1. Bennett, Jeffrey et al. The Cosmic Perspective. New York: Addison-Wesley Longman, 1999.
2. Padilla, Michael et al. Motion Force and Energy. Boston: Pearson Prentice Hall, 2005.
3. Padilla, Michael et al. Astronomy. Boston: Pearson Prentice Hall, 2005.
4. O'Conner, J.J., Robertson, E.F. "Sir Isaac Newton" MacTutor History of Mathematics. January 2000. 30 April 2007. http://www-history.mcs.st-andrews.ac.uk/Biographies/Newton.html.
5. O'Conner, J.J., Robertson, E.F. "Tycho Brahe" MacTutor History of Mathematics. April 2003. 30 April 2007. http://www-history.mcs.st-andrews.ac.uk/Biographies/Brahe.html.
6. Field, J.V.. "Johannes Kepler" MacTutor History of Mathematics. April 1999. 30 April 2007. http://www-history.mcs.st-andrews.ac.uk/Biographies/Kepler.html.
7. "Ancient Egyptian Astronomy" Ancient Egypt. 1 August 2001. 30 April 2007. http://aldokkan.com/science/astronomy.htp.
8. "Mayan Astronomy" 22 July 2005. 20 April 2007. michielb.nl/projects/maya astronomy/astronomy
9. "Newton's Law of Universal Gravitation" Wikimedia Foundation, Inc. 20 June 2007. 22 June 2007. http://en.wikipedia.org/wiki/Newton's_law_of_universal_gravitation.
10. "Ellipse/Proofs." Wikipedia. Wikipedia,2007. Answers.com 22 Jun. 2007.