Pentagon Tool

Turning the pentagonal nut atop a fire hydrant requires a special tool. An ordinary wrench, which operates best on polygonal nuts with parallel sides, won't do as well.

For more, see "Fire Hydrant Pentagons."

Photos by I. Peterson

Market Packing

Hexagonal close packing of "spheres" at a market in Tblisi, Georgia.

The displayed produce at the market illustrates a variety of packing strategies, from random to close, depending on the size and shape of the items and other constraints.

Photos by I. Peterson

Triking on Square Wheels

A tricycle with square wheels on a circular track at the National Museum of Mathematics in New York City.

Note the Fibonacci spiral pattern at the track's center.

For more on square wheels, see "Riding on Square Wheels."

Photos by I. Peterson

Tide Cycles

A wonderful visualization of tide cycles at the Exploratorium in San Francisco.

Each acrylic slice is a hanging record of tide levels for one day, so you can easily pick out the peak of high tide and the valley of low tide. These peaks and valleys shift from day to day and month to month, as influenced by the moon's gravity (and, to a lesser extent, by the sun's gravity). So the timing of high and low tides reflects cycles in the motion of the moon and sun. The highest and lowest tides occur each month at the time of the new moon and the full moon.

The Exploratorium exhibit, titled "Tidal Ribbon," represents the tidal record for one year (2011), as measured at the NOAA tide station near the Golden Gate Bridge.

Photos by I. Peterson

Octahedral Moondog

Located in the National Gallery of Art's Sculpture Garden, Tony Smith's Moondog is based on a lattice of tetrahedral and octahedral components (15 stretched octahedra and 10 tetrahedra). As described by George Hart, you can think of the structure as part of a diamond crystal lattice, with the smaller tetrahedral faces (visible as equilateral triangles) representing carbon atoms and the remaining "struts" as carbon-carbon bonds.

Photos by I. Peterson

DC Pi

Sighting of pi on Wisconsin Avenue in Washington, D.C.

For more on pi sites, see "Pi Places" and "Sliding Pi in Toronto."

Photo by I. Peterson