Night and day occur just like clockwork. Even though we’re not guaranteed a sunrise, we’ve come to expect the Sun to rise and set each day.
The transition from light to dark is full of mystery and never the same. In this article, we look at twilight and how long it takes after sunset for the world to get dark.
What Is Sunset?
It probably goes without saying that sunset is the time at the end of the day that the Sun disappears on the edge of the world.
During this time, the Earth’s evening sky typically goes through some colorful transitions. However, the actual moment of sunset is when the very top of the Sun’s disc dips below the horizon.
No matter where in the world you find yourself, the Sun will always set in the western sky. After the sun completely sets, evening transitions into a period of twilight until complete darkness sets in.
What Is Twilight?
On a clear evening, there’s a length of time between sunset and what we know to be the night. During this time, the Earth goes through a gradual period transitioning from light to dark. This stretch of time is known as twilight.
Earth’s twilight is broken into three phases, each depending on how far below the horizon the Sun has traveled. If you can imagine the Sun’s path around the Earth as a circle, we can measure in degrees how far the Sun is from the horizon at any given time, day or night.
To put those degrees into perspective, hold the three middle fingers of one hand out in front of you as far as you can. The distance between those three fingers is around five degrees.
Civil twilight is the brightest phase of twilight and occurs from the moment the Sun has completely disappeared until it is 6° below the horizon.
Although the Sun is no longer present, its presence remains. The sky may have turned a myriad of colors, but it is far from dark. Only the very brightest objects, such as the Moon, Venus, Jupiter, and Sirius, may become visible.
Individuals experiencing civil twilight see light start to fade but are still able to go about business without the need for artificial light. It’s called civil twilight because you can still see neighbors and be social during this stretch of time.
The term nautical twilight dates back to an era where sailors could use the stars during this time for navigation. Most stars are visible with the naked eye, the horizon is still visible, and there is still some natural light to be able to function.
Nautical twilight includes the time when the Sun travels from 6° below the horizon all the way to 12°.
Almost all the sunlight has faded away as the evening enters astronomical twilight. Astronomical twilight occurs as the Sun travels from 12° to 18° below the horizon.
At this point of twilight, most people would say it’s properly dark outside. However, this is not quite the case.
All but the faintest stars are visible at this point, and the designation between earth and sky is no longer apparent.
The end of the final phase of twilight ushers in darkness. Darkness, or nighttime, occurs when there is a complete absence of light energy. This is a period where no light from the Sun reaches this part of the Earth.
True night, or when darkness begins, starts the moment that the Sun moves more than 18° below the horizon.
How Long Does It Take To Go From Sunset to Darkness?
Surprisingly, this answer isn’t quite so easy to answer. At the equator, it can take as little as 30 or 40 minutes to achieve true darkness. In the far north or south, it can take upwards of a few hours!
There are a few factors that contribute to this wide range of times.
Location on Earth
At the equator, the Sun’s rays are very direct. Light travels like a narrow flashlight beam down to the Earth’s surface. In this part of the world, the Sun travels farther through the sky each day and covers the angle representing twilight in a much shorter period of time.
However, the further north or south you go, the more diagonal those rays become. The Sun’s shallow path through the sky means it takes longer for the run to reach those various stages of twilight, and ultimately darkness.
Time of Year
Another factor that plays a significant role in the length of twilight is the time of year.
Our Earth is tilted 23.5° from vertical on the plane the Solar System sits on. This tilt is what gives us our seasons.
In the summer months, the Earth is tilted toward the Sun. This means more exposure to the Sun’s light and longer days as a result. These longer days factor into longer periods of twilight, and therefore it takes longer for the sky to go completely dark.
Winter brings about the opposite effect. During this time of year, the Sun doesn’t rise nearly as high in the sky. Unfortunately, this also means it sets that much sooner each night.
Days are known to be short, and the Sun spends most of its time below the horizon. Twilight is also much shorter, meaning it gets dark that much faster.
Both location on Earth and time of year together can lead to some amazing extremes when it comes to day and night.
For instance, in the far northern reaches of countries such as Norway and Finland, the Sun can take hours to set on a spring or fall day. Once you add in the longest days in summer, there are times where the Sun doesn’t set at all.
That’s right, the Sun literally stays up in the sky for 24 hours a day and never dips below the horizon. There is no twilight and certainly no darkness during those summer days.
The same holds true for some of the Earth’s southernmost locations, especially in places like Antarctica.
Days of complete light may seem like a treat (unless you have trouble sleeping), but the opposite happens at these polar locations in the winter months.
These same locations experience very little or no Sun at all during the cold winter. There is no time of day where the Sun is visible or it is light outside. These areas are temporarily stuck in perpetual darkness.
How Does It Stay Light After Sunset?
Once the Sun dips below the horizon and we can no longer see the source of our celestial light, it stands to reason that the sky should turn instantly dark. We know this isn’t the case, as we’ve all experienced the various phases of twilight that occur after sunset.
If the Earth were a flat surface like the ancient peoples believed, the sky would turn instantly dark after sunset. Obviously, the Earth is spherical in its shape. Even after the Sun has disappeared from view, its rays are still present in our atmosphere for a short while. We’re able to take advantage of that sunlight during civil twilight to do things without the need for additional light.
As the Sun dips further, its light can still bounce off the gas molecules in our atmosphere. Gas molecules like oxygen and nitrogen are great at helping sunlight travel to places it should not be able to reach.
This sunlight isn’t nearly as strong as what we see during the day, but it gives us our darker phases of twilight where there is still some visibility.
The Many Colors of Sunset
Interestingly, this scattered sunlight is also the reason why we have such beautiful sunsets.
Sunlight is white light and is a combination of all the colors of the rainbow. Each color that we can see travels unimpeded from the Sun in a wave throughout space before it reaches our atmosphere.
Blue is the shortest color but is most prevalent when the Sun’s rays are more direct. This is why we have our amazing blue skies.
As the Sun begins to set at the end of the day, those light waves must travel further through our atmosphere to reach us. When this happens, that blue color starts to fall short, and other, longer colors emerge.
Thankfully, that’s when the other colors step in. The yellows, oranges, pinks, and reds that usually hide behind the blues come pouring out. Red is the longest wavelength and often lingers the latest into the night.
How Long After Sunset Do You Need To Wait Before You Can Stargaze?
If you’re a stargazer, you’ll want to wait until at least astronomical twilight to see a majority of the stars in the sky. Any time sooner and you’ll be treated to some of the brightest ones, but you’ll miss out on a full sky.
If you can wait until astronomical darkness, you’ll be blessed with the whole gambit of all the stars that can be seen with the naked eye. You might even be able to catch a nebula or the Andromeda galaxy if it’s the right time of year.
In a world where we can flip a switch and have complete darkness inside, it’s incredible that darkness outside is something so complex. Fortunately, the transition from light to dark is an incredible experience that all humans are able to enjoy.