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# Around and Around: What is the Shape of a Planet's Orbit?

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Source:
Author: Janice VanCleave

### PROBLEM

What is the shape of a planet's orbit?

### Materials

• black pen
• ruler
• 10-inch (25-cm) square of poster board
• sharpened pencil
• 8-inch (20-cm) piece of string adult helper

### Procedure

1. Use the pen to mark a dot in the center of the poster board. Label the dot Sun and draw rays around the dot. Mark a second dot 2 inches (2.5 cm) away from the first dot.
2. Ask an adult to use the pencil to make a hole through each dot on the poster board.
3. Insert the paper brads in the holes and secure them.
4. Tie the ends of the string together to form a loop, then place the loop around the brads.
5. Place the point of the pen against the inside of the loop, and with the string taut and the pen's point against the paper, move the pen around inside the loop until it is back at the starting point.

### Results

An oval shape is drawn.

### Why?

The oval shape drawn is an ellipse, which is the shape of the orbit of a planet. While a circle has one center point, an ellipse has two foci (points in line with each other and on either side of the center point of the ellipse), represented by the two brads. For each planet's orbit, the Sun is located at one focus, and nothing is at the other focus.

On a planet's orbit, the point closest to the Sun is called the perihelion and the point farthest from the Sun is called the aphelion. (In the figure the orbit shown is more elongated than are any of the planets' orbits.)

### LET'S EXPLORE

How does the focal distance (the distance between the foci) of an ellipse affect its shape? Repeat the previous experiment three times. First, leave the brads where they are and draw the ellipse with a red pen. Second, place the second brad 1 inch (2.5 cm) away from the first brad and use a blue pen to trace the ellipse. Third, place the second brad 3 inches (7.5 cm) away and use a green pen to draw the ellipse. Science Fair Hint: Title the series of drawings Shapes of Planetary Orbits. Label the drawings A, B, and C, with A being the most elongated ellipse and C the least. Prepare a legend, such as the one shown, indicating the focal distance of each orbit.

### SHOW TIME!

Use the equation below to calculate the focal distance of Earth, which would be F.

F=M1M2

where

M1 = 95 million miles (152 million km)

M2 = 92 million miles (147 million km)

Example:

F = 95 million miles (152 million km)

1. Using the focal distance of the orbit of each planet and the information from the previous experiment, prepare a listing of the planets' elliptical-shaped orbits in order from the least to the most elongated. The focal distance can be determined by subtracting a planet's minimum distance (M2 ) from its maximum distance (M1) from the Sun. (See the appendix for this information.)
2. Pluto is generally the farthest planet from the Sun. However, during part of Neptune's orbit, Neptune is the farthest planet from the Sun. Use the maximum and minimum distances of Neptune and Pluto from the Sun to make a diagram to explain this phenomenon.

### CHECK IT OUT!

Sir Isaac Newton (1642–1727), an English scientist, determined that planets stay in orbit because of the force of attraction between two bodies called gravity. Gravity also makes the planets move faster at the aphelion and slower at the perihelion of their orbit. For information about Newton's ideas about planetary movement, see pages 47–53 in David Filkin's Stephen Hawking's Universe (New York: Basic Books, 1997).