A spherical cow is a humorous metaphor for highly simplified scientific models of complex real life phenomena. The implication is that theoretical physicists will often reduce a problem to the simplest form they can imagine in order to make calculations more feasible, even though such simplification may hinder the model's application to reality.
The phrase comes from a joke that spoofs the simplifying assumptions that are sometimes used in theoretical physics. 
Milk production at a dairy farm was low, so the farmer wrote to the local university, asking for help from academia. A multidisciplinary team of professors was assembled, headed by a theoretical physicist, and two weeks of intensive on-site investigation took place. The scholars then returned to the university, notebooks crammed with data, where the task of writing the report was left to the team leader. Shortly thereafter the physicist returned to the farm, saying to the farmer, "I have the solution, but it works only in the case of spherical cows in a vacuum".
It is told in many variants, including a spherical horse in a vacuum, from a joke about a physicist who said he could predict the winner of any horse race, provided it involved perfectly elastic spherical horses moving through a vacuum. An early 1973 version was based on a chicken but there are many variations.
In an episode of the sitcom The Big Bang Theory, the joke is told by Dr. Leonard Hofstadter with a slight variation, the punchline mentioning "spherical chickens in a vacuum". Spherical chickens can be further traced back to a letter to the editor of the journal Science titled A Spherical Chicken from 1973.
In the 1997 British Channel Four series Brass Eye, host Chris Morris distributed flyers and got public opinion on the use of spherical cows as food, implying they are secretly being sold in British shops. Response was negative, but nobody shown on the program questioned the reality of spherical cows' existence.
- Shelton, Robin; Cliffe, J. Allie. [http://web.archive.org/web/19991009000912/http://lheawww.gsfc.nasa.gov/docs/xray/research/snrs/spherical_cow.html
- "The Sacred Spherical Cows of Physics"
- Washington Post: "The Coase Theorem"
- Kirkman, T. W. (1996). "Spherical Cow: A Simple Model". Statistics to Use. Retrieved 2007-02-19.
- Hefley, Bill; Hefley, William E.; Murphy, Wendy (1 February 2008). Service science, management and engineering: education for the 21st century. Springer. p. 80. ISBN 978-0-387-76577-8. Retrieved 28 September 2011.
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- Huva, Amy. "When Nerds go Viral". Vancouver Observer. Retrieved 6 May 2014.
- "Consider a Spherical Cow" University Science Books
- "Fedora 18 Is Codenamed The Spherical Cow". phoronix.com. 2012. Retrieved 2012-05-11.
- NASA:Exploration of the Universe Division - Supernova models as spherical cows
- Hubble Heritage Gallery Page: related history from Space Telescope Institute