Around and Around: Peri

peri (around) + metron (measure)
In geometry, the perimeter of a polygon is the sum of all its straight sides, while the circumference of a circle is diameter times pi. In general use, "perimeter" and "circumference" are synonyms, but "perimeter" is also used by the military to describe the "boundary of a defended position."

peri (around) + pherein (to carry) + y (suffix forming abstract or collective nouns)
In the past, the periphery covered the atmosphere around the earth; then its boundary got narrowed to the circumference or line around a circular body; now, it is similar to a perimeter, but its focus is outside rather than inside a boundary.

peri (around) + pherein (to carry) + y (suffix forming abstract or collective nouns) + al (suffix forming adjectives)
When not used as a noun to connect to computers, "peripheral" is an adjective that has both a literal and figurative meaning:
1) on or near an edge or constituting an outer boundary
2) related to the key issue but not of central importance

peri (around) + scope (suffix meaning "an instrument for seeing")
In the 15th century, Gutenberg marketed a periscope that allowed pilgrims at a religious festival to see over the crowd. Hundreds of years later, periscopes became parts of submarines, tanks, and rifles. The proper noun Periscope refers to an app whose goal, according to the designer, is "to see the world Live through someone else's eyes."

peri (around) + hodos (a going, way, journey) + ic (suffix forming adjectives)
In chemistry, the periodic table was so-named because it grouped the elements according to similar properties that recur. In biology, a woman's period usually recurs in a periodic cycle that's monthly. In grammar, a period periodically, but not always, marks the end of one description of events or interval of time and leads to the beginning of another.

peri (around) + patein (to walk) + ic (suffix forming adjectives)
To the Greeks, the word had a more philosophical sense: Aristotle used to walk around the gardens while he was teaching; thus, "peripatetic" was used as both an adjective and noun to refer to any Aristotelian ideas or disciples.

peri (around) + piptein (to fall)
The English spelling of the word is "peripety" but the idea is used most often in discussing or comparing to Greek tragedies. According to Aristotle, in order for peripeteia to be the most powerful part of a plot, it should be
1) within the rule of necessity or probability
2) accompanied by recognition or discovery of something previously unknown
3) a cause of changes within the character

peri (around) + stalsis (checking, constriction)
After food is chewed into a ball, it is swallowed and moved down through the esophagus into the stomach. In a peristaltic wave, the muscles contract and relax to move the ball along. This movement is in one direction only; thus, peristalsis is not connected to vomiting, which is caused by contraction of the abdominal muscles.

peri (around) + geo (earth)
A supermoon is technically called a perigee-syzygy, because it is an alignment of the earth, moon (when it is full), and sun that makes the moon seem huge. Some people believe a supermoon will lead to strange transformations or events on earth, but the most probable effect will be on the range of low and high ocean tides. The opposite of "perigee" is "apogee" ("apo" means "off, away from"), and an apogee-syzygy is also called a micromoon.