Tiptoe Into The Twilight Zone

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.

Gift at day's end

Clouds glow an intense pink in early twilight. 
Bob King

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:

Arriving at night by degrees

Twilight is the time when the Sun illuminates the lower atmosphere after sunset or before sunrise. It's divided into three varieties that depend on how far the Sun is below the horizon.
T. W. Carlson / Wikipedia

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.

Shadowed twilight

Earth's shadow topped by the Belt of Venus and the full Moon. Our planet's shadow is visible any clear evening "rising" in the east just after sunset or setting in the west shortly before sunrise.
Bob King

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.

How twilight stacks up

Layers of twilight seen in late civil and the first half of nautical twilight.
Bob King

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.

Splayed rays opposite the Sun

Sunlight shining from below the horizon between gaps in clouds (left) can create spectacular displays of crepuscular rays in the sunset direction that can extend clear across the sky to the opposite horizon, like the anti-crepuscular rays shown here. Their convergence is an optical illusion — the rays are really parallel, as seen in a photo taken from the International Space Station (right).
Bob King / NASA

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.

Purple pulchritude

The purple light in the western sky during late civil twilight. Although not always visible, the color is can be very luminous and a thrill to the eye.
Bob King

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.

Zippy equatorial twilights

Looking west at sunset in late August from a location on the equator. The Sun meets the western horizon and proceeds straight down, following its line of declination (vertical lines). After little more than an hour, it reaches –18 degrees below the horizon and true night begins.
Chris Marriott’s SkyMap

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.

Summer Sun slow to depart

At mid-latitudes in summer, lines of declination intersect the horizon at a shallow angle with the Sun's path nearly parallel to the horizon. The Earth must spin a while longer before the sun reaches a point –18° below the horizon, increasing the length of twilight by about an hour compared to winter (see below).
Chris Marriott's SkyMap

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.

Winter Sun's on the run

In winter, the sun’s path follows the more strongly downward-curving declination lines and reaches –18° much faster than in the summer, yielding shorter twilights.
Chris Marriott's SkyMap

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!

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Astronomy Blogs, Explore the Night with Bob King, Observing
Bob King

About Bob King

Amateur astronomer since childhood and long-time member of the American Association of Variable Star Observers (AAVSO), Bob King also teaches community education astronomy and writes the blog Astro Bob. The universe invites us on an adventure every single night. All we need do is look up. Check out my forthcoming book "Night Sky with the Naked Eye" (on Amazon and BN) about all the great things you can see at night without any special equipment.

3 thoughts on “Tiptoe Into The Twilight Zone

  1. vgent

    Thanks for the nicely illustrated article.

    I note that the definition of nautical twilight in the diagram (from Wikipedia) and in the text contradict each other. The diagram agrees with the definition in nautical handbooks (such as Bowditch and Dutton) in placing the lighter limit at sunset (or sunrise) while the text claims that the lighter limit is at a solar depth of 6 degrees below the horizon.

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