Credits

• Page 04: Gravity at Earth's Center Video

  © 2011, 2008 WGBH Educational Foundation. All Rights Reserved.

  ---

• Page 06: Galileo's Experiment on the Moon Video

  Source: NASA

  ---

• Page 11: Gravity at Earth's Center Video

  © 2011, 2008 WGBH Educational Foundation. All Rights Reserved.

  ---

• Page 11: Galileo's Experiment on the Moon Video

  Source: NASA

  ---