Reply To: Antitwilight Sky

[Mr Giovanni Di Giovanni]

Yes, the arc is called Earth’s Shadow, it persists until about 30 minutes after sunset and appears 30 minutes before the sun rises. However, this is not the true Earth’s Shadow, which may be visible, but with great difficulty, as a narrow ‘Dark Segment’ in contact with the horizon, and in any case only from high mountains. The antitwilight phenomenon is generated by the scattering (Rayleigh, and Mie) of sunlight that has passed through the atmosphere, grazing the planet. Therefore, position and intensity of colours contain information about the physical state of the atmosphere down to the synoptic scale (500-1000 km)
I have been systematically observing this phenomenon for about three years. Observation is cheap, easy and interesting. Light pollution and air turbulence do not affect it.
Perhaps, Leonardo da Vinci was the first to attempt a rational explanation of the colours in the sky. The French mathematician de Mairan in his Traité Physique et Historique de l’aurore boréale (page 400) expressed astonishment that such a striking effect, as old as the world, was only mentioned in Joh. Casp. Funccii and not in the books on Physics and Astronomy.