When you look at the Moon, sometimes, besides the illumination of the crescent of the moon, you are able to see the rest of the disk of our planet’s natural satellite. That dim illumination of the darker disk is called earthshine.
What causes it?
But what causes this illumination? We all know that sunlight hits Earth, there’s no question there. But Earth is a globe, and a fraction of the light that hits our planet, is reflected back into space (and in the scientific literature is known as earthlight). Some of this light that gets reflected into space, hits the moon as well, and it is this what causes the lighting up of the dark disk besides the crescent moon. And this dim light gets reflected back to Earth and it’s what our eyes percieve.
Sunlight hits Earth (yellow arrow). A portion of this light gets reflected back into space (white arrow) and can cause earthshine, which is reflected back to Earth (grey arrow).
When to look?
When the moon is full, we’ve all seen that it can light up pretty well entire regions on our planet, from where the full moon is visible. From the surface of the moon, Earth can be also illuminated by the Sun (just like other planets of our Solar System). When Earth is “full” (from the moon’s night sky perspective), it means that our planet can light up an entire region on the moon as well. Why would it only be the other way around, anyway? This also means that earthshine is visible at its maximum when Earth is almost “full”, so, when the Moon is a very thin crescent.
Planetshine
This phenomenon occurs on other celestial bodies as well, not only on Earth! When a planet is illuminated by its star, and if that planet has a natural satellite, the phenomenon takes plase in the same waym just like in the Sun-Earth-Moon system. The only difference is the name – it’s generally called planetshine. And, of course, light reflected from a planet in general, is called planetlight.
A new mystery of the Northern Lights has been solved!
At the moment, scientists know a lot about what causes the Aurora, how it forms, what colors it can have, they know how to forecast the Northern Lights, and many, many other interesting and useful things about this beautiful phenomenon. But there are still a few questions which scientists are trying to answer.
One of these questions found an answer very recently (on June 7, 2021), thanks to the work of physicists from UCLA, Wheaton College, the University of Iowa and the Space Science Institute!
What is the Aurora?
The temperature in the Sun’s core is huge (like millions of degrees Celsius). At this temperature, a nuclear fusion reaction of hydrogen occurs. As a result, electrically charged particles (electrons and protons) are thrown out from the Sun’s atmosphere and they escape into space and, at some point, they reach Earth and they interact with Earth’s magnetic field (called magnetosphere), creating a so-called disturbance in the magnetosphere. The electrons from the solar wind then accelerate along the magnetic field lines and then collide with the gases that make up Earth’s atmosphere (mostly oxygen and nitrogen). These collisions determine the emission of visible light. It is this visible light what we call the Aurora Borealis or the Northern Lights.
Mystery solved
The mystery – until a few days ago – was that it was not well understood how the acceleration of electrons along the magnetic field lines occurs.
But the mystery is now solved! After creating, with the help of special scientific equipment, the same conditions just like those in Earth’s magnetosphere, the research team concluded that the acceleration happens due to the fact that the electrons “surf” along the electric field of a wave (in this case a so-called Alfvén wave, see what this is below), which transfers the energy of the wave to the accelerated particles, through a process called Landau damping.
Put it simply and without fancy words, as one of the researchers that took part in this study said, “measurements revealed (that the) electrons undergo (…) acceleration by the Alfvén wave’s electric field, similar to a surfer catching a wave and being continually accelerated as the surfer moves along with the wave”.
Alfvén waves are launched along the magnetic field lines, towards our planet, at the moment of magnetic reconnection, which is a violent process when two magnetic field lines couple together (which happens during a geomagnetic storm for example). Electrons from the solar wind, which are ‘trapped’ in the magnetic field line, “surf” along the field line, being accelerated by this Alfvén wave.
The science behind the Aurora
In this article is presented only a very small part of the Aurora science.
Now you have the chance to completely understand this magnificent phenomenon, starting from beginner level! I designed for Aurora Labs the Learn the Aurora workshop (either the online version, or the workshop we can do together here in Vadsø) where you can learn, through simple experiments and explanations (suitable even for the non-scientists!), the complete science behind the Northern Lights!
So, be sure to take part in my online workshop now, during summer, and be prepared to see (and understand) this beautiful phenomenon next winter!
…Oh, and did you know Aurora Labs can even certify you as an expert in Northern Lights? 😉
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.
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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
