit.php 8. http://csep10.phys.utk.edu/astr161/lect/time/seasons.html 9. http://spaceplace.nasa.gov/seasons/en/ 10. http://en.wikipedia.org/wiki/day_length 11. http://www.livescience.com/38083-earth-core-day-length-pattern.html 12. https://pantherfile.uwm.edu/kahl/www/images/weather/other/analemma.html 13. http://scienceblogs.com/startswithabang/2009/08/26/why-our-analemma-looks-like-a/ Related Book Pages The following are the pages from your book that correspond to this lesson. Massachusetts Standards Comprehensive E.S. Book Intensive/Honors E.S. Book Meteorology/GIS Book pp. 622-625 pp. 370-373,759,783 NA The following are the Massachusetts Framework Standards that correspond to this lesson. Earth Science Learning Standard(s) 1.5, 4.2, 4.3 2
CCXI. Seasons of Earth A. Earth-Sun Relationships 1. Earth intercepts less than two-billionths of the Sun s energy a. This energy accounts for over 99.9% of the energy that heats Earth s surface b. Solar energy is not distributed equally and unequal heating drives ocean currents and winds. If the sun s energy were to stop, winds and ocean currents would stop also 2. Earth s Motions a. Rotation spinning of earth about its axis produces daily cycle of daylight and darkness i. one complete rotation actually takes 23 hours, 56 minutes and 4.1 seconds ii. the rotational speed is about 1000 miles per hour at the equator iii. rotational axis the imaginary line that passes through the poles of Earth and the planet rotates around this axis iv. Earth rotates in a counter-clockwise direction when viewed from the northern hemisphere. Earth rotates in a clockwise direction when viewed from the southern hemisphere. b. Revolution Earth s elliptical movement around the Sun at an average speed of about 100,000 kilometers per hour (60,000 mph) i. one complete revolution takes 365.2564 days which requires Leap Years every four years ii. Average Distance to Sun 150,000,000 km or 93,000,000 miles or 1 astronomical unit (AU) iii. Aphelion position in Earth s orbit when Earth is furthest from the sun; occurs on about July 4 th ; distance is 152,000,000 km iv. Perihelion position in Earth s orbit when Earth is closest to the sun; occurs on about January 3 rd ; distance is 147,000,000 km; Earth receives slightly more solar radiation at this time v. Earth revolves around the sun in a counter-clockwise direction as viewed from the northern hemisphere c. Axial Precession slow migration of Earth s axis that traces out a cone over a period of 26,000 years i. may influence global climate change ii. North Pole will point to star Vega in about 12,000 years B. Characteristics of the Seasons 1. Cause of Seasons a. Seasons are essentially caused by the gradual but continuous change in the duration of daylight and the altitude of the noon sun (i.e. the angle between the sun and the horizon) 2. Altitude of the Sun a. Because of Earth s spherical shape, solar energy is most concentrated at the surface of Earth when the sun s altitude is 90 b. The lower the altitude angle, the more spread out and less intense is the solar radiation that reaches Earth s surface because the energy is distributed over a larger area.
c. When the angle of the sun is low, the sunlight must pass through more atmospheric gases which causes the energy to be absorbed, reflected, and/or scattered more before reaching the surface. An angle of just 30 causes the light to pass through what is equivalent to 2 Earth atmospheres. An angle of 5 is equivalent to 11 atmospheres. d. Insolation the measurement of incoming solar radiation that is caused by the angle that sunlight strikes the surface of Earth Solar Altitude Equivalent Numbers of Atmospheres Sunlight Must Pass Through 90 1.00 80 1.02 70 1.06 60 1.15 50 1.31 40 1.56 30 2.00 20 2.92 10 5.70 5 10.80 0 (Horizon) 45.00 3. Earth s Orientation a. Earth s axis is not perpendicular to the plane of its orbit. It is tilted 23.5 from the perpendicular. This tilt is called the inclination of the axis or obliquity. The plane of the Earth s orbit is called the plane of the ecliptic or the orbital plane. b. The north pole of the axis always points toward Polaris, the North Star, so the orientation of Earth s axis relative to the sun s rays is always changing as Earth revolves around the sun. c. If the axis was not inclined, Earth would have no seasons. 4. Solstices and Equinoxes a. Vernal (Spring) equinox i. Usually occurs on March 20 or 21, although an equinox is really a time rather than a day ii. Marks the first day of spring in northern hemisphere iii. neither the north pole nor the south pole are tilted toward the sun; the axis is perpendicular to the ecliptic iv. day and night are of equal length all over the world; the Equator always has days and nights of equal length b. Summer Solstice i. Usually occurs on June 21 or 22, although the solstice is really a time rather than a day ii. Marks the first day of summer for the northern hemisphere iii. The axis of the northern hemisphere is tilted 23.5 in toward the sun iv. Vertical rays of the sun hit directly at latitude of 23.5 N (Tropic of Cancer) i. Longest day of the year for northern hemisphere; shortest day of the year for southern hemisphere; the Equator always has days and nights of equal length v. South Pole and all latitudes below Antarctic Circle (66.5 S) receive no sunlight during this day. c. Autumnal (Fall) equinox ii. Usually occurs on September 22 or 23, although an equinox is really a time rather than a day iii. Marks the first day of Fall in northern hemisphere i. neither the north pole nor the south pole are tilted toward the sun; the axis is perpendicular to the ecliptic iv. day and night are of equal length all over the world; the Equator always has days and nights of equal length d. Winter Solstice i. Usually occurs on December 21 or 22, although the solstice is really a time rather than a day ii. Marks the first day of winter for the northern hemisphere iii. The axis of the northern hemisphere is tilted 23.5 away from the sun iv. Vertical rays of the sun hit directly at latitude of 23.5 S (Tropic of Capricorn) v. Shortest day of the year for northern hemisphere; longest day of the year for southern hemisphere; the Equator always has days and nights of equal length v. North Pole and all latitudes above Arctic Circle (66.5 N) receive no sunlight during this day
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