The Antitwilight Arch: The Earth's Shadow

Scientific summary

Astronoo's article examines the optical phenomenon of the antitwilight arch, a visible manifestation of the Earth's shadow in the atmosphere. Based on the principles of Rayleigh and Mie scattering, it explains the formation of the dark band and the pinkish belt (Belt of Venus). The study of this effect, observable opposite the Sun, confirms the sphericity of the Earth and the structure of its shadow. It details the observation conditions, links with lunar eclipses and related phenomena such as anticrepuscular rays, offering a clear synthesis of this daily spectacle of nature.

What is the antitwilight arch and why is it the visible proof of the Earth's shadow?

This article is devoted to the antitwilight arch, also called the Belt of Venus. It is an atmospheric optical phenomenon that manifests itself as a dark band and a pinkish glow, observable on the horizon, opposite the Sun, at sunrise and sunset. But how is this spectacle visible proof of the Earth's shadow? The answer lies in the very mechanism of its formation. The shadow of our planet, a cone of darkness projected into space, meets the upper layers of the atmosphere. This geometric shadow, normally invisible, becomes tangible when sunlight, filtered by the atmosphere, delimits its boundary. The bluish or dark band we see is the direct shadow, while the pinkish band (the Belt of Venus) is produced by the backscattering of sunlight by molecules and atmospheric aerosols above this shadow. Thus, this daily and accessible phenomenon offers a spectacular demonstration of the Earth's sphericity and the complex interaction between light, the atmosphere and our planet.

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The phenomenon of the Earth's shadow

The antitwilight arch, also known as the Belt of Venus, is an atmospheric optical phenomenon that highlights the shadow cast by the Earth into its atmosphere.

This phenomenon, observable at twilight, on the opposite side from the Sun at sunrise or sunset, appears as a dark band topped by a pinkish glow, thus illustrating the sphericity of our planet.

Composition of the arch

The dark band corresponds to the Earth's shadow projected onto the atmosphere, while the colored band results from the backscattering of sunlight by molecules and aerosols present in the air.

Although spectacular, this phenomenon is not rare: it occurs at every sunrise or sunset, provided that the atmospheric conditions are favorable.

N.B.:
The name "Belt of Venus" comes from a historical association with the goddess Venus. The term has long been used in meteorology and astronomy to describe this optical phenomenon, with no direct connection to the planet Venus, but rather with the idea of beauty and grace.

Observation conditions

To observe this phenomenon, several conditions must be met:

Clear sky and an unobstructed horizon to the east (for the evening arch) or to the west (for the morning arch)
A relatively clear atmosphere with suspended particles to scatter the light, ideally with little pollution
A sky free of low clouds opposite the Sun
A precise moment: approximately 10 to 15 minutes after sunset or before sunrise

Role of latitude and season

Latitude and season influence the duration of twilight and therefore the visibility of the arch. In the tropics, twilight is shorter, making observation more fleeting, while at high latitudes, in summer, twilight can last several hours, offering a wider observation window.

Link between the Earth's shadow and the antitwilight arch

The Earth projects a cone of shadow into space, caused by the blocking of sunlight. This geometric shadow extends about 1,370,000 km (3.57 times the Earth-Moon distance). The Earth's shadow extends well beyond the Moon, which explains why the Moon can be completely immersed in this shadow during lunar eclipses.

From spatial shadow to atmospheric shadow

The antitwilight arch corresponds to the visible part of this shadow projected onto the lower layers of the Earth's atmosphere. More precisely, the dark band observed on the horizon, opposite the Sun, is the shadow itself, where direct light is blocked by the Earth. Above this dark zone appears the Belt of Venus, a pinkish band resulting from the scattering and backscattering of sunlight by atmospheric molecules and particles.

Thus, the antitwilight arch is the optical manifestation of the upper limit of the Earth's shadow projected onto the atmosphere. It is this superposition between the geometric shadow of the Earth and the phenomena of light scattering that creates the spectacle observable at twilight.

Anticrepuscular rays

Sometimes, dark and light rays seem to converge towards the antisolar point. This is a perspective effect: in reality these beams are parallel and are produced by shadows cast by clouds or mountains.

Origin and interpretation

Anticrepuscular rays are the extension of crepuscular rays observed on the Sun's side. The perspective effect makes them converge towards the antisolar point, offering a striking spectacle of optical geometry in the sky.

Comparative table of twilight phenomena

Twilight and anti-solar phenomena
Phenomenon	Mechanism	Color	Scale
Earth's shadow	Conical projection of the Earth's shadow onto the atmosphere	Bluish to dark	Narrow band at the horizon, antisolar side
Belt of Venus	Rayleigh and Mie scattering of sunlight in atmospheric particles	Pink to pale red	10-20° above the antisolar horizon
Anticrepuscular rays	Shadows of clouds or reliefs projected onto atmospheric layers, perspective effect	Light and dark contrast	Apparent convergence towards the antisolar point, covering a large portion of the sky
Antisolar arc	Reflection and refraction in atmospheric ice crystals	Rainbow colors	Celestial vault around the antisolar point
Circumhorizontal arc	Refraction in horizontal ice crystals	Vivid horizontal colored arc	Low in the sky, sometimes visible at the end of the day
Dark zone of the boundary layer	Greater absorption of sunlight in the lower atmospheric layers	Dark gray to dark blue	Horizontal band below the antitwilight arch
Civil, nautical, astronomical twilight	Progressive attenuation of indirect sunlight	Shades ranging from yellow to dark blue	Time interval after sunset or before sunrise

N.B.:
Observation of the antitwilight arch is optimal in clear weather, with an unobstructed horizon and low light pollution. Photography of the phenomenon often requires a long exposure to capture the subtle nuances of colors.

