| http://digfir-published.macmillanusa.com/astro_interactive_exercise/ie_an_explanation_of_superluminal_motion.html |
Chapter
1. An Explanation of Superluminal Motion
| 51413969b11c0f181b00000a |
| http://digfir-published.macmillanusa.com/astro_interactive_exercise/ie_an_explanation_of_superluminal_motion_dlap.xml | DLAP questions | 51413969b11c0f181b00000a |