Understanding the twilights

Understanding the twilights

Did you know that there exists three types of twilight and that these specific times of the night have properties and phenomena of their own?

What is the twilight?

By definition the twilight is a period of the circadian day that starts directly after the last light of sunset and finished when the sky is completely dark at nighttime. It’s composed of the dusk after sunset, and dawn right before sunrise. At sunset, the totality of the solar disc needs to be under the horizon for the twilight to begin. Now there are always controversies because the extreme atmospheric refraction near the horizon sometimes makes the Sun appear or reappear whereas it is actually already under the horizon (loupe effect). In the same way, the end of dusk marks the point where there the Sun no longer has any lighting effect on the sky, leaving a pitch black starry background. The twilight is always broken down into 3 categories with their own characteristics and phenomena: the civil twilight, thenautical twilightand the astronomical twilight. For the purpose of this article, we will talk about dusk, assuming that the same phenomena also appear in the same manner at through dawn.

Pink clouds right after sunset

The civil twilight

When the Sun is located between 0 and 6 degrees elevation under the horizon, we call it civil twilight. Right after the solar disc leaves the horizon at dusk, the sky is still extremely bright. Depending on the height and position of the clouds that might hang above your head, they usually still catch the light of our star. Because the sunrays that hit them cross a lot of atmosphere since the Sun grazes the Earth, the diffraction of light makes the white color shift to the reds. The clouds usually take on spectacular yellow, orange, red, pink, purple and blue colors in a consecutive manner. In photography, we call this fleeting moment the ‘golden hour’, and it is a very coveted time for photographers to hurry and take colorful landscape pictures.

10 minuets after sunset

The civil twilight usually creates so much light still that only the extremely bright deep-sky objects will be visible with naked eyes. For example, the moon is ordinarily always light enough (except on new moons) to be caught. Bright planets at opposition (at their closest distance from Earth) will even appear sometime before sunset or after sunrise! Unfortunately the civil twilight is still too bright to see light phenomena like the aurora or noctilucent clouds in most cases.

2,9% crescent moon at civil twilight

When the Sun is between 3 and 6 degrees below the horizon, something special happens. From that point forward, the total amount of brightness diminishes very quickly. You still easily see everything around you without difficulty but there’s a nice transformation occurring up above. If you look directly opposite to the sunset on clear nights, you will see a nice pink arc slowly rising, leaving a dark blue/purplish crescent behind: we call it the belt of Venus. It is simply the line that marks the limit between the last pink rays of Sun and the Earth’s shadow (blue crescent). When a full moon rises right in the gradient, it’s such a nice sight to be captured. Near the end of the civil dusk, the sky may be dim enough to see the brightest stars of the night sky, especially if they are opposite to the twilight crescent (Vega, Sirius…). Because the Earth’s shadow is starting to completely engulf the sky, it marks the beginning of what photographers dub ‘the blue hour’.

Belt of Venus

The nautical twilight

When the Sun lies between 6 and 12 degrees below the horizon line, we are in nautical twilight. At nautical dusk, the blue hour continues, leaving a bluer sky by the minute. The light decreases progressively so that more fainter stars start appearing. This time is particularly beautiful if you are located near snow-capped mountains because the snow will typically mirror a special light called Earthshine. It is not the rays that directly come from the Sun but it’s the bluish light that is still scattered by the distant Sun below the horizon. This vicarious reverberation can produce quite stunning scenes, lighting the foreground one last time before it is completely plunged into the shadows of the night.

Earthshine making snowy mountains glow while in the shadow of the Earth

During nautical twilight, there are several astronomical phenomena that are visible and that you should miss. Typically two-three days prior and after a new moon, our natural satellite will display a thin crescent (3-10%) near the horizon (near sunset point at dusk, and near sunrise point at dawn). Oftentimes the crescent is accompanied by the conjunctions of planets like Mars, Mercury, Saturn or Venus. Moreover that’s the perfect time to gaze at the dark side of the moon Usually invisible because left in the shadows of the Sun, it illuminates nicely when the alignment of the Earth and the Sun allows for the sun ray to be reflected off the Earth and onto the moon. We call this phenomenon Earthshine, and it is extremely photogenic. Altogether this spectacular scene can usually be observed right in the beautiful twilight gradient, spanning from deep red to orange, to yellow and eventually dark blue.

Moon, Earthshine and Venus over the Alps

The Sun is also too low to illuminate even the highest tropospheric clouds, but it is in the perfect position to reflect its light on the millions of tiny ice particles that form in the northern hemisphere summer mesosphere: noctilucent clouds. From their latin name meaning ‘night-shining’, these clouds at the edge of space are the highest on Earth and give the impression that they shine a silvery color against the dimming twilight sky. When enough moisture form at an altitude of 80km above our heard, ice crystal start nucleating and reflect the sunlight. Noctilucent clouds are usually visible from May to September in the northern hemisphere and October to Mars in the southern hemisphere, mostly around 45-80 degrees of latitude.

Noctilucent clouds

Finally the end of nautical twilight usually is the perfect time to start chasing the aurora. While some display can be very bright during geomagnetic storms, the sun rays largely outshine it. That’s why the aurora can only be seen around where the stars start to come out. However even then, the green lights tend to be quit washed out so you will probably have to wait a few more minutes to enter the astronomical twilight to see it better.

Faint aurora seen in the nautical twilight

The astronomical twilight

When the Sun dips below 12 degrees elevation and until 18 degrees, we enter astronomical twilight. During this time, it becomes harder and harder to discern the objects that surround us and the contrast between the brighter horizon and the darker one is sometimes so important that it becomes hard to drive. The Sun still leaves a lighter spot but the twilight crescent diminishes in size. The faintest stars start becoming visible, and most notably our own galaxy the milky way can finally be spotted with the naked eye.

Around the middle of astronomical twilight, it becomes quite hard for our eyes to notice the sunlight that’s still affecting the sky because it is very dim. If you take a picture however, you will still be able to see the typical bluish hue and the orange colors. While noctilucent clouds are best seen during nautical twilight, the course of the sun deeper below the horizon will make them ‘shrink’ until they seemingly disappear half-way through the end of astronomical twilight. However they are sometimes extremely bright that they will look like a weird-looking shiny lenticular cloud right above where the Sun is located.

If you are aurora chasing, astronomical twilight is your cue to being aware and active. If the green light barely shone through the nautical twilight, they will start revealing their true beauty and colors, especially during strong geomagnetic events or sub-storms at high latitudes.

For more contrast and colors, you will have to wait until complete darkness but some gorgeous pictures are taken at astronomical twilight. It gives the aurora a very bluish hue, so the green turns emerald and the reds turn purple.

Lastly, two additional phenomena can be spotted right around the end of astronomical dusk and beginning of astronomical dawn: airglow and zodiacal lights. The former usually glows faintly but gives the sky some extra colors due to UV-activated atoms in our upper atmosphere. Zodiacal lights are those triangular-shaped pillars of light following the ecliptic, shining when sunlight gets reflected off tiny inter-planetary dust particles.

Zodiacal light towards the end of astronomical dusk

When the Sun passes 18 degrees below the horizon, we officially enter nighttime and the Sun no longer has any effect on the sky. We are completely within the Earth’s shadow and this is the best time to observe or capture night-sky phenomena like the milky way, the aurora, airglow, the stars etc…

Milky way right after the start of astronomical dawn

 

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