During autumn, a typical day here in Vadsø starts with the Sun rising in the morning, just like anywhere else on Earth. But, after just a few hours of daylight, during which beautiful clouds color the heavens in white and greyish shades… it’s time for the Sun to set around noon, in its typical red and yellow sights, making room for the polar blue evenings.
And a short while after that… the amazing night show starts! Northern Lights turn the sky green, and, along with the stars and the Milky Way, they create an outworldly atmosphere, and you can easily imagine you’re… well… somewhere out of this world! Yes, it’s that beautiful!
Just look at my new video and picture yourself here, in the Arctic wilderness, with your head turned up towards the heavens, and just start dreaming!
In a previous article we saw what is twilight and what are the main types of twilight. As a reminder, twilight is the period of the day when a certain point on Earth is illuminated indirectly, by sunlight scattering, when the Sun is below the horizon, but not more than 18°, thus its rays are still visible, indirectly, for an observer located at that certain point.
Now, we’re going to look at how twilight occurs on Earth, when it occurs and… how it doesn’t occur at all in certain places!
Standard twilight occurrence
A standard twilight occurrence was described in our previous article:
During the course of a day, at sunrise, the Sun appears in the sky from the right (East) and, during the whole daytime, it shines its light directly onto the place on Earth where the observer is located. In the evening, at sunset, the Sun will again reach 0° on the left side (West) and it will slowly disappear under the horizon. This moment the evening twilight starts. Due to earth’s rotation, the Sun will continue to descend more and more under the horizon. But before our star reaches 18° under the horizon, there will still be distinguishable light from the Sun for the observer. When the Sun reaches 18°, dusk occurs, and the observer will not distinguish any indirect sunlight anymore and the astronomical night starts.
Because of Earth’s rotation, the Sun continues its trip and, very early in the morning, before it rises, it will reach again 18° under the horizon. At this moment, dawn occurs, and twilight starts again – this time we’re talking about the morning twilight. As time passes more, the Sun will ascend more, until it reaches again 0° and it rises the next day.
Twilight occurs thus during both periods of the day when the Sun is between 0° and 18° under the horizon.
This scenario is true for people living on Earth between approximately 50° North or South of the Equator at any time of the year. This is also valid for higher latitudes, but not around the summer solstice, when the Sun does not descend more than 18° under the horizon during the “night”, and thus there’s no real astronomical night between dusk and dawn.
Continuous twilight between sunset and sunrise
As written in the previous paragraph, above latitudes of approximately 50°N/S, around the date of the summer solstice, the Sun does not descend lower than 18° under the horizon, which means that, even if the Sun is under the horizon, its rays can still be seen, indirectly, during the whole night, which translates itself into a continuous twilight during the whole “night” hours.
In function of the latitude, there can be a continuous astronomical twilight, a continuous nautical twilight, or a continuous civil twilight between sunset and sunrise. This actually occurs in very popular and accessible places around the world, such as:
Continuous astronomical twilight: many European countries, such as northern UK, Ireland, the Netherlands, Germany and many other in the Northern Hemisphere or the Falkland Islands in the Southern Hemisphere;
Continuous nautical twilight: a great part of Russia and Canada, northern Denmark in the Northern Hemisphere or Ushuaia in Argentina in the Southern Hemisphere;
Continuous civil twilight: more northern parts of Russia (such as Sankt Petersburg), Northern Norway, Northern Sweden, Northern Finland.
White nights
A continuous civil twilight between sunset and sunrise is called a white night. The term white night also applies if a certain place does enter nautical twilight also, but if the Sun does not descend lower than 7°.
The white night constitutes a popular symbol for Sankt Petersburg in Russia, where, around the summer solstice, the Sun never goes lower than 7° under the horizon for several days.
A continuous nautical or astronomical twilight does not mean a white night occurs.
No astronomical day between morning and evening twilight
Within the two Polar Circles – Arctic and Antarctic – in wintertime, Polar Night occurs. The polar night means that the Sun does not rise above the horizon at all during 24 hours. But it may approach the horizon, above 18°, thus its rays are seen indirectly and twilight occurs during the “daytime” hours.
Again, in function of the actual latitude, during the normal “daytime” hours, there may be a continuous civil, nautical or astronomical twilight. Vadsø experiences a continuous civil twilight between approximately November 25 and January 17.
Church of Arctic Sea in Vadsø, at noon, during the Polar Night’s daytime civil twilight
No twilight at all
In polar regions, around the summer solstice, the Sun is up in the sky 24 hours a day, a period known as the Polar Day. The Sun that never sets for more than 24 hours is called the Midnight Sun, and it never disappears under the horizon for several days in a row. Higher the latitude, longer the period the Midnight Sun occurs.
As the Sun never goes under the horizon, these places experience no twilight at all during all these days.
This condition occurs here in Vadsø during the Midnight Sun period, and lasts more than 2 months, between approximately May 16 and July 26 each year.
