<p><strong>The <a href="http://news.nationalgeographic.com/news/2013/03/031314-pi-day-exploratorium-mathematics-pie-science/">celebration of Pi Day</a> raises the age-old question of whether perfect circles exist in the universe. </strong></p><p>A look at nature's stunning circular creations, like the remarkable rings of Saturn seen here, seems to make a compelling argument that Greek philosopher Plato was mistaken when he first observed that no one has ever seen a perfect circle—only imperfect approximations.</p><p>But <a href="http://ww2.odu.edu/~jadam/index.html">John Adam</a>, a mathematics professor at Old Dominion University and the author of <a href="http://press.princeton.edu/titles/7686.html"><em>Mathematics in Nature: Modeling Patterns in the Natural World</em></a>, said that no perfect circle can occur in nature "since a perfect circle is a geometrical idealization."</p><p>Though Saturn's rings appear to be circular thanks to the&nbsp;<a href="http://saturn.jpl.nasa.gov/faq/FAQSaturn/index.cfm#q13">shepherd moons</a> orbiting the planet, parts of the rings are bent by the pull of gravity from its other moons.</p><p>Here are some of nature's other (seemingly) perfect circles.</p><p><em>—Linda Poon</em></p>