crepusculum

Crepusculum is Latin for twilight, of uncertain ultimate origin — possibly related to creper (dark, obscure). The adjective crepuscularis meant 'of or relating to twilight.' English borrowed 'crepuscular' in the seventeenth century as a scientific term for animals active during the low-light periods at dawn and dusk, when neither daylight predators nor nighttime predators are fully active.

The crepuscular niche is a survival strategy. By being active when neither diurnal nor nocturnal predators are hunting at full capacity, crepuscular animals exploit a window of relative safety. Deer, rabbits, cats (domestic and wild), many species of mosquito, and most bat species are crepuscular. The strategy works because twilight light levels confuse visual predators adapted to either full daylight or full darkness.

The word has a literary life beyond biology. Crepuscular light, crepuscular atmosphere, crepuscular rays (the beams of sunlight that break through clouds near the horizon) — the word describes a quality of light that painters and photographers prize. The golden hour in photography is the crepuscular period. Turner, Monet, and the Impressionists built their visual language around crepuscular light.

Crepuscular is one of those English words that people collect. It appears on lists of 'most beautiful words,' alongside 'petrichor,' 'ephemeral,' and 'luminous.' The word sounds like what it means — the soft 'cr' opening, the liquid middle, the trailing '-ular' suffix. It is an argument for Latin in English: sometimes the borrowed word is more beautiful than any native one could be.