| http://digfir-published.macmillanusa.com/astro_interactive_exercise/ie_the_rotation_of_our_galaxy.html |
Chapter
1. The Rotation of Our Galaxy
| 51413892b11c0ff21a000007 |
| http://digfir-published.macmillanusa.com/astro_interactive_exercise/ie_the_rotation_of_our_galaxy_dlap.xml | DLAP questions | 51413892b11c0ff21a000007 |