Източници
http://astronomy4all.com http://calgary.rasc.ca/howfast.htm The RASC Calgary Centre - How Fast Are We Moving?
http://www.cosmos.1.bg
http://www.teachersdomain.org
http://iopscience.iop.org
http://spacefellowship.com
http://vselenata.net
http://www.binaryresearchinstitute.org
http://www.see.leeds.ac.uk
