Scientific Background Information  Go back to lesson. The understanding that students gain in grades K-4 provides the motivation and the basis from which they can begin to construct a model that explains the visual and physical relationships among earth, sun, moon, and the entire solar system.  Observation and satellite data allows students to conclude that earth is  moving, and that it has unique features that distinguish it from other planets in the solar system.  Activities with trajectories and orbits that use the earth-sun-moon-solar system allow students to develop the understanding that gravity is a force that holds all parts of the solar system together.  National Science in Education Standards: http://www.nap.edu/readingroom/books/nses/html/6d.html#es  It is difficult to observe the shapes of planetary orbits directly, since the earth (our observation platform)  is both rotating about its axis and moving in its own orbit about the sun.  Historically, it was not until well after Nicholas Copernicus's proposal of a heliocentric system (1543) that we began to understand the shapes of orbits.   Johannes Kepler (1571-1630), working from the observational data of Tycho Brahe (1546-1601), proposed three great laws of planetary motion:   1. The Law of Ellipses: The orbit of every planet is an ellipse with the sun at one focus.   2. The Law of Equal Areas: A straight line joining a planet and the sun sweeps out equal areas in equal times.  In simple terms, the speed of a planet is the largest when it is nearest to the Sun, and smallest when it is the farthest from the Sun.   3. The Harmonic Law: The squares of the orbital periods of the planets are proportional to the cubes of their respective mean distances from the Sun.  In simple terms, the closer a planet is to the sun, the more quickly it makes a complete orbit.  The relationships between Kepler's Laws and more fundamental laws of motion were discovered by Isaac Newton (1642-1727).  Newton proposed that a planet travels in an orbit about the sun because the sun exerts a force of gravitational attraction on it.  He showed that all three of Kepler's laws follow mathematically from his single Law of Universal Gravitation. he noted that, in general, orbits correspond to "conic sections" -- intersections of a plane with a right circular cone.   References  Other sites on Kepler and Newton:   http://www-groups.dcs.st-and.ac.uk/~history/Mathematicians/Kepler.html (Kepler)   http://scienceworld.wolfram.com/biography/Newton.html (Newton)