Scientific Background Information
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  Building a foundation for understanding of orbits of planets and other satellites begins with observation of objects in the sky and how they move.  Clouds move with the wind, airplanes and birds create their own direction.  The Sun, the Earth, the Moon, the stars and other plants have fixed laws that govern their movement.  Planetary orbits have a distinct elliptical shape.  In K-4, students begin by observing objects in the sky and how they move.  It is the teacher's job to elicit thinking about the patterns of these movements so that students have a firm basis in the upper grades for understanidng orbits.

By observing the day and night sky regularly, children in grades K-4 learn to identify sequences of changes and to look for patterns in these changes.  As they observe changes, such as the movement of an object's shadow during the course of a day, and the positions of the Sun and the Moon, they will find patterns in these movements.  They can draw the moon's shape for each evening on a calendar and then determine the pattern in the shapes over several weeks.  These understandings should be confined to observations, descriptions, and finding patterns.  Attempting to extend this understanding into explanations using models will be limited by the inability of young children to understand that Earth is approximately spherical.  They also have little understanding of gravity and usually have misconceptions about the properties of light that allow us to see objects such as the Moon.  Although children will say that they live on a ball, probing questions will reveal that their thinking may be very different.

Students can discover patterns in weather during the year by keeping a journal.   Younger students can draw a daily weather picture based on what they see outside.   Older students can keep simple charts and graphs from data they collect at a school weather station. 

Emphasis in grades K-4 should be on developing observation and description skills.  Younger children should be encouraged to talk about and draw what they see and think.  Older students can keep journals, use instruments, and record their observations and measurements.

National Science Education Standards

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. 
Sites discussing Kepler's Laws:

Other sites on Kepler and Newton: (Kepler) (Newton)