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|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.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.
Sites discussing Kepler's Laws:
Other sites on Kepler and Newton: