Astronomical Facts: How Long After Sunset Does It Get Dark?

It’s common knowledge that darkness comes after sunset, but have you ever thought about how long it takes to become dark after sunset? Very few people have taken time to think about what happens between sunset and nightfall. So, the question we’re going to address in this article is: How long after sunset does it get dark?

How Long Does It Take to Get Dark After Sunset?

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It takes approximately 70 to 140 minutes to get completely dark after sunset. However, this duration can be as low as 23 minutes in areas near the equator. But why does this happen? Why doesn’t it become dark right after sunset?

To understand this process, you have to be aware of the different stages involved. That’s why you hear people using different terms like sunset, dusk, twilight, night, etc. So, start by understanding what these terms refer to and what happens when the sun vanishes over the skyline.

Sunset is the time when the sun disappears over the skyline. In most places, the sunset displays a red spectacle that creates a stunning backdrop on the horizon. This scenery is quite therapeutic, especially if you’re having a romantic moment with your spouse.

However, the reddish backdrop doesn’t mean that the sun becomes red at sunset. It has everything to do with the earth’s atmosphere through which the sun travels and the tiny wavelength light colors spread in the air. At noon, the sun is directly above your head and its light travels a short distance through the air.

This means that very little sunlight is scattered in the atmosphere at noon. That’s why the sunlight appears to be bright yellow. But as time passes and the earth continues to shift, the sun moves away from you, causing the short wavelength blue light to be scattered all over the atmosphere.

That’s why the sun appears orange in the afternoon. At sunset, the sunlight has to travel a long distance to get to you. Therefore, the short wavelength light colors are scattered widely across the atmosphere.

So, only the longer wavelength light (red) colors are visible at this time. That’s why the sun appears red. Another reason why the sun appears red at sunset is that the sun sets in the west where weather systems usually converge from.

Therefore, when you see the red sky at sunset, it means that there’s a clean front on the west side and the next 12 hours are likely to be clear. Dusk, on the other hand, is the time when the sun hits 18 degrees under the horizon and you can see sunlight in the sky.

It’s safe to say that dusk precedes nightfall. Twilight is the point between sunset and dusk. At twilight, some sunlight is still visible in the sky. Twilight is divided into three categories: civil, nautical, and astronomical twilight.

The three phases occur twice a day (within 24 hours). It’s important to understand the three categories of twilight because they’ll help you understand what happens between sunset and nightfall.

Civil Twilight

Civil twilight occurs immediately after the sun slips beneath the horizon. It begins when the sun disappears to when the sun’s midpoint gets to six degrees beneath the horizon. You can measure the degrees of the sky using your fingers.

The six-degree measurement is slightly above your three fingers held at your arm’s length. At civil twilight, there’s enough sunlight but you might have to turn on the lights when driving on the road. You’ll also notice the street lights beginning to turn on.

For astronomers, civil twilight is the perfect time to view the brightest spheres. Additionally, civil twilight lasts longer in summer and winter for mid-latitudes and it’s shorter in spring and fall. The main explanation for this disparity is the fact that in spring and fall the sun sets directly in the west thus making a straight path downward.

This allows the sun to get to the six-degree mark faster than in summer and winter when the sun curves across the sky, dropping down at a more prominent angle. This makes the civil twilight last longer in summer and winter. On average, civil twilight in mid-latitudes lasts about 30 minutes.

In tropical areas, especially along the equator, civil twilight doesn’t vary between seasons. In this region, the sun cuts across the sky at a clean angle at sunset in almost a perpendicular way. Since the sun disappears faster along the equator, civil twilight in these areas is shorter.

Nautical Twilight

Nautical twilight picks up from where civil twilight stops. So, it begins when the sun is at six degrees below the horizon and ends at 12 degrees below the horizon. You can also tell the end of nautical twilight by observing the distant line between the sea and sky. This twilight ends when this line becomes blurry.

Another simple way to know when nautical twilight happens is to look out for more bright stars. This technique has been in use since the era of navigation. Sailors observed the stars to get directional cues.

Nautical twilight makes terrestrial objects more visible, but you may need a torch or any other artificial light to do anything outdoors. In summer, polar areas experience nautical twilight throughout the night because the sun doesn’t go beyond 12 degrees below the horizon.

So, it doesn’t reach astronomical twilight. In mid-latitudes, nautical twilight lasts for half an hour in spring, winter, and fall, while it lasts 45 minutes in summer.

Astronomical Twilight

Astronomical twilight is the final twilight stage. It’s also the darkest stage of twilight before nightfall. At this time, the sun drops from 12 degrees to 18 degrees below the horizon.

This brings total darkness, leaving the stars clearly visible. If you’re a stargazer, this is the right time to observe the faint stars and clusters in the sky. In mid-latitudes, astronomical twilight lasts about half an hour in fall and spring.

In summer, this stage of twilight lasts about an hour. In mid-latitudes, astronomical twilight starts an hour and a half after sunset. Therefore, it’s safe to conclude that, on average, it takes at least 70 minutes after sunset to get dark, depending on where you live on planet Earth.

If you watched twilight from outer space, you’d notice that there isn’t a sharp boundary on the surface of the earth to mark the shift from sunlight to darkness. Instead, this shift appears as a shadow line on the surface of the earth, which is commonly referred to as the terminator line.

This shadow covers a large portion of the earth’s surface and gradually transitions to total darkness as the earth rotates. However, the darkness doesn’t engulf the entire earth. When night falls on one part of the earth, the sun rises on the other.