References

FAQ: Everything you need to know about the antitwilight arch

What is the difference between the antitwilight arch and the Belt of Venus?

Both terms refer to the same optical phenomenon. The antitwilight arch is the scientific name that emphasizes the Earth's shadow, while the Belt of Venus is a poetic name referring to the pinkish color of the luminous band that surmounts the shadow. The term "Belt of Venus" was popularized in the 19th century by explorer and scientist John Tyndall, in reference to the Roman goddess of love and beauty, often associated with the pinkish glow of the sky.

Can the antitwilight arch be observed in the city?

Yes, but it is more difficult due to light pollution and buildings that obstruct the horizon. The best conditions are in the countryside, in the mountains or by the sea, with an unobstructed horizon and a clear atmosphere. Atmospheric pollution (aerosols, fine particles) can however intensify pinkish and orange colors, a phenomenon similar to that which makes sunsets more spectacular in urban areas. For optimal observation, move away from large cities and face east (for the evening arch) or west (for the morning arch).

Is the antitwilight arch related to the aurora borealis?

No, the mechanisms are totally different. The antitwilight arch is a sunlight scattering phenomenon in the lower layers of the Earth's atmosphere, while the aurora borealis (and australis) are caused by the interaction of charged particles emitted by the Sun (solar wind) with the Earth's magnetic field and atoms in the upper atmosphere (oxygen and nitrogen). Auroras generally occur at altitudes between 80 and 300 km, well above the Earth's shadow, and are mainly visible in polar regions. On the other hand, the antitwilight arch can be observed from any point on the globe, subject to favorable weather conditions.

Why is the Earth's shadow bluish and not black?

The shadow itself is black, because it corresponds to the area where direct sunlight is blocked by the Earth. However, it is observed through a layer of atmosphere that preferentially scatters blue light (this is the Rayleigh scattering phenomenon, the same that makes the sky blue during the day). This is why its upper limit, which corresponds to the highest part of the shadow, appears bluish or purplish. On the other hand, the lower zone, closer to the horizon, is crossed by a greater thickness of air laden with dust and water vapor, which absorbs more light and gives it a darker shade, sometimes grayish or brownish.

Is the antitwilight arch visible all year round?

Yes, the antitwilight arch is a daily phenomenon that occurs at every sunrise and sunset, everywhere on the planet. However, its visibility depends heavily on atmospheric conditions (clouds, mist, pollution). The best periods are often the intermediate seasons (spring and autumn), when the atmosphere is more stable and sunrises and sunsets are longer. In summer, twilights are shorter in the tropics, but much longer at high latitudes, sometimes offering spectacular antitwilight arches that can last for several hours.

What is the maximum height of the antitwilight arch in the sky?

The antitwilight arch generally extends from the horizon to an altitude of about 10 to 20 degrees above the horizon. The dark band of the Earth's shadow occupies the first degrees (about 0 to 5°), while the Belt of Venus rises to 10-20°. This height depends on latitude, season and atmospheric clarity. In the tropics, the band is often narrower and lower, while in temperate or high latitudes, it can rise higher and form a wider arc.

What is the link between the antitwilight arch and lunar eclipses?

During a lunar eclipse, the Moon passes through the conical shadow projected by the Earth into space. This shadow is the same one that, in the atmosphere, gives rise to the antitwilight arch. During an eclipse, the Moon may appear reddish ("Blood Moon") because sunlight is refracted by the Earth's atmosphere and filtered, allowing only red and orange wavelengths to pass through. This same process of scattering and refraction is at work in the Belt of Venus. By observing the antitwilight arch, we are therefore visualizing the atmospheric portion of the shadow that, a few days later, may plunge the Moon into darkness.

What other optical phenomena can be observed near the antitwilight arch?

Several phenomena can accompany the antitwilight arch:

Anticrepuscular rays: dark and light beams that appear to converge towards the antisolar point, due to shadows cast by clouds or reliefs.
The Globe of Venus (or "false aurora"): a reddish and diffuse glow observed just after sunset, sometimes confused with the arch.
The Brocken spectre: a shadow projected onto a bank of clouds or mist, surrounded by a luminous halo, observable from a summit or an airplane.
The green flash: a brief green flash that appears just above the Sun at the precise moment of its setting or rising, due to the differential atmospheric refraction of wavelengths.

Where can I find forecasts to observe the antitwilight arch?

There are no specific forecasts for the antitwilight arch, but you can use the following tools:

Weather apps (like Windy, Météo-France or AccuWeather) to check cloud cover and air quality.
Specialized websites on atmospheric optics like Atmospheric Optics (new window).
Planetarium apps (Stellarium, SkySafari) to know the precise times of sunrise and sunset and the direction of the antisolar point.
Live webcams located by the sea or in the mountains to visualize conditions in real time before going out.