Speedy Orbits

Kristen Harris

February 22, 2000

Brief Description:

Students will learn about the differences in the planets' orbits. They will discover that the closer a planet is to the sun, the shorter its orbit is, and the further a planet is from the sun, the longer its orbit is. The students will compare the sizes of the planets in relation to each other, and in relation to the sun. They will also notice the distance between planets.

Grade Level and Course of Study Content Standard:

This lesson is appropriate for fourth grade students. It will fulfill concept #8 in the fourth grade Alabama Course of Study which states that "students will develop initial knowledge of stars, planets, and moons in the Universe." Concept #10 which states that "students will know that our solar system is a sun-centered system" will also be covered.

Background Information:

The nine planets of our solar system are located in the following order from the sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto. A mneumonic device that is commonly used to remember the planets' order is My Very Excellent Mother Just Served Us Nine Pickles.

Copernicus discovered that our solar system is called a heliocentric model which means that is a sun-centered model. This means that all nine planets revolve around the sun. Revolution is the movement of one body in space around another body. The path a body follows is called its orbit. Until the early 1600's, scientists believed that the planets traveled in perfect circles around the sun. Johannes Kepler then discovered that the planets actually travel in elliptical orbits. He also proved that the sun is not in the exact center of these orbits, but it is actually off to one side of their paths.

Although scientists accepted Copernicus' and Kepler's theories, no one knew why the planets traveled in this manner. Isaac Newton was the first person to derive a theory. He showed that the planets travel around the sun because of its gravitational pull. This attraction exists because of the objects' mass. Therefore, the greater the mass, the greater the gravitational attraction.

Newton's first law states that an object in motion will continue to travel in a straight line unless acted upon by an outside force. This law also pertains to objects at rest; they will remain at rest unless acted upon by an outside force. Therefore, a planet will not change direction unless it is forced to. This explains inertia. The planets' inertia is what keeps the planets moving. Inertia combined with the sun's gravity is what causes the planets to travel around the sun.

This law of inertia can be applied to all objects in space. For example, our moon travels around Earth because of the moon's inertia and the Earth's gravitational pull.

Each planet takes a different amount of time to travel around the sun. This is called the planet's period of revolution. The closer a planet is to the sun, the shorter its period of revolution. The farther away a planet is from the sun, the longer its period of revolution.

Planet Period of Revolution
Mercury 88 days
Venus 224.7 days
Earth 365 days
Mars 687 days
Jupiter 11.9 years
Saturn 29.5 years
Uranus 84 years
Neptune 164.8 years
Pluto 247.7 years

Pluto's orbit crosses over Neptune's orbit. Therefore, for a few years, Pluto is the eighth planet, and Neptune is the ninth planet.

Mercury is the first planet from the sun, and the second smallest planet in our solar system. It is named after the Roman god of commerce and thievery. It has a rocky surface with many craters. The temperature on Mercury is either very hot or very cold. If a person was on the side of the planet facing the sun, he would melt. Its atmosphere is very thin which must replenish itself often because it is so hot.

Venus, the second planet from the sun, is named after the goddess of love and beauty. The surface is very rocky with many volcanoes. The surface is covered by clouds composed of sulfuric acid. These clouds cause tremendous air pressure which could crush a person. The temperature is extremely hot, and averages around 900 degrees Fahrenheit.

Earth is the third planet, and the only planet with known life. Its name derives from English and German roots. The weather on Earth varies. It is cold at both the poles, and gets warmer as you approach the equator. The atmosphere traps heat which allows life to survive.

The fourth planet is Mars, named after the Roman god of war. It has two moons which were actually asteroids that were trapped by Mars' gravitational pull. Mars has a rocky dusty surface, and often has huge dust storms which can cover the entire planet. The atmosphere is extremely thin, and the temperature is very low.

Jupiter, the fifth planet from the sun, was named after the Roman's supreme god. Jupiter is the largest planet; it is so large that 1,300 Earths could fit inside of it. It is a gaseous planet and consists mostly of hydrogen and helium. There are constant storms in Jupiter's atmosphere. The most famous storm is the Great Red Spot which is visible in photographs of Jupiter.

The sixth planet, Saturn, was named after the Roman god of agriculture. It is also a gaseous planet, made of mostly hydrogen and helium. It has powerful storms with winds up to 1000 miles per hour. Saturn is surrounded by spectacular rings which actually consist of numerous snowballs. Saturn has at lease eighteen moons which are located within Saturn's rings.

