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, 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: = Summer Solstice WS = Winter 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 9 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 : 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 at the Summer Solstice. Step : Rotate the earth so that it corresponds to the Summer Solstice and place the observer at o N latitude (Providence). Complete the illustration. 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 5: Rotate the earth so that it corresponds to the Summer Solstice and place the observer at 7 o N latitude (Arctic Circle). Complete the illustration. Verify the expression: Latitude of observer + Altitude of the sun @ equinox = 9 o Complete the illustration. Step : Place an observer at any latitude in the Northern Hemisphere. Is this relationship true for Step 1 above as well? Seasons
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 1 17 calendar year. 1 17 Spring begins, March, 1 Spring begins, March, 17 Summer begins, June 1, 1 Summer begins, June 1, 17 Autumn begins, September, 1 Autumn begins, September 3, 17 Winter begins, December 1, 1 Winter begins, December 1, 17 March, 1 June 1, 1 June 1, 1 September, 1 Number of days: Number of days: September, 1 December 1, 1 December 1, 1 March, 17 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 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. http://astro.unl.edu/classaction/animations/coordsmotion/daylighthoursexplorer.html 1. Use the Latitude slider to set the latitude for Providence, RI.. The graph shows how many hours of daylight throughout the entire year. Record the data from the graph for the number of daylight hours in Providence at the beginning of each season. Adjust the latitude at each of the other locations, recording the hours of daylight for the entire year. Hours of Daylight Location Latitude Vernal Equinox Summer Solstice Autumnal Equinox Winter Solstice Yearly Average Barrow, AK 71. o N Fairbanks, AK 5. o N Providence, RI 1.5 o N Atlanta, GA 33. o N Honolulu, HI 1. o N Buenos Aires Argentina 33. 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? 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? Seasons
Fairbanks, AK 1 1 1 1 Latitude: Providence, RI 1 1 1 1 Latitude: Miami, FL 1 1 1 1 Latitude: Seasons 5
Atlanta, GA 1 1 1 1 Latitude: Honolulu, HI 1 1 1 1 Latitude: Equinox Daylight: hrs Buenos Aires, Brazil 1 1 1 1 Latitude: Seasons
Step 1: Observer at the Equator at the Vernal Equinox Step : Observer at Tropic of Cancer at the Summer Solstice Step 3: Observer at Tropic of Capricorn at the Winter Solstice Seasons 7
Step : Observer at Providence at the Summer Solstice Step 5: Observer at Arctic Circle at the Summer Solstice Step : Observer at any latitude at the Vernal Equinox Seasons