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 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.

 Click to Run

Source: My Solar System, PhET Interactive Simulations, 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 into your notes.
3.  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.

4. Enter the first mass for the sun (body 1) and the planet (body 2) from the data table.
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 3-6 to complete the data table. Then answer the following questions in your notes.

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?

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.