Me on the shore of the Varanger Fjord, during twilight
This is not an article about the famous TV show with vampires! In the next few minutes we’ll talk about what the actual twilight period of the day is, when and how it occurs, and what each type of twilight looks like.
What exactly is twilight?
Twilight is the period of the day when a certain point on Earth is illuminated indirectly, by sunlight scattering, when the Sun is below the horizon, but not more than 18°, thus its rays are still visible, indirectly, for an observer located at that certain point. The lower the Sun is below the horizon, the dimmer the twilight. When the Sun reaches 18° below the horizon, the sunrays’ brightness is gone completely and the “real” pitch-black nighttime starts. (Of course, when talking about the Sun being “18° under the horizon”, we mean its “geometrical centre!”)
The image below presents schematically an observer, his line of sight and the position of the (geometrical centre of the) Sun, at various times of the day:
During the course of a day, at sunrise, the Sun appears in the sky from the right (East) and, during the whole daytime, it shines its light directly onto the place on Earth where the observer is located. In the evening, at sunset, the Sun will again reach 0° on the left side (West) and it will slowly disappear under the horizon. This moment the evening twilight starts. Due to earth’s rotation, the Sun will continue to descend more and more under the horizon. But before our star reaches 18° under the horizon, there will still be distinguishable light from the Sun for the observer. When the Sun reaches 18°, dusk occurs, and the observer will not distinguish any indirect sunlight anymore and the astronomical night starts.
Because of Earth’s rotation, the Sun continues its trip and, very early in the morning, before it rises, it will reach again 18° under the horizon. At this moment, dawn occurs, and twilight starts again – this time we’re talking about the morning twilight. As time passes more, the Sun will ascend more, until it reaches again 0° and it rises the next day.
Twilight occurs thus during both periods of the day when the Sun is between 0° and 18° under the horizon.
In function of the position of the Sun below the horizon during twilight, due to the difference of indirect illumination, we distinguish three types of twilight: ● civil twilight – when the Sun is between 0° and 6° under the horizon, ● nautical twilight – when the Sun is between 6° and 12° under the horizon, ● astronomical twilight – when the sun is between 12° and 18° under the horizon.
Civil twilight
Civil twilight at the border of Vadsø Municipality
Evening civil twilight occurs right after sunset and it lasts until the Sun reaches 6° under the horizon. Morning civil twilight occurs before sunrise, when the Sun is 6° and less under the horizon.
During civil twilight, there’s enough indirect light from the Sun so that artificial illumination is not needed. Objects and landscapes are still visible to the unaided eye. This period is especially sought for by photographers, as the lighting creates an amazing effect in pictures. In polar regions, this is when you can experience the beautiful polar blue.
The first bright stars and planets appear in the sky during civil twilight. Venus is usually seen at this time, hence its name “evening star” or “morning star”.
Nautical twilight
Long exposure photo during nautical twilight
Evening nautical twilight occurs when the evening civil twilight ends, thus when the Sun is lower than 6° below the horizon, and it lasts till our star reaches 12° under the horizon. In the morning, nautical twilight occurs when the Sun is 12° below the horizon and until morning civil twilight starts.
The name “nautical twilight” comes from the fact that sailors could still distinguish a visible horizon at sea, and they were still able to navigate thanks to the brightest stars that are perfectly visible in the sky during the nautical dusk. When not at sea, in places where light pollution is absent and when certain atmospheric conditions are met, the unaided human eye may still distinguish shapes or silhouettes of objects.
Long exposure of a nautical twilight scene, on one of Vadsø’s streets
Astronomical twilight
Evening astronomical twilight occurs when the Sun is below 12° relative to the horizon and just before the astronomical night starts – thus when the Sun reaches the 18° point under the horizon. In the morning, astronomical dawn marks the time when the first indirect sunrays appear and until the morning nautical twilight starts.
The end of the evening astronomical twilight marks the moment when the faintest stars and other faint astronomical objects become visible. And, of course, they will stay visible until astronomical dawn. The unaided human eye will generally not be able to detect any light however, and it will consider the sky entirely dark.
***
In the next part, we will look at how twilight occurs in different locations on Earth. Did you know that, here in Vadsø, when the Polar Night starts (around 25 November each year), we experience a continuous civil twilight from around 8 o’clock in the morning and till around 14h? Find out other cool twilight facts here!
You now know why sunsets and sunrises are red (if not, read this article and find out!). But did you know that another beautiful light phenomenon occurs at sunset and sunrise, besides the beautiful reddish skies? It actually occurs opposite of where the Sun is setting or rising, thus opposite of where all the beautiful reddish colors light up the sky! So, next time you have a clear sky, try looking away from the nice sunset, in order to see the anti-twilight arch! But what is this anti-twilight arch?
Let’s explain its name first
The Belt of Venus, which is a stylized name for the anti-twilight arch, is a pink glow above the horizon, right opposite of where the Sun sets/rises. This opposite place of the Sun is actually an imaginary point, which we will call from now on the antisolar point (anti means opposite).
