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Physical Science 1421 Seasons Equipment Needed Quantity Equipment Needed Quantity Seasons Simulator (NAAP) - UNL Astronomy 1 Seasons Excel File 1 The purpose of this laboratory activity is to study how the position of the Earth in space, the Earth’s tilt, direct light, and indirect light combine to produce a phenomenon that we call Seasons. Part 1: Background Position of Earth in Space The shape of the Earth is an ellipsoid; however for most practical educational purposes we approximate its shape to a sphere. It can be divided into two hemispheres, the Northern Hemisphere and Southern Hemisphere. On the top of the Northern Hemisphere is the North Pole and the bottom of the Southern Hemisphere is the South Pole. The Earth’s axis is an imaginary line running from the North to the South poles. The Earth spins on its own axis all the time, turning around completely once every 24 hours. The rotation of the Earth about its axis is what provides us with the alternation between day and night. What do you think makes the seasons happen? Direct and Indirect Light The Earth is actually tilted 23.5 o from the plane of the ecliptic (the plane the planets orbit about the Sun). It should look like this: Figure 1 the Earth

Figure 2 Light as it hits the surface. In the left (Figure 2), the light is striking the paper at an angle. That is, it is shining on the surface with indirect light. In the right, the light is striking the paper perpendicularly. That is, it is shining as a direct light. Direct light produces more heat (gives more rays per area and therefore more energy per area) than indirect light (where light striking a surface at an angle spreads out). Length of Day and Night During the year the length of a day changes, due to the tilt of the Earth. The Sun rises earlier in the summer than in the winter and sets later in the summer than in the winter. The table below gives the time of sunrise and sunset for three latitudes. 0 o latitude 30 o N latitude 60 o N latitude Date Sunrise Sunset Sunrise Sunset Sunrise Sunset Jan. 1 6:00 18:08 6:56 17:12 9:02 15:06 Feb. 1 6:10 18:18 6:51 17:37 8:15 16:13 Mar. 1 6:09 18:16 6:28 17:58 6:55 17:30 Apr. 1 6:00 18:07 5:49 18:19 5:24 18:45 May 1 5:53 18:00 5:17 18:37 3:56 19:59 June 1 5:54 18:01 5:00 18:56 2:48 21:08 July 1 6:00 18:08 5:02 19:05 2:40 21:26 Aug. 1 6:02 18:10 5:18 18:55 3:36 20:34 Sep. 1 5:56 18:03 5:44 18:22 4:50 19:10 Oct. 1 5:46 17:53 5:53 17:45 6:03 17:36 Nov. 1 5:40 17:48 6:13 17:14 7:24 16:04 Dec. 1 5:45 17:54 6:38 17:00 8:35 15:03 Table 1 Sunrise and sunset times at different latitudes throughout the year The Seasons As mentioned in the previous section, the Earth’s rotation axis is tilted 23.5 o throughout the year pointing the same direction in space. The Earth moves around the Sun once every year. It follows an elliptical orbit. This means that it goes almost in a circle around the Sun, but gets a little further away at some times. The path it takes is like an oval. p. 2