THE SEASONS To observers on earth, it appears that the earth stands still and everything else moves around it. Thus, in trying to imagine how the universe works, it made good sense to people in ancient times to start with those apparent truths. The ancient Greek thinkers, particularly Aristotle, set a pattern that was to last for about 2,000 years; a large, stationary earth at the center of the universe, or geocentric solar system. Positioned around the earth were the sun, moon, planets, and tiny stars arrayed in a perfect sphere. Shortly after the discovery of the Americas, a Polish astronomer, Nicolaus Copernicus, proposed a different model to the universe. Discarding the premise of a stationary earth, he showed that if the earth and planets all circled the sun, or heliocentric solar system, the motion of the planets could be predicted more accurately than an earth-centered model. His model violated the prevailing common-sense notions in that it required the apparently immobile earth to spin completely around on its axis once-a-day and that, worst of all, the earth losing its place as the center of the universe and taking its place around the sun orbiting with the other known planets. As astronomical measurements continued to become more precise, it became clear that a refinement of Copernicus model was required. A German astronomer, Johannes Kepler, who lived during the same time as Galileo, developed a mathematical model of planetary motion that discarded the notion of a circular motion of the planets. He postulated three laws, the most revolutionary of which was that planets move in elliptical orbits at predictable but varying speeds. Although Kepler s Three Laws of Motion turned out to be correct, Kepler offered no explanation why the planets would move in this way. In this laboratory we will make use of computer simulations in order to stand outside of the earth to visualize the motions of the earth about the sun to account for the changing seasons. PART I: THE EARTH S ORBIT & THE SEASONS Open the laboratory webpage located at the University of Nebraska. http://astro.unl.edu/naap/motion1/animations/seasons_ecliptic.html Before you start the animation read the following descriptors of the panels. There are three main panels on the screen: LEFT PANEL: Orbit View. This panel shows the perspective of the positions of the Earth and Sun as the Earth moves along its yearly orbit around the Sun. Click and drag the earth. This changes the location of the Earth in its orbit. Notice that the date changes (lower panel) as well as the direction of the sunlight hitting the Earth s surface. Click and drag the orbit. This changes the perspective, so you can look straight down on the Earth and Sun, or look at them head on. This shows the earth as it orbits the sun. As it goes through its orbit, the date and seasons display in text below the action. Note the date and the location of the Earth relative to the Sun on those days. Note: SS = Summer Solstice WS = Winger Solstice VE = Vernal Equinox AE = Autumnal Equinox Seasons 1
UPPER RIGHT PANEL: View from the Sun. This view shows the earth as seen from the sun. It illustrates how the light rays hit the surface of the earth depending upon the latitude of the observer and the time of year. Click and drag the observer. Change the latitude to above and below the equator and note the change in orientation of the light rays with respect to the earth. Click and drag the date. Change the date on the lower panel to observe the seasonal effects on the angle at which the light rays strike the earth s surface. LOWER RIGHT PANEL: Sunlight Angle. This view shows the angle with which rays of sunlight are striking the earth. It lists the noon sun s angle with respect to the horizon (its altitude). Use the diagrams at the end of the laboratory to illustrate the position of the earth and sun to meet the following conditions: Verify that when the observer is at a latitude within the tropic, at some date during the year the rays of the sun are directly overhead making an angle of 90 o with the ground. Step 1: Rotate the earth so that it corresponds to the Vernal Equinox. Place the observer at the equator. Complete the illustration. Step 2: Rotate the earth so that it corresponds to the Summer Solstice. Place the observer at the Tropic of Cancer. Complete the illustration. Step 3: Rotate the earth so that it corresponds to the Winter Solstice. Place the observer at the Tropic of Capricorn. Complete the illustration. Verify that an observer in Providence does not observe the sun directly overhead. Step 4: Leave the earth at the Winter Solstice and place the observer at 42 o N latitude (Providence). Complete the illustration. Step 5: Rotate the earth so that it corresponds to the Summer Solstice and place the observer at 42 o N latitude (Providence). Complete the illustration. Step 6: Rotate the earth so that it corresponds to the Winter Solstice and place the observer at 42 o N latitude (Providence). Complete the illustration. Verify the expression: Latitude of observer + Altitude of the sun @ equinox = 90 o Complete the illustration. Step 7: Place an observer at any latitude in the Northern Hemisphere. Verify that an observer above the Arctic Circle experiences a polar day during the summer and a polar night during the winter. Complete the illustration. Step 8 & 9: Seasons 2
