Have you ever wondered, as dusk settles in, precisely “What Time Will It Get Dark Tonight?” It’s a question that blends everyday curiosity with a touch of astronomical intrigue. For stargazers and night sky enthusiasts, knowing when true darkness descends is crucial for planning observations. Head out too early, and the fainter celestial wonders will remain hidden. But how do you determine the perfect moment?
If you’re seeking a quick estimate for the contiguous United States, here’s a general guideline:
- Southern States: Expect darkness to fall approximately 70 minutes after sunset.
- Northern States: Darkness arrives a bit later, usually around 100 minutes (1 hour and 40 minutes) after sunset.
However, this is a simplified answer. The exact timing varies depending on your specific latitude, the season, and even your personal definition of “dark.”
To truly understand when night begins, we need to delve into the fascinating world of twilight and the science behind how our sky transitions from day to night. Let’s explore the different stages of twilight and the factors that influence when darkness finally blankets the sky.
Decoding the Twilight Spectrum: Civil, Nautical, and Astronomical
Twilight isn’t a single event, but rather a series of phases, each marked by the sun’s position below the horizon. Astronomers have categorized twilight into three distinct stages, each with unique characteristics:
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Civil Twilight: This is the brightest phase of twilight, commencing at sunset (when the sun’s center dips below 0° elevation) and extending until the sun is 6° below the horizon.
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Nautical Twilight: As civil twilight fades, nautical twilight begins. This phase lasts from when the sun is 6° below the horizon until it reaches 12° below the horizon.
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Astronomical Twilight: The final and darkest stage of twilight, astronomical twilight starts when the sun is 12° below the horizon and concludes when it reaches 18° below the horizon.
Once the sun descends more than 18° below the horizon, astronomical twilight ends, and true night begins. At this point, the sun’s light no longer directly illuminates your location, creating optimal conditions for stargazing and night sky photography.
Experiencing Twilight: What Can You See?
Each twilight phase offers a different experience, both on the ground and in the sky above. Without precise instruments to measure the sun’s position, we can use observable conditions to distinguish between these twilight stages:
Civil Twilight: Still Plenty of Light
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Ground Level: During civil twilight, the world remains easily visible without artificial light. You can comfortably read outdoors, for instance. As this phase progresses, you’ll notice the need for car headlights and the illumination of streetlights.
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Sky View: The sky is still bright, and only the most luminous celestial objects become visible. Planets like Venus and Mercury might make their appearance during this time.
Nautical Twilight: Shadows Lengthen, Stars Emerge
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Ground Level: Light diminishes noticeably during nautical twilight. Objects become increasingly difficult to distinguish, appearing as silhouettes against the fading light. Outdoor activities require artificial illumination. Color vision wanes as light levels drop, and the world begins to take on shades of gray.
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Sky View: Stars begin to emerge in greater numbers, and constellations become clearly discernible. The sky itself deepens to a dark blue, transitioning to black near the horizon opposite the sunset.
Astronomical Twilight: Approaching True Darkness
Night: The Full Canvas of Stars
For most casual observers, the difference between astronomical twilight and night is subtle, primarily noticeable as a slight residual sky glow near the sunset horizon.
Once night arrives, the limitations on visible celestial objects are lifted (except for interference from the Moon). The full splendor of the night sky, with fainter stars and deep-sky objects, becomes accessible for observation.
It’s important to remember that these twilight phases are symmetrical. As sunrise approaches, night transitions back through astronomical, nautical, and civil twilight, culminating in the dawn.
Why Darkness Delays Its Arrival After Sunset
It’s a natural question to ask: “If the sun has set, why isn’t it instantly dark?”
During civil twilight, even after sunset, ambient light allows you to see clearly without artificial illumination.
Imagine a flat Earth, as ancient cultures once conceived. If the sun merely dipped below the edge of this disc-shaped world, darkness would indeed descend instantaneously.
However, our reality is different. Earth is a sphere enveloped by a substantial atmosphere, and the sun resides approximately 93 million miles away. When the sun sets below your horizon, it continues to illuminate the atmosphere high above the Earth’s surface.
This phenomenon is readily apparent from an airplane at dusk or dawn. While the sun has set for those on the ground, passengers at high altitude can still witness sunlight. The sunlight interacts with the atmosphere, which is composed of gas molecules like oxygen and nitrogen. These molecules cause sunlight to scatter in various directions. This scattered sunlight reaches our eyes, providing illumination even after the sun has physically disappeared below the horizon.
This scattering of sunlight within the atmosphere after sunset is the very reason we experience the three distinct phases of twilight.
The amount of atmosphere illuminated by the sun decreases as the sun dips further below the horizon, leading to the progression of twilight phases. (Source: Wikipedia)
- At sunset, the entire atmosphere above you is still illuminated by the sun. However, the amount of lit atmosphere rapidly decreases as the sun descends.
- By the end of civil twilight (sun 6° below the horizon), the atmosphere directly overhead (at the zenith) is no longer sunlit. However, about a third of the atmosphere above the horizon remains illuminated, contributing to the lighter blue skies we observe.
