Throughout most of the solar system, the Sun sets and — wham! — like a power failure, there’s instant blackness. In our neck of the woods, Mars almost succeeds, but its thin air can’t produce a palette of saturated colors. That leaves us to ponder the phenomenon, along with bats and other crepuscular characters that cleverly avoid both day and night predators.
“Crepuscular” is a wonderful word that means “pertaining to twilight.” It can describe Mercury, mosquitoes, or those fantasyland beams streaming out of the sunset, called “crepuscular rays.” It has such a great sound, I say “crepuscular” as often as possible, even when it’s inappropriate.
The fun starts with the Sun balanced atop the horizon, where its rays traverse 13 times more air than when it’s high up. This causes nearly all of its blue light to scatter out, leaving only the longer wavelengths. That’s why the setting Sun is orange. Even rainbows just before sunset have little or no blue.
Next, the Sun’s disk transits the horizon, which takes 2 minutes in the tropics and 3 for nearly everyone else, thanks to its angled path in temperate countries.
Then comes twilight, which sounds vague, but is actually very precisely defined. And there’s really not one, but three.
Civil twilight begins at sunset and is the period of intense colors. Rayleigh scattering of solar wavelengths through the variety of low air thicknesses near the skyline causes these colors. Civil twilight officially ends when the Sun has plunged 6° below the horizon, equal to 12 times its own width. That’s when streetlights must be on, according to most municipal ordinances.
Astronomical twilight continues still longer, until the Sun falls 18° below the horizon, letting the faintest stars emerge. Its conclusion heralds the arrival of full darkness, when toes stub and scopes swing toward galaxies.
Twilight’s duration is expressed in degrees instead of time for good reason: Its length varies. Depending on the season and latitude, civil twilight can expire in less than half an hour or linger through the night. It’s always shortest in the tropics, where 24 minutes is typical. A shame: Swaying palms go nicely with twilight’s mild air and vivid colors. From Seattle, civil twilight averages about 36 minutes. In Fairbanks, Alaska, it’s a full hour around now, and it simply never ends after sunsets between mid-May and the end of July. Those arriving then and hoping to see aurorae are wasting their time.
Thanks to twilight’s slow pace, we rarely sense how dim the light has become. Photochemical changes in the eyes, pupil dilation from 2.5 to nearly 7mm, and the shift to scotopic rod-cell vision make the transition so smooth, we’d never guess that ambient illumination during early astronomical twilight has grown 500,000 times dimmer than when the Sun was shining. In modern times, of course, artificial lights have modified the passage to night. Urban eyes rarely experience full darkness. Major cities offer constant illumination resembling mid-nautical twilight, and the Milky Way has long been reduced to a staged curiosity that draws appreciative gasps at planetaria.
Real twilight, fortunately, is far from extinct, and it plays host to marvels not seen any other time. The most reliable spectacle of civil twilight is the twilight wedge —Earth’s shadow as a striking gray-blue band atop the eastern horizon just after sunset. Other goodies include crepuscular rays, antisolar rays — which converge just above the horizon opposite the newly vanished Sun — and the appearance of Venus and Mercury.
Nautical twilight is the realm of emerging Earth satellites and the brighter stars. Finally, the darkest part of astronomical twilight is when the legendary zodiacal light lights up the ecliptic near the horizon. Its eerie glow is widest at the bottom, narrowest on top. This month, the zodiacal light is best seen before dawn in the Northern Hemisphere, and after sunset in the southern.
In morning twilight, we’re presented with a wonderful chance to glimpse Mercury, whose best before-dawn appearance of 2007 begins the last days of October and continues for 3 weeks. Morning twilight is also now home to Venus, which in early October shines at its brightest of the year at magnitude –4.5. Count on numerous UFO reports by sleepy characters in the wee hours misidentifying Venus, which won’t be this conspicuous again until 2015.
They’ll think they’ve seen something from the twilight zone. And they’ll be right.
