Twilight. Gloaming. Dusk. Blue Hour — all names for that colorful and contemplative time between day and night. We explore twilight's brief but fascinating sights and learn why it gets shorter as summer turns to fall.
Twilight takes us gently into that good night. I wouldn't mind spending time on the Moon, but I'd miss dusk and dawn. Many of us cherish the interval after sunset or before sunrise as an opportunity for contemplation. Evening twilight may bring a welcome relief from a hot summer day or anticipation of a clear night for skywatching. At dawn, the world comes back to life, starting from stillness and ending with sunrise and the sound of traffic.
Darkness comes earlier now than it did in June and July. Not only have sunset and sunrise times conspired to extend the night at the expense of day, but twilights are shorter, too. Back in June, twilight lingered nearly three hours, leaving a skimpy three hours of darkness before the start of dawn.
That's since been shaved down to 1 hour and 51 minutes at my latitude and will reach a minimum of 1 hour and 42 minutes in early September.
Twilight comes in three different flavors depending upon the Sun's altitude below the horizon:
Civil twilight begins at sunset and ends when the center of the Sun is 6° below the horizon. There’s still enough sunlight reflecting off the atmosphere to see your way around and recognize faces and landmarks. The crescent Moon and Venus first appear in the west (when visible). Assuming the evening sky is free of haze, scattered light from the just-set Sun colors the western horizon bright yellow, which deepens to orange and orange-red as the minutes pass.
In the east, Earth's dusky shadow rises, topped by a pink sash known as the Belt of Venus. Because sunlight must travel a long, horizontal path through the thickest part of the atmosphere at and after sunset, the great density of air molecules and aerosols scatter away all the violets and blues.
Reds, oranges, and yellows remain to liven the sky and turn the underbellies of higher clouds vivid scarlet and pink. Lower clouds are rarely illuminated with the intensity of higher cloud formations because sunlight is greatly attenuated by dust and haze in the lowest part of the atmosphere during its downward journey. Be sure to watch for the "purple light" about 20-30° above the western horizon about 30 minutes after sunset; it's caused by sunlight reflected from aerosols in the lower stratosphere mingling with the still-blue sky. Extra amounts of dust in the atmosphere from volcanic eruptions and dust storms can intensify the effect.
Nautical twilight spans the time when the center of sun is between 6° and 12° below the horizon, or about 45-60 minutes after sunset. The horizon is indistinct but still visible, and brighter stars like Vega and Arcturus make their appearance. Sailors can still discern the horizon while using the stars to determine their position at sea, hence the name.
While warmer colors rule throughout much of dusk, you'll notice a surprising sight near the end of nautical twilight — a layer of pale green merging with the deepening blue a short distance above the western horizon.
By the end of nautical twilight, the western sky is still faintly aglow, but many constellations and even the summer-fall Milky Way are weakly visible. True night with no trace of sun-touched atmosphere begins when Sun's center reaches 18° below the horizon. This marks the end of astronomical twilight. From a dark-sky site, stars of magnitude +6 (or fainter) are visible, and we can begin looking for our favorite faint fuzzies without crepuscular compromise. Before dawn, each interval plays in reverse until the moment of sunrise.
Why Twilight Length Varies
Twilight length depends on the Sun's distance below the horizon; the faster it plummets to –18°, the briefer the twilight. The shortest twilights are found at the equator (1 hour and 10 minutes), where the Sun’s path is nearly always vertical to the horizon, and longest at the poles, where its path is nearly parallel to the horizon. In the far north, twilight lingers six weeks before the annual sunrise on the first day of spring and for six weeks again after sunset on the fall equinox. For mid-latitudes it falls in between the two extremes.
Let's look at the Sun's path below the horizon to get a clearer pictures of twilight's changing length. The Sun’s position in the sky is defined by its declination (the celestial equivalent of latitude) and right ascension (similar to longitude). After sunset, the Sun continues to move along its line of declination. At the equator, those lines are perpendicular (or nearly so) to the horizon; once the Sun sets, it quickly sinks to –18° and twilight ends.
Away from the equator declination lines intersect the horizon at a shallower angle. To reach the required –18°, the Earth must rotate a little longer to "move" the Sun far enough below the horizon for night to begin. That means a longer twilight.
From mid-latitudes, when the summer Sun reaches its maximum northerly declination, the lines of declination curve upward below the horizon. This lessens or flattens the angle of the sun as it travels below the horizon, increasing the time it takes to dip to –18° (twilight’s end).
In winter, the declination lines curve downward below the horizon at a steeper angle, sending the sun fleeing into the night faster than during the summer and putting an end to twilight in about an hour and a half. In late August, the lines of declination are beginning their downward trend with an ever-shrinking twilight the result.
While I cherish the peace and transition twilight brings, I'm always happy to see it shorten up in late summer. Night sky observing can begin earlier, which means a better night's sleep!