PART II: KEPLER S SECOND LAW: DISTANCE VS SPEED The dates of the equinoxes and solstices are given in the table below. Determine the number of days for each season during the 2016 2017 calendar year. 2011 2012 Spring begins, March 20, 2016 Spring begins, March 20, 2017 Summer begins, June 21, 2016 Summer begins, June 21, 2017 Autumn begins, September 22, 2016 Autumn begins, September 23, 2017 Winter begins, December 21, 2016 Winter begins, December 21, 2017 March 20, 2016 June 21, 2016 June 21, 2016 September 22, 2016 Number of days: Number of days: September 22, 2016 December 21, 2016 December 21, 2016 March 20, 2017 Number of days: Number of days: Which season has the greatest number of days? What does this imply about the speed of the earth as it orbits the sun at this location? Which season has the fewest number of days? What does this imply about the speed of the earth as it orbits the sun at this location? It is the gravitational attraction between the sun and the earth that keeps the earth in its orbit. Remember Newton s Second Law of Motion, it states that in order to accelerate a mass a force must be applied to it. What can you say about the relationship between the magnitude of this force and the distance the earth is from the sun? Construct a statement (or series of statements) that illustrates the relationship between the duration of winter and summer, the distance from the sun, and speed of the earth as it orbits the sun. Most people would expect that the earth is closer to the sun during the summer and farther from the sun in the winter. As you have seen this is not true. What factor is responsible for the degree of heating the earth s surface as the earth orbits the sun? Seasons 3
PART III: EFFECTS OF LATITUDE ON DAYLIGHT HOURS In the 2002 thriller Insomnia, Al Pacino plays a Los Angeles detective whose murder investigation takes him to northern Alaska in the summer. As the arctic Sun never sets, he finds himself unable to sleep, and he is gradually driven to madness by the pressures of the investigation and the unrelenting sunshine. Is it true that there is endless daylight in the arctic summer? What happens in the winter? In this section you will investigate seasonal changes for several locations on Earth. Go to Science Gizmos, Grades 6 8 Open Seasons Around the World 1. On the DESCRIPTION pane, use the Latitude slider to set the latitude for Providence, RI. 2. Set the speed slider to its maximum value. Select the YEAR GRAPH tab and click Play ( ). The simulation will automatically pause at the end of one year. The graphs show how many hours of daylight there are and the total solar energy that impacts that location in a day (we will return to this graph in a later laboratory). 3. Record the data from the graph for the number of daylight hours in Providence at the beginning of each season. Reset the Gizmo and allow the program to run for a full year at each of the other locations, recording the hours of daylight at the completion of one year. Hours of Daylight Location Latitude Vernal Equinox Summer Solstice Autumnal Equinox Winter Solstice Barrow, AK 71.0 o N Fairbanks, AK 65.0 o N Providence, RI 41.5 o N Atlanta, GA 33.0 o N Honolulu, HI 21.0 o N Buenos Aires Argentina 33.0 o S Examine the data for locations in the northern hemisphere, how does changing the latitude of the location effect the number of hours of daylight at the equinox? Does Fairbanks experience a polar day or night? Use the data to support your answer. Does Barrow experience a polar day or night? Why is this answer different for Barrow than for Fairbanks? Seasons 4
Construct a general statement that describes the changes in the hours of daylight at the Winter and Summer Solstices as the latitude increases (more northerly). Compare the data for Atlanta and Buenos Aires. What s similar about their hours of daylight during the year, and how does the data differ? CHANGING EARTH'S AXIS As you have seen, the hours of daylight are different for different locations, we have seasons because of the tilt of Earth's axis. What would happen if the Earth were more or less tilted? To find out, try the following steps. 1. Select the DESCRIPTION tab and set the Latitude to the latitude of Providence (41.5 o N). Click and drag the Earth axis angle slider back and forth, observing the effect on the diagram of the Earth. 2. Set the Earth axis angle to 0. Set the speed to maximum and click Play ( ). Select the YEAR GRAPH tab and observe the resulting graphs for the hours of daylight and record the data from the graph for the number of daylight hours at the beginning of each season. 4. Reset the Gizmo and allow the program to run for a full year at the other axis angles. Hours of Daylight Earth s Axis Vernal Equinox Summer Solstice Autumnal Equinox Winter Solstice 0 o 47.0 o -23.5 o How are the hours of daylight altered by changing the earth s axis to 0 o? How would this affect seasonal changes worldwide? How are the hours of daylight altered when increasing the tilt of the earth s axis to 47 o? How would affect seasonal changes worldwide? How does reversing the tilt alter the daylight and seasonal changes worldwide? Seasons 5
24 22 20 18 16 14 12 10 8 6 4 2 0 Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec Fairbanks, AK Latitude: Summer Daylight: hrs Summer Night: hrs Winter Daylight: hrs Winter Night: hrs Equinox Daylight: hrs 24 22 20 18 16 14 12 10 8 6 4 2 0 Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec Providence, RI Latitude: Summer Daylight: hrs Summer Night: hrs Winter Daylight: hrs Winter Night: hrs Equinox Daylight: hrs 24 22 20 18 16 14 12 10 8 6 4 2 0 Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec Miami, FL Latitude: Summer Daylight: hrs Summer Night: hrs Winter Daylight: hrs Winter Night: hrs Equinox Daylight: hrs Seasons 6
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