When will it be dark?
“How long does it take to get dark after sunset?” is a really smart question to ask as an astronomer!
Well, you need to know when it’s going to be properly dark so you can figure out when it’s worth getting outside and taking a look at the night sky. Go too early and you won’t see all that you hoped for… but when is too early?
If you’re in the US and want a quick answer, here it is…
It will be dark enough to see deep space objects
- For southern states: around 70 minutes after sunset
- For northern states: around 100 minutes (1 hour 40 minutes) after sunset
There’s variation in that for your particular latitude, the season and how dark you want it to be, though.
So, if you want to discover more about how long it takes to get dark after the sun sets, then read on…
What is Astronomical Twilight?
You may have heard of astronomical twilight (yes, it is a real thing) but it is only the last and darkest of three recognised twilight phases.
Each twilight phase is determined by how far the sun is below the horizon:
- Civil Twilight – begins at sunset (the centre of the sun is at 0° elevation) until it is 6° below the horizon
- Nautical Twilight – starts when civil twilight ends and lasts until the centre of the sun is 12° below the horizon
- Astronomical Twilight – the last and darkest of the twilights. It begins when the sun is 12° below the horizon and ends when it reaches 18° below
Once the sun moves more than 18° below the horizon, it is officially ‘night’ and there is no sunlight lighting your location, perfect for taking photos of the night sky.
What Can I See During Twilight?
Each of the three twilights has different levels of seeing, both down here on Earth and up in the night sky. Since we don’t know how far below the horizon the sun is, this simple guide will help you judge based on local conditions:
- On the ground, you can still see everything clearly without extra illumination, you can still read a book, for example. Towards the end of this period, car headlamps need to be switched on and streetlamps will be lit.
- In the sky, only the brightest objects will be visible, such as Venus and Mercury
- On the ground, light has faded now and objects become hard to distinguish from the background or each other. They seem more like silhouettes. You’ll need additional lighting to carry on outdoor activities.Your color-seeing gives way to shades of gray as there is no longer enough light for your eyes to see in color.
- In the sky, the stars come out now and constellations can be clearly seen. The sky itself will turn dark blue, and be black at the horizon opposite where the sun set.
- On the ground, it is what we would all call night. It is truly dark. You will perhaps see the sky as dark blue or off-black at the horizon nearest the sun, otherwise the sky will be black.
- In the sky, you will now be able to see deep space objects (with one of these telescopes), although seeing is compromised a little nearest the horizon where the sun set earlier.
For most of us, there is little to tell different between astronomical twilight and night, except for slight differences in sky brightness at the horizon of the setting sun.
At this point, the sun places no limit on the objects you can see (except by reflecting off the Moon of course).
The reverse of these phases is true as we head towards sunrise. Night gives way to astronomical twilight, then nautical twilight and finally civilian twilight, which ends with the sun rising.
Why is it Not Dark as Soon as the Sun Sets?
It’s a reasonable question to ask.
If the source of our daylight has gone below the horizon, why does it take so long to actually get dark?
Why is it not dark as soon as the sun sets?
If we lived on a flat disc (like our ancestors believed) and the sun brushed the edge of it as it set, it would indeed get night-dark instantly the sun was out of sight.
But, we don’t.
The Earth is a sphere, has a deep atmosphere and our sun is around 93 million miles away. When it sets below the horizon, the sun continues to shine on the atmosphere above the Earth’s surface.
If you’ve ever taken a flight at the start or end of a day, you’ve probably experienced this for yourself.
Imagine you’re on a plane high above your friend on the ground below. As the sun sets below the horizon for them, you will still see it above the horizon. It is still shining on the atmosphere you are flying in but is not lighting the ground beneath you.
The sunlight hits the atmosphere full of gas molecules like oxygen and nitrogen. Those gases cause light to bounce around, or ‘scatter’. Some of the scattered light hits our eyes and so we see the world lit, for a short time, after the sun has set.
The sun continuing to shine on the atmosphere after it sets below the horizon is why we have three different twilights:
How much atmosphere is lit by the sun after sunset (source)
- At the moment of sunset, all of the atmosphere above our heads is still lit by the sun, but the amount shrinks rapidly throughout civil twilight
- At the end of civil twilight (i.e. sun is 6° below the horizon), none of the atmosphere above our heads (the zenith) is now lit by the sun. However, still a third of the atmosphere above the horizon is lit by the sun, which we see as lighter blue skies
- By the start of astronomical twilight (12° below the horizon), only the very highest layers of the atmosphere at the horizon where the sun set are still lit
- When astronomical twilight gives way to night at 18° below the horizon, none of the atmosphere we can see is lit by the sun
Now you understand the concept of twilight, how we see them and what causes them, let’s return to our original question…
How Long Does it Take to Get Dark After Sunset?
