Children 8-14 model Earth’s axial tilt and movement around our Sun to reinforce their knowledge about the “fixed” North Star and the apparent movement of the zodiac and stars through the year. This activity is an extension to Zodiac Track. Before You Begin Obtain night sky charts for your location for different seasons of the year. The charts should show the Zodiac constellations. Sky and Telescope offers an interactive Sky Chart that can be generated and printed easily using a local zip code. The Activity Review the signs of the zodiac with the children. Invite the children to illustrate what is happening using the constellations on the wall and using themselves to represent Earth. Why do the signs of the zodiac move across the sky at night? (Because Earth is spinning on its axis, causing distant stars to appear to move) Do the constellations spin around Earth (No) What is Earth’s motion? What is missing from this model of Earth and the constellations? (Our Sun!) What can be used to represent the Sun? (The lamp) Where is our Sun relative to Earth and the constellations? (Our Sun is the closest star to Earth; the constellations are also made of stars, but these stars are much farther away — and at varying distances —relative to our Sun) It is important for the children to understand that the stars that we see making up a constellation exist in three dimensions. (The stars are not in the same plane — as with a drawing — although we perceive them that way from our location on Earth) Have one child at a time (or small groups of children) stand in the center of the room (under Polaris) representing Earth. Place the lamp between the child (Earth) and the constellations on one wall. Turn on the lamp. Have the child rotate. What does the child observe about her ability to see the constellations behind the lamp compared to the constellations not behind the light? How does this translate to a model of our Sun and Earth?(Our bright Sun obscures the constellations — and the other stars! — during the day). Allow other children to try the experiment and make observations. Based on the children’s observations, what natural events does their model help them explain? Earth is surrounded by stars that are distant to our solar system. Earth rotates on its axis. Each location faces our Sun in the daytime and faces away from our Sun at night. When we are at a location facing our Sun, it is daylight and we do not see the stars. Do we see all the zodiac constellations in a single night? (No). Why not? Distribute the “night sky charts.” What zodiac constellations are visible in the night sky during each of the seasons? Is there a pattern to which constellations we see? (Yes, certain constellations are visible in the night sky during certain months; the pattern repeats annually) Why do we see certain constellations during some times of the year and others during different times? Invite the children to experiment with positions and movement of Earth and the Sun relative to the constellations to see if they can create a model that replicates the changing nighttime viewing of the zodiac signs through the year. Remind them that in their model the Earth should rotate once a day; Earth’s northern axis points to Polaris; and the Earth revolves around our Sun once a year. What is changing at the same (annual) timescale that we are observing the changing zodiac?(Earth is revolving around our Sun once each year) Do the constellations appear to change positions in the night sky as Earth travels around our Sun throughout the year? (Yes, the constellations will appear in different positions at different times of the year) Are the constellations themselves moving? (No) What causes this apparent change in positions? (The different orbital positions of the Earth throughout the year cause us to view the night sky from different perspectives) Do all stars appear in different positions at different times of the year? (From a northern hemisphere perspective, all stars appear to change positions except Polaris, the “Pole Star,”which is the only star that appears “fixed”) Why is Polaris the only star that appears “fixed”? (Their answer should include the idea that, because our axis points to Polaris, it appears to be fixed, similar to the way the center of a spinning disk appears to remain fixed while its periphery moves) Ask the groups to share their models when they think they can illustrate why the zodiac constellations observed in the night sky change through the year. Have all the children participate in reviewing the models. The children should have observed that when they are facing the Sun during the day, the Sun “blocks out” the incoming light from distant stars. As Earth moves around our Sun during the year, the Sun blocks different parts of the celestial heavens from our vision. This is why the observable zodiac constellations in our night sky change throughout the year, repeating the cycle each year. Reinforce the difference between the apparent annual movement of the constellations in the night sky and their nightly progression, as presented in Zodiac Track. Each night the stars and constellations will “travel across the sky” because of Earth’s rotation. However, the time of year — and therefore the position of Earth in its revolution around our Sun — will determine which constellations are visible and where you will see them.   Last updated January 9, 2007 Who? Ages 8–14 How Long? 60 minutes What's Needed? • Bright lamp without shade (a flashlight cannot be substituted) placed near one of the walls in front of the constellation • Constellations from Zodiac Track activity, hung on walls around room • Polaris hung from the ceiling in the middle of the room For each group of children: • Night Sky Charts for different seasons Connections to the National Science Standard(s) Standards A, B&D (grades K–-4): Understand and communicate that objects in the sky, such as stars, have patterns of movement on a nightly and yearly scale, caused by Earth’s rotation and orbit. The position of objects in the sky can be described by locating them relative to another object or the background.  Standards A&D (grades 5–8): Understand and explain that the objects in our solar system are in regular and predictable motion. Apparent movement of the stars in the night sky is due to Earth’s own rotation on its axis and its orbit around the Sun.