Use your words

I’m trying to think of a word that I’m not sure even exists. It describes the phenomenon of two occurrences—two blinking lights, two beeping sounds, etc.—that each happen at regular but different intervals. Because of the difference between these intervals, the two occurrences will seem to happen in unison, then gradually fall out of sync before again returning to a synchronous instance. For example, blinking light ‘A’ flashes every 9 seconds and light ‘B’ flashes every 10 seconds. At various points in time (in this case, every 90 seconds), both lights will flash simultaneously; but at the next flash, light ‘A’ will flash one second sooner than ‘B’. For a time, the amount of time between light ‘A’ and light ‘B’ will increase with each flash, until the flashes are substantially opposed—light ‘A’ will flash, 5 seconds later light ‘B’ will flash, and 4 seconds after that light ‘A’ will flash again. The flashes will then occur with successively diminishing amounts of time between them until they once again flash in unison.

You’ll see examples of this with those battery-powered yellow construction lights, lights on antennae or the spires of two different buildings (and sometimes the same building), two faucets dripping, the back-up-beeping noises of construction equipment, two people dribbling basketballs, etc. Any two things that happen regularly but at different periods will come in and out of sync.

It seems that there must be a word for something as common as this, but I’ve Googled all sorts of variations on this description without any satisfaction. The closest I can get is “phase synchronization”, which I’m not happy with. There is a rather interesting conversation on the subject at ask.metafilter.

catenary

Another favorite obscure word for a rather familiar phenomenon is “catenary“.

In mathematics, the catenary is the shape of a hanging flexible chain or cable when supported at its ends and acted upon by a uniform gravitational force (its own weight). The chain is steepest near the points of suspension because this part of the chain has the most weight pulling down on it. Toward the bottom, the slope of the chain decreases because the chain is supporting less weight.

The catenary is the ideal form for an arch which supports only itself. If made of individual elements whose contacting surfaces are perpendicular to the curve of the arch, no significant shear forces will be present at the joints, and the thrust into the ground will be directly along the line of the arch. The Gateway Arch in Saint Louis, Missouri, United States follows the form of an inverted catenary.
Wikipedia

Catenary curves are not to be confused with the arc formed by the cable of a suspension bridge; that is a parabolic curve.