Observation of Earth’s shadow over antisolar sky during sunset & sunrise

2025 February 7

Giovanni di Giovanni

The colours of the sky have been studied for over 50 years, providing important information about the physical state of the atmosphere. In this work, we use simple digital photographs to detect the colours, structures, and brightness of the sky opposite the Sun at sunset and sunrise. A theoretical formula is proposed to process the sky brightness profiles measured on the antisolar vertical and calculate the total optical depth. Interesting results have emerged from these studies conducted in central Italy over the years 2020–2024. First, the transparency of the air was found to follow an almost constant temporal trend. Second, the optical depth was anti-correlated with variations in the amount of total ozone in the air. Third, the optical depth values associated with antitwilight colours made it possible to identify three zones in the local sky with different turbulence. Finally, a correlation between optical depth and light pollution was identified. We emphasise that our study only deals with physico-chemical aspects of the atmosphere, and not with its dynamics; it is not meteorology.

The antisolar sky at sunset & sunrise

Immediately after sunset, the eastern sky is characterised by a broad, dark, iron-grey arc overlaid by a bright pinkish band. The arc is called Earth’s shadow (Figures 1 & 2). The structure appears at twilight on the antisolar horizon (‘antitwilight’) and disappears after about 30 minutes while artificial lighting is still off. A similar phenomenon can be observed at sunrise. When the sky is clear and cloudless, various details are evident both to the naked eye and in panoramic photographs taken even with a simple smartphone. The colours degrade from blue at the zenith to red, turquoise, and grey at the Earth’s shadow above the horizon. Antitwilight observed from sea level is indistinguishable from that observed from mountains and aeroplanes (Figures 3 & 4).^1,2

Very frequently, along with Earth’s shadow, the sky also contains faint but distinct dark beams that appear to diverge from the antisolar point (Figures 5 & 6). Early observers thought it was a perspective effect or optical illusion.³ In 1885, the Italian astronomer Annibale Riccò proved that such rays are produced by the shadow cast on the atmosphere by clouds or mountains near the horizon where the Sun has set.⁴ In a paper from 1928, Rougier, at the time director of Bordeaux Observatory, cited an observation made by the famous André Danjon of the shadow cast in the morning by Mont Blanc above the local atmosphere.⁵ A similar observation was recently repeated by Nick James, the current director of the BAA’s Comet Section (Figure 7).

Currently, antitwilight sky observations are almost completely neglected. However, this activity can be a new opportunity for amateur astronomers and photographers (Figure 8).

At sunset and sunrise, the sun’s rays travel a long path through the atmosphere. Thus, the colours and brightness of the sky are rich in information about molecules and pollution from aerosols in the troposphere and stratosphere. The objective of the present work is the systematic photographic observation of the antitwilight sky at sunset and sunrise, including measurements of its brightness and an assessment of the transparency of the local atmosphere and its annual variations. We also compare these properties with the total ozone content in the atmosphere.

Definitions

The following bands appear about 10 minutes after sunset (or before sunrise). The names recently given by Lynch are appropriate and the most suitable for our study (Figure 9):¹

–  The Horizon Band (HB) is whitish but faint and appears just below the Blue Band (barely visible in Figures 2 & 9).

–  The Blue Band (BB) is the most evident and it extends 150–160° around the horizon. This band is characterised by an ashen or dark, turquoise-blue colour and is not very bright, although its visibility depends on the aerosol load in the troposphere.

–  The Belt of Venus (BV) is very broad, pinkish, and more vivid in the morning, distinctly outlined at the bottom and very diffuse on the top.

–  The Upper Sky (US) is blue and always clearly visible. It extends to the zenith and serves as a background to Earth’s shadow.

–  The Dark Segment is a very dark feature and is in contact with the horizon. It extends vertically for less than 3° and is seen by only a few well-positioned observers. It is regarded as Earth’s true shadow.⁶

Historical background

The colours of the sky have always attracted attention from great scientific minds, such as Newton, Clausius, and many others. Leonardo da Vinci was perhaps the first to attempt a logical explanation for them, in the early 16th century.⁷ The name ‘anticrépuscule’ was given by Johann Kaspar Funck in the early 18th century, in his book De Coloribus Coeli.⁸

The first extensive but qualitative descriptions of the phenomenon appear in the Traité physique et historique de l’aurore boréale, by the French mathematician Jean-Jacques de Mairan, in 1733.⁹ De Mairan expressed astonishment that such a striking effect, as old as the world, had not yet been described in physics and astronomy books. He surmised that, after sunset, sunlight was reflected from the upper atmosphere back into the antisolar air, forming luminous structures seen from the ground.

In his Traité, de Mairan cites the Jesuit priest François Noël as the originator of the term ‘second crépuscule’. Noël called it the ‘secundum Solis crepusculum’: the second twilight (or anti-twilight).¹⁰ In 1785, the botanist and mountaineer Horace Bénédict de Saussure observed Earth’s shadow in Switzerland from the Faulhorn, a mountain 2685-m high, but without interpreting the phenomenon.¹¹ Kaemtz (1843) performed brightness estimates and measurements of the positions and extensions of light arcs in the anti-solar sky at sunset and sunrise and attributed the dark arc to the cone of shadow that Earth casts on the atmosphere.¹² In autumn of the years 1832 and 1833, Bravais made observations from the Faulhorn and also conducted many precise measurements of the brightness, extensions and heights of the pink and violet stripes in the antitwilight sky that appear immediately after sunset.¹³ In 1871, the first satisfactory explanation of the blue colour of the sky was given by the celebrated British physicist Lord Rayleigh.¹⁴ Comprehensive explanations are given on many websites.¹⁵

The last observations of the 19th century were carried out during the Antarctic expedition of 1898. The geologist Henryk Arctowsky first described the anti-twilight phenomenon seen from south-polar latitudes during the months that the scientific ship Belgica was stuck in the ice.¹⁶ Despite these observations, the true nature of the phenomenon had still not been clearly explained by the beginning of the 20th century. Only after the Great War did astronomers and geophysicists take a more concrete interest in crepuscular phenomena. In 1925 Sept, the Swiss Meteorological Commission organised a project of simultaneous observations. Stations were set up in Berne, Neuenegg, Beatenberg, Davos, Faulhorn, Jungfraujoch, and at the meteorological observatory in Potsdam. Unfortunately, only three observations were concluded successfully due to persistently cloudy skies, and the project fell through.

Extensive descriptions and reviews of studies carried out up to the middle of the last century are scattered in the bibliography, and easily found on the internet.^17–25 For example, Dubois, a famous observer of lunar eclipses, concluded based on spectroscopic measurements that the colour of the BB is a consequence of the attenuation of the yellow and red spectral components by atmospheric ozone.^26,27

The amateur interested in observing the colours of the sky will find valuable references in the photographic reviews by Minnaert and Lynch.^28,29 The optical problems of the anti-twilight are comprehensibly exposed in the recent Lynch article.¹ For a more rigorous presentation on atmospheric optics, we refer to the articles by Lee and the large volumes by Rozemberg and Timofejev.^30,31,32

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