"Why is the Earth round?". Ask A Scientist. Argonne National Laboratory, Division of Educational Programs. Archived from the original on 21 September 2008.
Taylor, Barry N., ed. (2001). "The International System of Units (SI)". NIST Special Publication 330(PDF). United States Department of Commerce: National Institute of Standards and Technology. p. 29. Retrieved 2012-03-08.{{cite book}}: CS1 maint: publisher location (link)
"Why is the Earth round?". Ask A Scientist. Argonne National Laboratory, Division of Educational Programs. Archived from the original on 21 September 2008.