- At the onset of astronomical twilight (sun 12° below the horizon), only the highest atmospheric layers near the sunset horizon are still catching sunlight.
- When night commences (sun 18° below the horizon), the atmosphere we observe is no longer illuminated by the sun.
With a clearer understanding of twilight and its causes, let’s revisit our initial question:
Calculating Darkness: Latitude and Seasonal Influences
Two primary factors govern how long it takes to get dark after sunset:
- Latitude: The closer you are to the equator, the faster darkness descends after sunset.
- Season: Darkness arrives slightly later in summer compared to winter, and in extreme northern latitudes, true darkness may not occur at all during summer months.
Latitude’s Role in Twilight Duration
At the equator, the sun’s path across the sky appears almost vertical. It rises directly in the east, reaches its zenith (highest point) overhead at midday, and sets precisely in the west. The sun’s descent below the horizon is also nearly perpendicular. This steep angle means the sun quickly passes through the 6°, 12°, and 18° twilight thresholds.
Conversely, closer to the poles, the sun’s trajectory across the sky is shallower. It rises in the northeast (in the Northern Hemisphere), arcs across the sky at a lower angle, never reaching directly overhead, and sets in the northwest. This more oblique angle means the sun takes a longer, more gradual path to reach 6°, 12°, and 18° below the horizon, thus extending the duration of twilight.
Visualizing this can be challenging, but resources like this explanation from Cornell University offer insightful perspectives. Let’s illustrate this with a concrete example.
Twilight Times Across Latitudes: An Equinox Comparison
Consider the March equinox (around March 20th), when day length is nearly uniform across the globe. The table below compares the time elapsed from sunset to nightfall for various cities at different latitudes: Quito, Ecuador (near the equator), Key West, Florida (southern US), Kansas City, Kansas (central US), and Anchorage, Alaska (northern US).
| City | Civil Twilight (Sunset) | Nautical Twilight | Astronomical Twilight | True Night |
|---|---|---|---|---|
| Quito, Ecuador | 18:25 | 18:45 (+20 mins) | 19:09 (+44 mins) | 19:33 (+68 mins) |
| Key West, Florida | 19:39 | 20:02 (+23 mins) | 20:28 (+51 mins) | 20:55 (+76 mins) |
| Kansas City, Kansas | 19:32 | 19:58 (+26 mins) | 20:30 (+58 mins) | 21:02 (+90 mins) |
| Anchorage, Alaska | 20:18 | 21:02 (+44 mins) | 21:54 (+96 mins) | 22:53 (+155 mins) |
As the table clearly demonstrates, darkness arrives much quicker at the equator (Quito) than even in Key West. Moving further north, the time to darkness progressively increases. Nautical twilight begins just 20 minutes after sunset in Quito, compared to 23 minutes in Key West, 26 minutes in Kansas City, and a significantly longer 44 minutes in Anchorage – all on the same day.
When considering “true night” as our benchmark for complete darkness, the differences become even more pronounced. In Quito, true night arrives a little over an hour (68 minutes) after sunset at the spring equinox. Moving north, this duration extends to 76 minutes in Key West, 90 minutes in Kansas City, and a remarkable two and a half hours in Anchorage.
Seasonal Variations in Twilight Duration
While latitude is the dominant factor, the season also subtly influences twilight duration. For any given location, twilight tends to be slightly longer closer to the summer solstice. However, this seasonal difference is relatively small, amounting to only a few minutes in the mainland USA.
The most dramatic seasonal impact on darkness is experienced in far northern latitudes. In regions sufficiently far north, there are periods during the summer when true night simply doesn’t occur.
Alaska provides a prime example. For much of June, the sun never descends more than 6° below the horizon. Consequently, it remains in civil twilight throughout the night, offering enough ambient light to read outdoors even at midnight!
While the North Pole experiences 24-hour daylight in summer, even in the contiguous US, the summer solstice affects twilight duration. Unless you reside in the extreme northernmost reaches of the country (within about 15 miles of the Canadian border), you will still experience true astronomical night every day of the year.
Even Seattle, the northernmost major city in the continental US, enjoys approximately 2 hours of true night on the longest day of the year. However, just a short distance north, in Bellingham, Washington, a different scenario unfolds. For about two weeks in mid-June, astronomical twilight persists throughout the night, transitioning directly into morning twilight without a period of true darkness.
Darkness After Sunset: A Recap
So, to answer the question, “what time will it get dark tonight?”
For the contiguous 48 states, expect darkness to arrive roughly 70 to 100 minutes after sunset. The further north you are, the longer the twilight period and the later true darkness sets in. If you live near the Canadian border, be aware that mid-June might bring nights without complete darkness. However, across most of the US, you can count on experiencing true night every night of the year.
To pinpoint the twilight times for your specific location and date, utilize resources like Time and Date’s sunrise and sunset calculator. This tool will provide precise timings for civil, nautical, and astronomical twilight, allowing you to accurately determine “what time will it get dark tonight” and plan your stargazing or evening activities accordingly.
*Excluding Alaska, where twilight durations are significantly longer, and true night may be absent during June.
