After sunset, how long does it take before it's completely dark?
When does nightfall occur
Time from sunset to complete darkness is a very astute inquiry for an astronomer to ponder.
If you want to go outside and observe the night sky, you'll need to know when it will be sufficiently dark to do so. You won't get to see everything if you show up too soon, but when is it too soon to go?
Here's the short answer for our American readers:
The sky will be dark enough for stargazing.
- Southern states: 70 minutes after sundown
- Around 100 minutes (1 hour 40 minutes) after sunset, for the northern states.
Of course, that depends on the season, your latitude, and how dark you want it to get.
In that case, keep reading if you're curious about how long it takes for the sky to completely darken after sunset.
Just what is astronomical twilight, anyway?
Although astronomical twilight does exist, it is the third and darkest of the three twilight phases commonly discussed.
The depth of the sun's horizon below which each twilight phase occurs:
- From the time the sun goes down (when its center is at 0 degrees elevation) until it dips 6 degrees below the horizon, we are in a state of civil twilight.
- After the end of civil twilight, the sun will be 12 degrees below the horizon, ushering in the period known as "nautical twilight."
- The darkest and longest twilight of the year is astronomical twilight. At 12 degrees below the horizon, it starts, and at 18 degrees below, it ends.
When the sun has descended below the horizon by 18 degrees, we can safely call it night, and the darkness will be ideal for photographing the stars.
At Dusk, I Can See
Each of the three twilights has different levels of seeing, both down here on Earth and up in the night sky This simple guide will help you make an educated guess based on local conditions, as we have no way of knowing how far below the horizon the sun actually is.
- It is not necessary to turn on any additional lights in order to see clearly while standing on the ground; a book, for instance, can still be read. By the time this time period ends, it will be dark enough that car headlights must be turned on and streetlights will be turned on.
- Venus and Mercury, among the brightest stars, will be the only ones visible.
- The sun has set, and it's getting too dark to make out individual objects on the ground. Their form is more implied than shown. There needs to be more light so that outdoor activities can be performed. When there isn't enough light, your color vision fades and is replaced by grayscale.
- At this hour, the sky becomes dark enough to make out individual stars and constellations. The sun sets and the sky turns a deep blue that eventually becomes black at the horizon.
- We can all agree that it is night on the ground. Really, this night is pitch black. At the horizon closest to the sun, you may see the sky as dark blue or off-black; elsewhere, it will be black.
- Now that the sun has set, deep space objects can be seen in the sky (with one of these telescopes), though viewing conditions are slightly diminished close to the horizon.
For the majority of us, the transition from astronomical twilight to night is virtually imperceptible, with the exception of the very slight decrease in sky brightness at the horizon where the sun sets.
The sun no longer obscures the view (except, of course, when reflected off the Moon).
As we progress toward sunrise, the opposite is true. With the dawn comes the transition from night to astronomical twilight, then nautical twilight, and finally civilian twilight.
When the Sun Goes Down, Why Doesn't Night Fall Instantly?
An understandable concern has been raised.
When the sun goes down, why does it take so long for it to be completely dark?
When the sun goes down, why does it not instantly become night?
If the Earth were flat (as our forefathers thought), and the sun dipped below the horizon as it set, then it would be completely dark as soon as the sun went down.
But we don't
We live on a spherical planet with a thick atmosphere, approximately 93.7 million miles from the sun. The sun's rays continue to illuminate Earth's atmosphere even after they have dipped below the horizon.
Those who have ever taken a flight at sunrise or sunset can attest to this fact.
Let's say you and a friend are in an airplane and you're looking down at them from 30,000 feet. Although the sun will be below their horizon at sunset, you will still be able to see it above yours. Even though the sun is out, it does not illuminate the ground below you as you fly.
When air molecules like oxygen and nitrogen are hit by sunlight, they release their gases. In the presence of those gases, illumination is scattered. The world remains bright to our eyes because some of the scattered light reaches them, even though the sun has set.
There are three types of twilight because the sun's rays continue to illuminate the atmosphere even after they have dropped below the horizon.
What percentage of the atmosphere is still visible after the sun has set?
- At sunset, the entire upper atmosphere is still illuminated by the sun, but this amount gradually decreases throughout civil twilight.
