This image from my recent ride along the Bolinas Ridge captures the specific time of the evening — civil twilight — when ambient light takes on a lovely bluish hue prized by painters and photographers.
But twilight (both morning and evening) is divided, more scientifically, into civil, nautical, and astronomical segments based upon the number of degrees the center of the sun is below the horizon (6, 6-to-12, and 12-to-18 degrees respectively).
Evening nautical twilight (nautical dusk) is said to end once sea navigation via the horizon line is no longer possible. Evening astronomical twilight (astronomical dusk) ends once the dimmest celestial objects (e.g. galaxies and nebulae) become properly visible to astronomical observation.
Commentary: in the context of the virtual world of Texting, Tweeting, and Tagging (from which I claim no exception) it feels palpably iconoclastic (if not downright subversive) to patiently witness our solar system’s sun disappearing ever so gently beyond the planet’s watery horizon.
Featured Comment by Shoeless Joe: “There is a biological connection, too, as the rods and cones in our eyes perceive light differently at twilight. It is called the Purkinje Shift (or Purkinje Effect). Wikipedia has a good article on it. One of the beauties of cycling is that it affords many opportunities to “patiently witness” the world around us in a way that is unique to the sport.”