How do I Explain the Orbit of a Planet?

Constructing an Orbital Diagram

• 1

  Tack the sheet of paper to the piece of cardboard. Place the two tacks approximately 3 inches apart and near the center of where the orbit display will go. Label one of the tacks to represent the sun; it does not matter which.

• 2

  Tie the string into a loop and wrap it around the tacks. Use the pencil tip to pull the string so that the pencil and tacks make a triangle.

• 3

  Trace out a circular path with the pencil while keeping the string tightly wound around the tacks, though loose enough to allow the string to move with the pencil. Keep the pencil tip flush with the inside of the string. Follow the path until the pencil returns to where you began tracing.

• 4

  Write down key terms on the model to better explain the diagram and its relation to a planet's orbit. Label the ellipse, an elongated or flattened circle and the shape of a planetary orbit. Mark the perihelion, the part of the orbit where the planet is closest to the tack representing the sun, and the aphelion, the part of the orbit where the planet is farthest from the sun. Label the two tacks as focus points; within a planet's orbit, the sun is always one of the two focus points. Explain that the distance between the second point inside the ellipse and any point on the ellipse--added to the distance between the sun tack and the same point on the ellipse--is always the same, no matter which point on the ellipse or orbit you choose. This characteristic is what defines these areas as focus points.

• 5

  Explain the rest of the orbit's characteristics. Mention that the sun is not at the center of a planet's orbit, which means that the distance between a planet and the sun is always changing throughout its orbit. Explain that a planet's orbital speed is constantly changing depending on its distance from the sun. The speed is highest at perihelion and lowest at aphelion.