The Seasons Autumn Winter Summer Sun Spring Winter Spring Summer Autumn Celestial Celestial Celestial Celestial Equator Equator Equator Equator Winter Observations What is the orientation of the earth’s axis in relation to the sun when the northern hemisphere experiences winter? (Pointed away) What is the sun’s position in relation to the celestial equator during the winter? (23 degrees below) Where on the globe (in latitude) would the sun appear directly overhead on the winter solstice? (Tropic of Capricorn, or 23 degrees south of the equator) 1 Summer Observations What is the orientation of the earth’s axis in relation to the sun when the northern hemisphere experiences summer? (Pointed towards) What is the sun’s position in relation to the celestial equator during the summer? (23 degrees above) Where on the globe (in latitude) would the sun appear directly overhead on the summer solstice? (Tropic of Cancer, or 23 degrees north of the equator) Comparison of Winter & Summer Compare the area of illumination for the winter and summer solstices. Which season experiences a greater intensity (greater concentration) of light? Summer Run the simulation again without stopping and note the distance and the speed of the earth as it orbits the sun. While the simulation is not to scale and greatly exaggerates the motion of the earth, it accurately depicts the changes in distance and speed of the earth as it orbits the sun. Comparison of speeds between summer and winter: The earth orbits the earth faster in the winter than the summer. Comparison of distances between summer and winter: The earth is closer to the sun in the winter than in the summer. The dates of the equinoxes and solstices are given in the table below. Determine the number of days for each season during 2011. 2011 2012 Spring begins, March 20, 2011 Spring begins, March 20, 2012 Summer begins, June 21, 2011 Summer begins, June 21, 2012 Autumn begins, September 22, 2011 Autumn begins, September 23, 2012 Winter begins, December 21, 2011 Winter begins, December 21, 2012 March 20, 2011 – June 21, 2011 June 21, 2011 – September 22, 2011 Number of days: 93 Number of days: 93 September 22, 2011 – December 21, 2011 December 21, 2005 – March 20, 2012 Number of days: 90 Number of days: 89 What is the relationship between the speed of the earth in its orbit with the length of the season? Winter is the shortest season. The faster the orbital speed the shorter the season. 2 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? The closer to the earth to the sun, the greater the gravitational force between the earth and the sun. Newton’s Second Law states that the greater the force the greater the acceleration. 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. The earth is slightly closer to the sun and has a greater orbital speed in the winter. Since the earth is moving faster during the winter, it takes a shorter time to travel ¼ the way around 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? The tilt of the earth’s axis. This factor changes the concentration of the sun’s rays that reach the earth’s surface. At low latitudes (the tropics) the light is more directly overhead (perpendicular to the earth’s surface) and more concentrated. At higher latitudes the sunlight hits the earth’s surface more obliquely. This increases the area of illumination thus the amount of light per area is less at higher latitudes. Comparison of Solstice and Equinox Observations Activate the simulation and stop the orbiting of the earth at two locations, the Autumnal equinox (fall) and the Vernal (spring) equinox. What is the sun’s position in relation to the celestial equator during the equinoxes? Where on the globe would the sun appear directly overhead on the summer solstice? (Use a globe for reference.) The sun is directly on the celestial equator, thus the sun is directly overhead on the equator. Compare the area of illumination at the equinox to the summer and winter solstices. The area of illumination at the equinox is less than at the winter solstice and more than at the summer solstice. Notice that the earth’s axis always points to the same direction in space. The northern pole always points to a location in the sky where we find the North Star, or Polaris. In a later laboratory we will observe the motion of the stars around Polaris to show that the location of Polaris in the sky does not change either during the course of the evening or during the progression of the seasons. 3 Use the globe and small yellow sphere as the sun and physically manipulate the orientation of the earth as it orbits the sun. Make a complete seasonal cycle. Use the small flashlight to simulate the illumination of the sun. Place it perpendicular to the globe’s surface and notice the area of illumination on the globe. Now change the position of the flashlight so it is more oblique to the globe’s surface. What happens to the area of illumination? When the flashlight is perpendicular to the earth’s surface the area of illumination is smaller and the intensity of illumination is greater. As the flashlight is tilted to the earth’s surface, the area of illumination is greater and correspondingly the intensity of the light is less. 4 .