- The end of civil twilight (i.e. e sun is 6 degrees below the horizon), the sky above us (the zenith) is no longer illuminated by the sun. Yet, we can see that the sky is lighter blue because the sun is still lighting up a third of the atmosphere above the horizon.
- The sun sets 12 degrees below the horizon, marking the beginning of astronomical twilight, by which time only the uppermost layers of the atmosphere are illuminated.
- At 18 degrees below the horizon, astronomical twilight turns to night, and the sun is no longer visible above the horizon.
Now that you know what twilight is, how to recognize it, and what causes it, let's go back to our original question...
The answer to that question depends on two things:
- How quickly it gets dark after sunset depends on your latitude, which is greater the closer you are to the equator.
- It takes longer for it to get dark in the summer (and it might not get truly dark at all) because of the season.
Effect of Latitude on How Long It Takes for It to Get Dark After Sunset
At the equator, the sun seems to rise and set on a horizontal plane. Straight up in the sky from the east, it travels directly overhead (i e the highest point of the sky) at noon and sinks directly to the horizon in the west.
It looks like the sun is setting perpendicular to the horizon. At the equator, the sun sets more slowly than it does at 6 degrees, 12 degrees, and 18 degrees below the horizon.
When you get nearer the poles, the sun moves at a more leisurely angle through the sky. It rises in the east, curves slightly, never quite reaches the midday zenith, and sets in the west.
To put it another way, because the horizon isn't as steep, it takes longer for the sun to sink to 6, 12, and 18 degrees below the horizon.
This is difficult to picture (but explained wonderfully here), so let me give you an example.
How long it takes for it to become completely dark after sunset
Take the March equinox (around the 20th) as an example, when the length of the day is approximately the same for everyone.
Check out the table below to see how the times of sunset and nightfall in Quito, Ecuador (near the equator), Key West, Florida (southern US), Kansas City, Missouri (central US), and Anchorage, Alaska (northern US) compare.
As can be seen, the equator has a much quicker nightfall than even Key West. The further north we travel from the equator, the longer that period of time will be.
Quito, on the equator, has the shortest duration of nautical twilight at 20 minutes after sunset, compared to Key West (23 minutes), Kansas City (26 minutes), and Anchorage (44 minutes).
And keep in mind that all of these times are on the same day.
If we use the term "night" to define when it is completely dark, we see even more variation.
At the spring equinox in Quito, it takes just over an hour (68 minutes) for it to become completely dark. From there, the time to get to Key West is 76 minutes, to Kansas City is 90 minutes, and to Anchorage it's almost 2.5 hours.
Seasonal Variation in the Length of Time It Takes Before It Gets Dark After Sunset
As you can see, the further you are from the equator on any given day, the longer it will take for it to get dark.
However, the closer we get to summer, the later it takes to get dark everywhere. For the continental United States, the time difference is negligible.
In the far north, there are times when there is no true 'night' at all during the summer, which has the greatest impact on the darkness of the season.
Consider the state of Alaska. Throughout the month of June, the sun never dips lower than 6 degrees. This means that you can read outside without a flashlight at all hours of the night because it never gets dark.
You probably already know that in the summer, the north pole experiences 24 hours of daylight, but you might not know that the impact of summer is less pronounced in the contiguous United States.
True 'night' occurs every day of the year, unless you happen to reside in the very far north of the country (within about 15 miles of the Canadian border).
Even in Seattle, the northernmost major city in the United States, there are still two hours of darkness on the longest day of the year.
Bellingham, in the northwest corner of Washington state, is only a few miles north of where things are different. There is no night for about two weeks in the middle of June, when astronomical twilight lasts until the sun rises again.
A Review of the Duration of Nightfall Following Sunset
There you have it; the whole and final solution
In conclusion, the time it takes for the 48 contiguous states to reach complete darkness after sunset varies from 70 to 100 minutes.
True nighttime darkness takes longer to set in the northern hemisphere after sunset.
In the middle of June, if you live near the northern border with Canada, you won't get any real darkness at all. Everywhere else* has at least some nights that are completely dark throughout the entire year.
The sunrise and sunset time for any given day can be found using the sunrise and sunset time calculator on Time and Date.
With the exception of Alaska, where the process can take twice as long and it never gets truly dark in June,
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