The solar system is not simply a collection of stationary planets and other bodies around the sun. Each one moves according to strict physical laws. The way the tilted Earth moves causes the seasons and day and night.

But why do the planets continue to revolve around the sun? What holds them in their orbit? Why don’t the planets collide into the sun or fly off into space?

The answer to all these questions is gravity.

If you were asked, "what is gravity?" you would probably answer something like “the force that holds me on the Earth,” and you would be correct. So how does gravity keep the planets in orbit? What factors affect the strength of gravity?

Let’s explore this last question so that we can get a better understanding of how gravity affects our solar system.

The following is a simulation from the University of Colorado at Boulder.

Directions:

1. Select the following options on the simulation.
1. From the Select Preset pull down choose Sun and planet.
2. Make sure the following are checked: System Centered, Show Traces, Show Grid.
3. Move the green slider all the way to accurate.
2. Copy the following table onto your paper.

Mass of Sun (body 1)	Mass of Planet   (body 2)	Time of One Orbit (seconds)
200	10
400	10
600	10
800	10
1000	10
200	1
200	20
200	50
200	100
200	200




The simulation should look like the one below if you have selected the correct options.


3. Enter the first mass for the sun (body 1) and the planet (body 2) from the data table.
4. Press the start button and let the planet (body 2) complete one orbit around the sun. Press stop when it returns back to the starting point.
5. Record the time in seconds it took the planet to complete one orbit in the data table.
6. Click the reset button.
7. Repeat steps 3-6 to complete the data table. Then answer the following questions on your paper.

Questions:

1. What does the time of orbit represent in this simulation?
2. When is the planet orbit the fastest?
3. What makes the length of the time of orbit increase or decrease?

Check Your Answers

1. The time of orbit would represent a planetary year.
2. The planet orbited the fastest when the sun had a mass of 1000 and the planet had a mass of 100.
3. The mass of the sun and the mass of the planet are the factors that changed the time of orbit.

The mass of the bodies affects the force of gravity. The sun is a giant mass of burning plasma. It is so massive that it exerts a very strong gravitational field on the planets, forcing them to orbit around the sun. However, that is only part of the explanation. What else affects the strength of gravity and the orbit of the planets?

Let’s use the same interactive to explore this question.

Directions:

1. Copy the following table onto your paper.

Position of Planet   (body 2)	Time of One Orbit (seconds)
50
75
100
125
150
175
200
225

2. Select the following options on the simulation.
1. From the Select Preset pull down choose Sun and planet.
2. Make sure the following are checked: System Centered, Show Traces, Show Grid
3. Move the green slider all the way to accurate.

The simulation should look like the one below if you have selected the correct options.


3. In this exercise you will leave the masses of the sun and planet as they are. The only thing you will be changing is the position (x) of the planet (body 2). You will not be changing the y value.
4. Change the position of the planet by entering the first planet position into the x value. You should see that planet moved closer to the sun.
5. Press the start button and let the planet (body 2) complete one orbit around the sun. Press stop when it returns back to the starting point.
6. Record the time in seconds it took the planet to complete one orbit in the data table.
7. Click the reset button.
8. Repeat steps 4-7 to complete the data table. Then answer the following questions on your paper.

Questions:

1. When is the planet orbit the fastest?
2. What makes the planet orbit time increase or decrease?

Check Your Answers

1. When the distance of the planet was set at 50 (In other words, when it was closest to the sun.)
2. The distance the bodies are from each other