There are two factors that determine the answer to that question:
- Your latitude – the closer you are to the equator, the quicker it gets dark after sunset
- The Season – In summer it takes longer to get dark than winter (and may not get truly dark at all)
The Impact of Latitude on Time to Darkness After Sunset
The sun appears to rise and set in a straight line at the equator. It rises in the east, goes straight overhead (i.e. the zenith) at midday and sets straight down in the west.
The sun appears to set at right angles to the horizon. It sinks past 6°, 12° and 18° below the horizon much faster than it does nearer to the poles.
Closer to the poles, the sun takes a more leisurely angle through the sky. It rises near the east, does not go straight up, does not get as high as the zenith at midday and sets near the west.
This flatter angle means it takes longer for the sun to reach 6°, 12° and 18° below the horizon… so it takes longer to get dark.
This can be hard to visualise (there’s a great explanation here) so it’s simpler to show you with a specific example.
How Long it Takes to Get Dark After the Sun Sets
Let’s take the March equinox (around March 20th) where the length of day is pretty much the same for everyone.
In the table below, we’ve compared the time from sunset to night for the towns of Quito in Ecuador (which is near the equator), Key West (south US), Kansas City (central US) and Anchorage (north US).
|City||Civil Twilight (Sunset)||Nautical Twilight||Astronomical Twilight||True Night|
|Quito, Ecuador||18:25||18:45 (+20mins)||19:09 (+44mins)||19:33 (+68mins)|
|Key West, Florida||19:39||20:02 (+23mins)||20:28 (+51mins)||20:55 (+76mins)|
|Kansas City, Kansas||19:32||19:58 (+26mins)||20:30 (+58mins)||21:02 (+90mins)|
|Anchorage, Alaska||20:18||21:02 (+44mins)||21:54 (+96mins)||22:53 (+155mins)|
You can see clearly that it takes a lot less time to get dark at the equator than it does even in Key West. As we move northwards, away from the equator, that time gets longer still.
At the equator, in Quito, nautical twilight starts only 20 minutes after sunset, compared with 23 minutes in Key West, 26 minutes in Kansas City, and 44 minutes in Anchorage.
And, remember, these times are all the same day.
If we say that ‘night’ is our measure of when it is truly dark, then the differences are even greater.
In Quito, it only takes just over an hour (68 minutes) to get dark after sunset at the spring equinox. Move north from there and it takes 76 minutes in Key West, 90 minutes in Kansas City and almost 2 and a half hours up in Anchorage.
The Impact of Season on Time to Darkness After Sunset
On any given day, as you’ve just seen, it will take longer to get dark the further away you are from the equator.
Whatever your location, though, it takes slightly longer to get dark the closer to summer we are. The difference is small, being only a few minutes for mainland USA.
The biggest impact for summer darkness is that, if you live far enough north, there are periods when there is no true ‘night’ at all.
Take Alaska, for example. For most of June, the sun is never more than 6° below the horizon. Which means it never gets past civil twilight, so you can read outside all night without an extra source of light!
You probably know that the north pole has 24hr daylight in summer, but come down to the contiguous states and the summer impact is not as marked.
Unless you live in the extreme north of the country (within about 15 miles of the Canadian border), you’ll experience true, dark ‘night’ every single day of the year.
For example, even the continental US’s northernmost big city, Seattle, has 2 hours of night on the longest day of the year.
Yet, just a few miles north, at Bellingham in the north-west corner of Washington state, it’s a different story. They have around two weeks in mid-June where astronomical twilight doesn’t end before the sun begins to rise again, there is no ‘night’.
Recap of How Long Darkness Takes After Sunset
So, there you have it, a complete answer.
In summary, for the 48 contiguous states, it takes anywhere from 70 to 100 minutes for it to get dark after sunset.
The further north you are, the longer it takes for true darkness to arrive after sundown.
If you live at the northern border with Canada, you will experience nights in mid-June where it never gets truly dark at all. Everywhere else* does experience at least some truly dark nighttime every day of the year.
If you want to discover the twilight times for your location on any given day, then check out Time and Date’s sunrise and sunset calculator.
*Outside of Alaska, where it can take twice as long and never gets truly dark in June