Uranus, the seventh planet from the sun, was named after the Greek god of the Heavens. It is composed mostly of rock and various ices. Its axis spins parallel to its elliptical orbit causing its poles to be warmer than its equator. Uranus has rings which are composed of large particles and fine dust.

The ninth planet is Pluto which was named after the Roman god of darkness. It is the smallest planet and has one moon. Its temperature is extremely cold, and its surface is frozen and dark because it is so far from the sun.

Concepts Covered in the Lesson:

All of the planets travel around the sun. pg. 1 # 8 and #10

Pluto takes longer to travel around the sun. pg. 1 #8

Mercury has the smallest orbit. pg. 1 #8

Venus and Earth are located close to each other. pg. 1 #8

Jupiter is much bigger than Mars. Pg. 1 #8


23 strings with a 1" washer attached to one end, and a Guthrie's sauce cup

attached in the center

1 large balloon

1 basketball

soccer ball

2 baseballs

2 tennis balls

1 1 ½ " styrofoam balls

2 1" styrofoam balls

placecard for each planet



1. Prior to the lesson, the teacher will set up the investigation area outside. Each planet will be marked on the ground in proportion to the planets' location in space. Mercury 0.4 paces; Venus 0.7 paces; Earth 1 pace; Mars 1.5 paces; Jupiter 5.2 paces; Saturn 9.5 paces; Uranus 19.1 paces; Neptune 30.1 paces; Pluto 39.3 paces. A ball, which is proportional to the planet's actual size, will also be placed at each of the planet's locations. Mercury-1" styrofoam ball; Venus-tennis ball; Earth-tennis ball; Mars-1 ½" styrofoam ball; Jupiter-basketball; Saturn-soccer ball; Uranus-baseball; Neptune-baseball; Pluto-1"styrofoam ball.

2. The class will be divided into two groups. Each group will be divided into the nine planets and the sun. Therefore, the class will consist of two solar systems.

3. The teacher will explain the activity that they will participate in and each person's role. They will discuss the following rules: when participating in the second activity, everyone should have their own personal space. Each person should be careful to not hit anyone with his orbitor and only perform the activities when instructed to do so by the teacher.

4. The class will go outside to the designated area. The teacher will show the students the balls and which planet each one represents. They will be asked to observe the difference in size and make comparisons. Then, the first group will walk to their planetary spots. They will walk towards the sun and observe which planets reach the sun first and which ones take the longest to reach the sun. They will also observe which planets are closer to each other than other planets.

5. The planets will then return to their places. This time, they will walk the orbits of the planets. The students will observe which planets completed their orbit first, and which planets completed their orbit last.

6. Group 2 will perform the same activity as group 1 observes.

7. If time permits, the next activity will be performed. Students will find their own personal space (about a 3ft. circle). The orbit strings will be distributed. Each student will hold the cup in one hand, and the excess string in the other hand. He will swing the string while holding the cup. As he swings, he will pull the excess string towards the ground. He will observe what happens.


1. The class will return to the room. The teacher will ask the students what they observed during the first activity. The teacher will ask questions relating to the planets' sizes, their orbit paths, and location in relation to each other. The teacher will look for responses such as :Jupiter is much bigger than Pluto; Mercury takes the least amount of time to orbit the sun; Venus and Earth are much closer than Jupiter and Uranus.

2. The teacher will ask the class what they observed as they swung the orbit strings. The teacher will look for responses such as: the closer the cup, the faster the string would swing.

3. After the class has discussed these concepts, the teacher will show the class a chart of the planets' revolution lengths and each student will try to match each orbit to the correct planet. They will record their answers in chart form which will be turned in. After the papers have been collected, the teacher will draw the chart on the board. Students will be chosen to write the correct planets and their revolutions starting with Mercury, and ending with Pluto.


Students will be assessed by observing their participation during the investigation and colloquium lessons. The students' answers from the orbit chart will also be observed for understanding.


Project Star. President and Fellows of Harvard College, 1993

Science Insights: Exploring Earth and Space, Addison-Wesley Publishing Co. 1996

Science Is…, Susan V. Bosak. 1991.

Science Mini-Unit: Our Solar System, Doloris Pebble and Nancy Christen


www.nasa.gov/kids.html--This site is useful for both teachers and students. It

allows viewers to learn about the solar system and has many colorful

pictures. It also links to teacher sites which are also sponsored by NASA.


Process Skills:

drawing conclusions



drawing inferences