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.
Though Saturn's rings appear to be circular thanks to the shepherd moons orbiting the planet, parts of the rings are bent by the pull of gravity from its other moons.
Here are some of nature's other (seemingly) perfect circles.
Image courtesy SSI/NASA
A Circular Arrangement
In the cross section of a plant stem, the vascular tissues that conduct water and support the plant are arranged in circles.
Adam noted that in nature, many things form circularly because it is the most efficient way to minimize or maximize specific processes under certain constraints. In mathematics, he said, a circle allows for the greatest area for any given perimeter and the least perimeter for any given area, compared to other polygons.
"Obviously, the particular circumstances dictate what other forces or organizing constraints may be present, and that will in turn dictate the shape of the pattern," he added.
Photograph by Greg Dale, National Geographic
The Eyes (Almost) Have It
A closeup of a human eye reveals that the pupil is almost a perfect circle.
Humans, like other vertebrates that are mainly active during the daytime, have round pupils that are designed to be effective in bright light. Nocturnal animals, meanwhile, have slit pupils to protect their big lenses and sensitive retinas, which allow them to hunt during the night.
But pupil shapes go beyond just round and slit. Some vertebrates' pupils are shaped like a heart or a crescent, while others' are shaped like a keyhole.
Photograph by Suren Manvelyan, Visuals Unlimited
A (Near) Perfect Arc
The arc of a rainbow, according to Adam, is the second closest thing to a perfect circle in nature, after the ripple (see next photo). But the arc can never appear to exceed a semicircle unless a person views it from the height of a raincloud with the sun behind him or her.
The arc forms when sunlight passes through falling raindrops, and the the different wavelengths of the light bends at different angles—with red light bending most and violet light bending least. The refracting light appears to beam at different places in the sky, coming together to create an arc.
Photograph by Ralph Lee Hopkins, National Geographic
Nature's Best Circle: The Ripple
"The closest thing to perfect circles in nature is probably circular wave patterns on a pond or puddle after a raindrop falls on it," Adam said.
Even when a noncircular object is thrown into water, the resulting ripples will form in concentric circles. Initially, the ripples expand accordingly to the object's shape. But as they spread farther outward, curves fill in the uneven spaces, leaving the ripples to "morph" into circular shapes.
Photograph from Ocean/Corbis
Photographer Joel Sartore captures an up-close image of the 200 to 300 fertile, dark brown disks located at the center of a black-eyed Susan.
As a composite flower, the black-eyed Susan is actually a plant within a group of plants. On the outer layer, each individual petal is a ray flower, and the dark center is essentially a cluster of small disk flowers that produce seeds.
Photograph by Joel Sartore, National Geographic
When scientists first found the desert grasslands of Namibia dotted with tens of thousands of grassless spots, they were baffled by the strange patterns and their peculiar behavior.
While the spots, known as fairy circles, typically last 24 to 75 years before vanishing as vegetation grows back, the lifespan of the largest spots can be much longer, according to the New York Times.
The size of these naturally occurring patches, which reveal Namibia's red soil, range from 6 to 30 feet (2 to 9 meters). At the perimeter, grass tends to grow taller, further emphasizing the presence of these bizarre circles.
Photograph by George Steinmetz, National Geographic