Shadow Bands
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Shadow bands are thin, wavy lines of alternating light and dark that can be seen moving and undulating in parallel on plain-coloured surfaces immediately before and after a total solar eclipse. They are caused by the refraction by Earth's atmospheric turbulence of the solar crescent as it thins to a narrow slit, which increasingly collimates the light reaching Earth in the minute just before and after totality. The shadows' detailed structure is due to random patterns of fine air
turbulence In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to laminar flow, which occurs when a fluid flows in parallel layers with no disruption between ...
that refract the collimated sunlight arriving from the narrow eclipse crescent. The bands' rapid sliding motion is due to shifting air currents combined with the angular motion of the Sun projecting through higher altitudes. The degree of collimation in the light gradually increases as the crescent thins, until the solar disk is completely covered and the eclipse is total. Stars twinkle for the same reason. All light passing through the Earth's atmosphere encounters tiny disturbances in temperature, pressure, and humidity. These disturbances change the air's refractive index, so that the light essentially passes through innumerable tiny prisms. Thus, the entire sky essentially "dances" randomly at the scale of the disturbances. However, they are so small that even the visible disk of planets (Venus, Jupiter, etc) are larger than the "dancing" scale, and so their brightness appears to remain steady. Only stars are visibly affected, because they are so far away that they are essentially pinpoints of light. This is why astronomers sometimes use the phrase, "stars twinkle, planets don't". Similarly, shadow bands are essentially the "twinkling" of the Sun's thin crescent in the seconds surrounding totality.


History

* In 1820, Hermann Goldschmidt of Germany noted shadow bands visible just before and after totality at some eclipses. * In 1842, George B. Airy, the English astronomer royal, saw his first total eclipse of the Sun. He recalled shadow bands as one of the highlights: "As the totality approached, a strange fluctuation of light was seen upon the walls and the ground, so striking that in some places children ran after it and tried to catch it with their hands." * In 1905, Catherine Octavia Stevens observed shadow bands at the start of the total eclipse of August 30 at Cas Català,
Majorca Mallorca, or Majorca, is the largest of the Balearic Islands, which are part of Spain, and the List of islands in the Mediterranean#By area, seventh largest island in the Mediterranean Sea. The capital of the island, Palma, Majorca, Palma, i ...
. "As to the character of their appearance and mode of progression, it was observed that they swept along with a flight that was at once rapid and orderly, there was no confusion of the wavy lines with one another, but all bore along in one and the same direction in parallel formation, traversing the ground as water-wave reflections may be seen to do on the under surface of a boat, only that there seemed in the case of the shadow-bands to be a more distinct expression of a forward movement." Clouds prevented observations after totality. * 22 years after observing the same eclipse from
Zaragoza Zaragoza (), traditionally known in English as Saragossa ( ), is the capital city of the province of Zaragoza and of the autonomous communities of Spain, autonomous community of Aragon, Spain. It lies by the Ebro river and its tributaries, the ...
, the British amateur astronomer Percy Mayow Ryves wrote in the
Monthly Notices of the Royal Astronomical Society ''Monthly Notices of the Royal Astronomical Society'' (MNRAS) is a peer-reviewed scientific journal in astronomy, astrophysics and related fields. It publishes original research in two formats: papers (of any length) and letters (limited to ...
that the experience had convinced him 'that the shadow bands were the result of light from a very small source, a small segment of the Sun’s disc, revealing the irregularities of density in the atmosphere, in a manner similar to that in which the irregularities in the glass of a window pane are revealed by the light of the planet Venus.' * In 2008, British astrophysicist Stuart Eves speculated that shadow bands might be an effect of
infrasound Infrasound, sometimes referred to as low frequency sound or incorrectly subsonic (subsonic being a descriptor for "less than the speed of sound"), describes sound waves with a Audio frequency, frequency below the lower limit of human audibility ...
, which involves the shadow of the Moon travelling at supersonic speed and inducing an atmospheric shock wave. However, astronomy professor Barrie Jones, an expert on shadow bands, stated, "The cceptedtheory works; there's no need to seek an alternative." * In 2024, students at the
University of Pittsburgh The University of Pittsburgh (Pitt) is a Commonwealth System of Higher Education, state-related research university in Pittsburgh, Pennsylvania, United States. The university is composed of seventeen undergraduate and graduate schools and colle ...
devised an empirical test to collect evidence for both the atmospheric turbulence theory and Eves's moon slit theory. The test of atmospheric turbulence involved using a high-altitude balloon with weather instruments to measure relationships between humidity, temperature, and
barometric pressure Atmospheric pressure, also known as air pressure or barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1,013.2 ...
in the atmosphere and the shadow bands on the ground. The test of the alternative theory involved sending another high-altitude balloon 90,000 feet above Concan, Texas to detect light patterns indicating shadow bands outside of the atmosphere.


References

{{reflist Light Solar eclipses