As an astronomer, it is important to know when it will get properly dark so that you can look at the stunning night sky with all the lovely stars, planets and other celestial bodies.
So, in this article, we’ll discuss all there is to know about how long after sunset is it dark.
Ready? Let’s get started…
How Long Does It Take to Get Dark After Sunset: Short Summary
The short answer to this question, “how long after sunset is it dark?” depends on your proximity to the equator.
The closer you are to the equator, the quicker it becomes dark after sunset and the farther you are from the equator, the longer it takes for it to become dark. In the US, it can take between 60 to 100 minutes for it to become “truly dark.”
So, the time it takes to become dark depends on various factors, including your location and seasons.
What Is Sunset?
Sunset is the time of the day or the actual moment when the top of the sun dips below the horizon. And, wherever you are in the world, the sun always sets in the west.
At the time of sunset, the evening sky displays beautiful colors. And, after the sun sets completely, the evening changes to a period of twilight, which comprises various stages after which, complete darkness or night sets in.
What Is Twilight?
The length of time between sunset and night, when the sky transitions gradually from light to dark is known as twilight. It is the period of time when the sun has set but when the sunlight is still visible.
There are three phases of twilight i.e., civil twilight, nautical twilight and astronomical twilight. Each phase of twilight depends on how far the sun has traveled below the horizon, which is measured in degrees.
The phases of twilight are determined by various factors and offer a different experience for astronomers.
Also known as the “magic hour” or “golden hour”, the phase that starts at sunset until the sun’s center is at 6 degrees below the horizon is known as civil twilight.
In the civil twilight phase, objects on the ground are visible and if there is no fog, you can see these without the need for any artificial light. You can easily see the planets, Mercury and Venus, during this phase.
During the golden hour, the white light changes to red and the clouds take on unique colors like orange, red, yellow, pink, blue and purple, which makes this time perfect for photography.
The nautical twilight phase begins when the sun’s geometric center is between 6 and 12 degrees below the horizon. Most of the stars can be seen with the naked eye during this phase.
This phase is called the “nautical phase” because sailors use the visible well-known stars as a guide. This phase is the best time to see the dark part of the moon, which is usually invisible because it is in the sun’s shadow.
In this phase, the alignment of the sun and earth is such that the sun’s rays are reflected off the Earth on the moon, causing the phenomenon referred to as “Earthshine.”
This phase starts when the sun’s center is at 12 degrees until it is at 18 degrees. True night begins after this period. Astronomical twilight is best for astronomers to view the sky and also for astrophotography.
During the astronomical twilight, you can see the Milky Way with the naked eye and even see very dim stars and you can also start to see the shiny green hues of the gorgeous Aurora Borealis.
When the sun moves 18 degrees below the horizon, there is no sunlight in the area and the period after this is “night.”
What Can You See During Twilight?
During the different phases of twilight, the level of visibility differs and you can see different objects both on Earth as well as in the sky.
- On the Ground: You can see all objects on the ground quite clearly without the need for any additional illumination. For instance, during this time, you can read a book easily. But towards the end of the civil twilight phase, you will need to turn on the streetlights or headlamps of your vehicle.
- In the Sky: You can see the brightest planets in the sky i.e., Mercury and Venus.
- On the Ground: During this phase, the light is faded. Objects look like silhouettes and it is difficult to distinguish the objects from their background or one another. You will need additional illumination or lights for outdoor activities. Your eyes cannot distinguish colors and you can only see shades of gray.
- In the Sky: The sky turns dark blue and black at the horizon at the part opposite where the sun has set. You can see the stars and constellations clearly in the sky.
- On the Ground: During this time, it is completely dark. The sky is completely black, but you may see the sky dark blue at the horizon closest to the sun.
- In the Sky: You can see deep space objects in the sky using a telescope, but you may not be able to see them very clearly in the area when the sun has set earlier on.
Often, people cannot tell the difference between astronomical twilight and night. There are very slight differences between the two phases in terms of the brightness of the sky at the horizon, where the sun sets.
Why Doesn’t It Get Dark Right When The Sun Sets?
Often, people ask, why does it not get dark as soon as the sun sets? Why does it take so long to actually become dark? This is because the shape of the Earth is not flat but spherical.
The Earth has a dense atmosphere and the distance of the sun from the Earth is around 93 million miles.
