Regents Earth Science Name: Unit 6: Astronomy Date: Section: LAB # Reasons for the Seasons Introduction: The units of time that mankind has devised are all imaginary. We base them on seasonal changes and the position of the earth relative to the sun. The period of a day before the sun moves to its highest position (noon) is known as am, which stands for ante meridian. When we talk of am, it is the morning hours, before (ante) the sun reaches its high point. On the other side of noon, pm, means post meridian or afternoon, past the point when the sun has reached its peak. The time required for the earth to complete an orbit (revolution) is one (1) year. A year is the time required for the earth, sun and a distant star to line up and requires 365.2422 days. A day is the amount of time required for the earth to spin (rotate) enough so that the sun will move from the meridian and return to that point. A day requires 23 hours, 56 minutes and 4 seconds. Each year is divided into four (4) parts called seasons. Each season is marked by an astronomical event. These events are the sun crossing the equator (twice), and when it completes both the northerly and southerly most motion. The reason Earth has four (4) seasons is often misunderstood. Many people incorrectly conclude that our weather is warmer during the summer, because we are closer to the sun at that time. However, we are actually slightly closer to the sun during the winter months!! The cause of Earth s seasons is the angle at which it rotates (spins), known as parallelism of axis, as it revolves (orbits) around the sun. Most objects rotate around an axis that is straight up and down, just like when a basketball player spins a ball on his finger. But Earth s axis is tilted, as shown in the diagram below, and this tilt causes the seasons. Reason for the Seasons5/18/2010 1
Materials: Flashlight, plastic ball, classroom globe and toothpick On the diagram below, label each of the parallels and the axis. Give both the name and latitude. Reason for the Seasons5/18/2010 2
Diagram 2 illustrates the positions of the earth in its orbit around the sun on each of the first days of the four seasons. At each position, label the following: date, season, event, and the parallel of latitude where the sun s vertical (direct) rays strike at noon on that day. Diagram 3 illustrates where the vertical (direct) rays of the sun strike the earth at the beginning of each season. For example, on June 21 the vertical rays of the sun are at the Tropic of Cancer (23 1/2 0 N). Reason for the Seasons5/18/2010 3
Procedures: Part A Teacher Demonstration 1. Observe as your teacher shines the flashlight on the plastic ball. 2. Since the ball is tilted away from the flashlight, this represents winter. 3. Record the shape of the squares at the equator and at the poles. 4. Carefully observe as the Earth rotates (spins) on its axis. 5. Record how the shadow of the toothpick changes throughout the day. 6. Observe as your teacher tilts the Earth toward the flashlight to represent summer. 7. Record the shape of the squares at the equator and at the poles. 8. Observe and record how of the toothpick changes. Winter Observations Shape of Squares at Equator Shape of Squares at Pole Shadow of Toothpick Summer Observations Shape of Squares at Equator Shape of Squares at Pole Shadow of Toothpick Reason for the Seasons5/18/2010 4
Questions: 1. When it is winter in the Northern Hemisphere, which areas on Earth get the most concentrated amount of light? Which areas get the most concentrated light when it is summer in the Northern Hemisphere? 2. If the squares projected on the ball become larger, what can you conclude about the amount of heat distributed in each square? 3. What time of year will the toothpick s shadow be longest? When will the shadow be shortest? 4. How are the amounts of heat and light received in a square related to the angle of the sun s rays? 5. How can you use your observations to explain what causes the seasons? Part B: 1. From a height of 15 cm, shine the flashlight straight down at the grid on the following page (90 0 angle). 2. Trace the lighted area with a pencil and label it #1. 3. Keeping the flashlight 15 cm above your paper, change the angle of the flashlight to about 30 0. 4. Trace the lighted area and label it #2. Reason for the Seasons5/18/2010 5
Questions: 1. Determine the number of blocks that were lit by the flashlight. (A) # of Full Blocks + 1/2 X # of Partial Blocks = Approximate Area (B) # of Full Blocks + 1/2 X # of Partial Blocks = Approximate Area 2. At which angle did the flashlight illuminate the most area? 3. What does this say about the intensity of light at each angle? 4. Explain how this activity demonstrates the cause of seasons on Earth. Reason for the Seasons5/18/2010 6
Conclusion: 1. What are the tree basic causes of seasons? 2. Why are the vertical rays of the sun a determining factor in the seasons? 3. At what rate do the sun s rays change each season (how much latitude)? 4. At what rate do the sun s rays change each year? 5. At what rate do the sun s rays change each day? 6. Where will the vertical rays be at the start of the next season? 7. Where were the sun s vertical rays at the start of the season that we are currently in? 8. How many days are we from a change of season? 9. Where are the vertical rays located today? 10. What is a solstice? 11. What is the origin of the word solstice? Sol = stice = 12. What is an equinox? 13. What is the origin of the word equinox? Equi = nox = 14. Define zenith - 15. Define Altitude - 16. How do the seasons change in the temperate (30 0 60 0 ) latitudes? 17. How do the seasons change in the equatorial regions? 18. How do the seasons change in the Polar Regions? Reason for the Seasons5/18/2010 7
Earth's tilt and the seasons - what if...? questions 19. What happens at the December solstice? 20. What happens at the September equinox? 21. What temperature changes occur in different regions from the June solstice through to the December solstice? Just imagine, what would happen if the tilt of the Earth's axis was just slightly different from what is now. 22. The Earth has a tilt of 23.5 0. What would the consequences be if the inclination increased to 35 0? 23. Explain the consequence if the Earth s orbit became more elliptical than it is now. Reason for the Seasons5/18/2010 8