Essay:Sources and references
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We want to provide a trustworthy encyclopedia. It's a good idea to let the reader know where we got the information, so they can decide how reliable that source is. NASA is a good source for astronomy and space travel. The United Nations may or may not be a reliable source of economic, political or scientific data.
Whatever federal information the U.S. government creates and releases into the public domain is really our stuff (as taxpayers). There is no law requiring credit to be given for copying public domain writing; by definition it is "ownerless" (public) property.
However, it is usually a good service to our readers, if we cite the agency and/or author of public domain works. If only because it makes the article more trustworthy. But also because it gives the reader a hint of where to go for more information.