The phenomenon takes place during twilight – thus before sunrise or after sunset respectively. It is represented by a pink glow that surrounds, just like an arch, the horizon opposite of where sunsets and sunrises occur.
So there you have it – the anti-twilight arch!
Concerning the name “Belt of Venus”, contrary of what you might guess, it’s got absolutely nothing to do with the planet Venus, or any of its belts or rings (…which do not exist anyway!). Planet Venus has a smaller orbit around the Sun than Earth does, and this makes Venus visible to our eyes only around sunsets and sunrises, similar to how the antitwilight arch becomes visible at sunset and sunrise. This is the only association that the Belt of Venus might have with the actual planet. The name Belt of Venus is, in fact, inspired from the girdle which was supposedly worn by the goddess Venus and which might resemble the pinkish arch around the antisolar point, at twilight.
So what exactly is this Belt of Venus?
After sunset (or before sunrise), the Sun is below the horizon, relative to an observer on Earth. In the figure below, the observer’s line of sight is represented by the thin grey line and the Sun is below this line of sight, thus below the observer’s horizon. The dotted circle around Earth represents our planet’s atmosphere. Even though the Sun is below the horizon, right after sunset, light rays from the Sun still make way to get to the observer and even further (red arrow), till above the antisolar point, where they get backscattered off Earth’s atmosphere (pink arrow). This region, where the backscattering takes place, has a belt shape, and this belt is nothing else but the antitwilight arch!
In this figure, notice that the backscattering “point”, at the tip of the red arrow, is a little above the line of sight for the observer. The small region between the backscattering point and the line of sight, is represented by a dark belt, which is our planet’s shadow.
What will you, as observer, see? Well, if you look right opposite where the Sun is setting, you will notice, right after sunset, a faint pinkish light, stretching around the antisolar point, like a belt, or arch. As time passes, this pinkish glow will rise. Right underneath it, you will see a darker belt, which is nothing else than Earth’s shadow! As time passes further, the pinkish glow will rise even more, as will our planet’s shadow, until night will take over entirely and it will become pitch black outside.
This effect is sometimes very faint, and in order to get a good view, you will need, first of all, a clear sky. Best is also to have a clear horizon above the antisolar point as well, in order to distinguish this effect as better as possible.
So, now that you know about the antitwilight arch, I dare you to ignore a beautiful sunset and look right opposite! But I promise that if you do, you will get to see another magnificent optical phenomenon, less known, but of equal beauty! Have you already seen the Belt of Venus?
The Belt of Venus. Image credit: Kent Duryee (https://commons.wikimedia.org/wiki/File:Belt-of-venus.jpg), „Belt-of-venus“, https://creativecommons.org/licenses/by/4.0/legalcode
Beautiful sunset over the Dammen Pond on the Vadsøya island
This period in Vadsø, as in much of the Arctic region, nights begin getting longer and longer. It’s not yet the Polar Night, which means that everyday, beautiful sunsets and sunrises mark the beginning and the end of the dark, cold Arctic nights. But what makes sunsets so beautiful? Why does the sky and the Sun turn red?
In order to answer this, we need to review the same concepts we took into account when answering the question “why is the sky blue?“.
Light is an electromagnetic wave, just like radio waves, microwaves, and even the radiation resulted from radioactivity! The only difference between all these different electromagnetic waves is their wavelength.
Even the light that we actually perceive with our own eyes is made up of multiple wavelengths. And to each and all of these wavelengths of light corresponds a different colour! So, the light that comes to us from the Sun and which we see, is made up of multiple colours! Of all colours, to be exact!
Just like an ocean’s waves, light travels the same way: in waves! Blue light travels in shorter waves (with a shorter wavelength) and red light travels in longer waves (longer wavelengths).
When the sunlight, with all its colours, reaches Earth, it meets the planet’s atmosphere! Thus, it starts interacting with various particles in the air, such as tiny ice crystals, dust, water droplets and even gas molecules that make up the air itself! And once the light waves interact with these particles, it gets scattered!
For a wave to interact with a particle, the two must be of the same order of size. Smaller particles scatter short wavelength light (blue) stronger. Small air molecules, which make up the entire atmosphere, scatter the blue component of sunlight the most, and in all directions, because of its short wavelength! And this is why, during a sunny day, everywhere you look, the sky is blue!
At sunrise and sunsets however, the Sun, relative to us, finds itself at low positions in the sky. From these low positions in the sky, sunlight needs to travel longer distances, through thicker amounts of the atmosphere in order to reach our eyes.
At sunset and sunrise, sunlight travels a longer distance (pictured as the thick yellow arrow) through the atmosphere until it reaches our eyes.
Because of this, the light gets scattered more strongly by the atmosphere. Blue light, which gets scattered easiest, is in fact scattered so much, that it is mostly removed before it actually reaches our eyes. Which in turn means that there is more red light (which gets scattered the least) left for our eyes to see.
Me at sunset, on the shore of the beautiful Varanger fjord.
