Lunar and Planetary Institute

 Children explore how Earth's axial tilt and revolution around our Sun lead to differences in day length through the year. Invite the children to create a model of the Earth using the Styrofoam ball with toothpicks for Earth's axes (see The Cycle of Light). Do the axes of our Earth point directly up and down? What does the globe show us? Our Earth is tilted approximately 23.5 degrees — so our axes point “to the side.” Ask them to draw the equator and then to draw an approximate outline of their continent and place a dot roughly where they live. Refer to the globe as needed. Review the causes of day and night (see The Cycle of Light). Based on their experience, does daylight length change throughout the year or stay the same? When do we have times of longer daylight? Shorter? Is there a consistent pattern? The period of daylight tends to be longer in the summer and shorter in the winter. Can they explain this annual pattern? Invite the children to experiment with their Styrofoam Earth and flashlight Sun to “explain” differences in daylight length through a year. One child will hold the Sun and another will slowly move Earth around it. The other children in the group will evaluate the results and suggest model changes. Are the children remembering to keep the Earth rotating on its axis? If the Earth rotates once a day, what happens once a year to explain the observed annual changes in daylight length? (Earth travels, or revolves, around our fixed Sun) Is their Earth axis tilted? Does the direction of tilt change? Draw the children's attention to the north polar region, where day and night differences throughout the year are extreme. They may be familiar with the fact that the north pole experiences months of darkness in the winter and months of daylight in the summer. Do the children's models show how this happens? Simply spinning on our axis (rotating) does not explain months of darkness or light at the poles. The long polar night and day cycles also can help the children fix the direction of the axis so it points consistently in a single direction for the movement of Earth around the Sun. Once the polar days and nights are established, do their models produce the more subtle day length changes for their location in a revolution around our Sun? Invite the groups to demonstrate their models to show that the annual pattern of changing day length on Earth is caused by Earth's annual revolution around the Sun with its axis pointing to a fixed position in space. More Activities Last updated January 4, 2007 Who? Ages 10–14 How Long? 60 minutes What's Needed? • 1 globe For groups of 2 to 4 children • 1 Styrofoam ball (6–9 inches) • 1 marking pen • 2 toothpicks • 1 flashlight Connections to the National Science Standard(s) Standards A&D (grades 5–8):Understand and communicate the scientific explanation of how objects in the solar system have regular and predictable motion, such as the apparent daily motion of the Sun in the day/night cycle, and annually across seasons.