1. Choose the simulation of the Sun and Earth from the option box at the top right. 2. Turn on Gravity Force, Path, and Grid.
3. What do the blue arrows indicate?
4. Using just the images, compare the size (mass) of Earth and the Sun.
5. Use the sliders to increase the masses of Earth and the Sun.
6. What happens to the blue arrows when you make this adjustment?
7. Reset the size of Earth and the Sun.
8. Click the blue play button at the bottom of the screen.
9. What does the gray line represent?
10. Which object orbits the other?

simulation 2

1. Choose the simulation of the Sun, Earth, and Moon from the option box at the top right.
2. Using just the images, compare the size (mass) of the Sun, Earth, and Moon.
3. Click the blue play button at the bottom of the screen.
4. What does the purple line represent?
5. Why does Earth have a circular path while the Moon has a “flower-shaped” path?

simulation 3

1. Choose the simulation of Earth and the Moon.
2. Using just the images, compare the size (mass) of Earth and the Moon.
3. Click the blue play button at the bottom of the screen.
4. Which object orbits around the other?
5. Look closely at Earth. What do you observe about its motion?
6. Increase the mass of the Moon to its maximum. What do you observe about the motion of
Earth now?
7. Use the Sim Speed slider at the bottom of the screen to speed up the simulation.
8. What do you observe about the motion of the Earth-Moon system?
9. Why do you think this movement is happening?

Simulation 4
1. Choose the simulation of Earth and the satellite.
2. Using just the images, compare the size (mass) of Earth and the satellite.
3. Click the blue play button at the bottom of the screen.
4. Which object orbits around the other?
5. Turn off Gravity. What do you observe?

Conclusion Questions
1. How do you think astronomers can determine the gravitational force between two celestial
objects without measuring it directly?
2. Scientists are beginning to develop the technology to observe other Sun-like stars distant
from our solar system. They can even detect large Jupiter-like planets orbiting some of
these stars. Other stars appear to have no planets, but astronomers can see the star wobble
a bit in space. What do you think this means?