Western Slope Skies - The ThreeTwilights
If you are the average person, the most beautiful time of day is sunrise or sunset— resplendent, dazzling, the stuff of innumerable poems, paintings, and beachside vacation photos. But my favorite time of day is twilight, when light and shadow play in their own entrancing drama.
Technically, twilight happens when the Sun is between zero and eighteen degrees below the horizon, due to the Earth’s rotation. This range is evenly divided into three phases-- civil, nautical, and astronomical. We experience three twilights twice daily - in this order in evening, in reverse order in morning. Evening twilight is called dusk, morning twilight, dawn. Let’s look closer at each phase, in dusk.
Civil dusk is brightest, the Sun within six degrees under the horizon. You can easily see, travel, and perform other civil activities outdoors. The phenomenon of Rayleigh scattering reigns, where the atmosphere differentially deflects various wavelengths of sunlight. Hence, coppery sky and amber afterglow in the west, and the salmon pink Belt of Venus in the east. The Belt heralds Earth’s encroaching shadow, visible in hazy violet ascending beneath. Only the brightest stars, planets, and the illuminated Moon are apparent.
Nautical dusk follows, as the Sun slips between six and twelve degrees sub-horizon. Sunlight has considerably dimmed, and artificial lighting will be desirable outdoors. The horizon grows less distinct; in traditional seafaring, accurate sight navigation becomes trickier (the nautical sense of this twilight). The western sky fades in scarlet and ochre, and eastward Earth’s shadow climbs higher. Cerulean light envelops you, signaling the legendary blue hour prized by visual artists for its tranquil, pensive tone. Actually only 20-30 minutes long, blue hour is caused by Chappuis absorption of most sunlight wavelengths by ozone. The starry host gradually strengthens enough to make out familiar constellations.
Astronomical dusk takes stage, once the Sun descends below twelve degrees. Sunlight is nearly extinguished. Artificial lighting now becomes a mixed blessing— providing safety and security on the ground, but producing yellowy skyglow in the lower atmosphere. A pervasive form of light pollution, skyglow can easily obscure the sky, confounding celestial observation and disorienting nocturnal wildlife. Fortunately, it can be mitigated (e.g., hoods over streetlamps), and global advocacy exists for dark sky preservation. Under ideal seeing conditions, your naked eye can perceive objects as faint as the planet Uranus. However, you will want a telescope to spot nebulae, galaxies, and star clusters.
Officially, night falls when the Sun is beyond eighteen degrees sub-horizon. Dusk ends, the sky its darkest until dawn. Then twilight returns in reverse, ending night as daybreak nears.
Proverbially, there is nothing new under the Sun. But under twilight, there is ample natural splendor that never grows old.
Western Slope Skies is produced by the Black Canyon Astronomical Society and KVNF Community Radio. This feature was written and voiced by Michael T. Williams.