Even when the sun sets and goes below the horizon, it continues to shine on the atmosphere that is above the surface of the Earth, which consists of gas molecules including oxygen and nitrogen.
When the light hits the gas molecules, they cause it to disperse or bounce around. And, even after the sun has set, the world appears to be lit up briefly as this dispersed light hits your eyes.
Below is the amount of atmosphere that is lit by the sun even after it has set:
- When the sun sets, the whole atmosphere above your head is lit. But the amount of light fades quickly through the civil twilight phase.
- When the civil twilight ends and the sun is 6 degrees below the horizon, the atmosphere above your head is not lit. But the atmosphere above the horizon is still lit and you can see the light blue sky.
- Then the sun goes 12 degrees below the horizon i.e., during the astronomical twilight, only the highest atmosphere layers where the sun has set are lit.
- When the sun dips below 18 degrees and it becomes night, no part of the atmosphere is lit by the sun.
How Long Does It Take to Get Dark After Sunset?
This is quite a tricky question to answer because the amount of time it takes to become truly dark after sunset depends on various factors including:
At the equator, the sun seems to rise and set in a straight line. The sun rises in the east, travels to the highest point, i.e., overhead at noon and sets in the west. It appears that the sun is setting at 90 degrees to the horizon.
It goes past 6, 12 and 18 degrees much faster at the equator than at the poles. But when you’re closer to the poles, the rays of the sun become diagonal. The sun rises near the east; it does not travel straight up or goes exactly to the zenith at noon and it sets near the east.
The shallow path of the sun at a flatter angle essentially means that it takes a lot longer for it to reach 6, 12 and 18 degrees below the horizon. And so, it takes a lot longer to become dark after the sun has set at the poles.
The length of twilight and the length of time taken for it to become dark after sunset is also impacted by the time of the year. The Earth is tilted 23.5 degrees from the vertical on the plane, which causes seasons.
During the summer, the Earth tilts towards the sun, causing more exposure to sunlight and in turn, longer days. The longer days mean longer twilight periods and so it takes a lot longer for the sky to become completely dark even after sunset.
The opposite happens during winter. The sun does not rise very high in the sky and this means that it sets a lot sooner every night.
The days in winter are shorter and most of the time, the sun is below the horizon. This means that twilight is shorter and it gets dark very quickly.
In terms of day and night, the time of the year, as well as your location, can cause extremes. For example, in countries in the far north or south like Finland, Norway and Antarctica, during spring and fall, the sun takes several hours to set.
To this, when you add the very long summer days, then there are times when the sun does not set at all. The sun never goes below the horizon and remains in the sky for 24 hours a day.
So, during the summer days, in these countries, there is no twilight and no darkness.
And, the opposite occurs in these places during the winter months.
These regions get very little or no sun at all. The sun is not visible at any time of the day, there is no light and these regions are plunged into darkness at all times.
What Is The Period/Light Called After Sunset?
The period after sunset or before sunlight is known as twilight, when the atmosphere is somewhat illuminated by the sun and it is not completely lit or fully dark.
How Long Do I Need To Wait After Sunset Before I Can See Stars?
You will need to wait until true darkness, or at least until astronomical twilight, to see most of the stars in the sky. Before this time, you will be able to only see the brightest stars. But if you wait until astronomical darkness, then you can see most of the stars that can be viewed with the naked eye.
How Long Does It Take To Get Dark In The U.S.?
If you live in the U.S., then you may not have dramatic intervals between sunset and darkness. Depending on where you are in the country and the time of the year, the time between sunset and darkness will range between 1 and 2 hours. As you move towards the north, the longer it will take between sunset and darkness.
Summary & Conclusion
So, if you’re wondering how long it takes to get dark after sunset, for the 48 contiguous states in the US, it can take between 60 to 100 minutes to get dark after sunset. And, as you travel north, the longer it takes to become truly dark after sunset.
The time can vary because of factors like the location of the place and the time of the year. And, if you want to know at what time it will get completely dark after sunset, you can use an astronomy calculator to find this out.
The astronomy calculator makes use of your current location and helps to calculate the sunset, sunrise, moonset and moonrise in your city. And, once you’re able to determine how long it takes to become dark after sunset, you can view the stunning beauty of the night sky.