The Antitwilight Arch: Earth's Shadow

The Phenomenon of Earth's Shadow

The antitwilight arch, also known as the Belt of Venus, is an atmospheric optical phenomenon that reveals the shadow our planet casts into the atmosphere. Visible at twilight, opposite the sun at sunrise or sunset, this dark band topped by a pinkish glow offers a spectacular demonstration of the Earth's roundness.

The dark band corresponds to the Earth's shadow projected onto the atmosphere, while the colored band is due to the backscattering of sunlight by molecules and aerosols. The antitwilight arch is not extremely rare; it is a relatively common phenomenon that occurs at every sunrise or sunset under favorable atmospheric conditions.

Observation Conditions

To observe this phenomenon, several conditions must be met:

• A clear sky and an unobstructed horizon to the east (for the evening arch) or west (for the morning arch)
• A relatively clear atmosphere with suspended particles to diffuse the light, ideally with little pollution
• A sky without low clouds opposite the Sun
• A precise moment: about 10 to 15 minutes after sunset or before sunrise

Link Between 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.

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 diffusion 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 superimposition between the Earth's geometric shadow and the phenomena of light diffusion 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.

Comparative Table of Twilight Phenomena