Zodiacal or zodiac light is a faint luminous phenomenon that originates in our solar system. The best time for its observation is in spring and autumn, but even then many things have to fit together.
Interplanetary dust and the ecliptic
Not only do planets, planetoids, and comets revolve around the Sun, but so does a lot of dust. Most of these bodies move in a common plane around the sun, the ecliptic. This is also true for the fine dust particles. Viewed from Earth, we perceive the ecliptic as an imaginary line near which the Sun and the planets move across the sky during the course of a year. It runs thereby by the always same constellations, the signs of the zodiac. And contrary to astrology, in reality there are not 12, but 13 of them. The zodiac is also called Zodiak (from the Greek Zodiakos).
In a really dark and moonless night, after the end of the evening or before the beginning of the dawn, one can observe a delicate glow in the form of a cone above the horizon in the area of the ecliptic. The latter must not be too inclined for this, but must point into the sky at as steep an angle as possible. At the equator this first condition is given every day, with us twice a year – namely in spring (optimum March 1) as well as in autumn (optimum October 15). In spring the zodiacal light can be observed in the evening, as soon as the sun has sunk low enough below the horizon. In autumn it is the other way round, here the observation time is the early morning before sunrise.
Fig. 1: Zodiakallicht am Cerro Paranal in Chile; Source: Wikipedia
Scattering and reflection
The dust particles scatter the sunlight, especially the forward sc attering is relevant(Mie scattering). This forward scattering is responsible for the light cone mentioned above. Under optimal conditions the brightness can reach that of the Milky Way, but for this the night must be perfectly clear and very dark. However, the light of the moon or artificial light pollution often ensure that the zodiacal light remains invisible or becomes visible only photographically with appropriate exposure time. Thus, in the vicinity of cities, this phenomenon usually eludes the human eye. Besides the primary cone, however, there are other luminous phenomena that can be seen only in the darkest places on Earth. Starting from the top of the cone, the so-called light bridge stretches across the sky, and at the point opposite the sun, the Gegenschein is found. At the Gegenschein the light of the sun is not only scattered, but also reflected back to us. Our cover picture shows an example of cone, light bridge and Gegenschein.
Fig. 2: The faint Gegenschein above the VLT, also Cerro Paranal in Chile; Source: Wikipedia
The interplanetary dust becomes visible for us in form of the zodiacal light. But it still holds one or the other secret. It is not static, because by certain effects it is permanently removed from the solar system – but also newly formed. Above a certain size, particles are slowed down by the Poynting-Robertson effect, they leave their orbit and finally burn up in the sun. Very fine dust, on the other hand, is virtually "blown" out of the solar system by radiation pressure. Nevertheless, dust is present at all times. The formation of new dust was a mystery for a long time and is still not conclusively explained. During the Juno mission to Jupiter, however, there was increasing evidence that Mars and its two small moons, among others, probably contribute to this process.
Another One Bites the Dust
Among others, a certain Brian May also devoted himself to this interplanetary dust. However, he is less known as an astrophysicist, but rather as the guitarist of the cult band Queen! Brian May began in 1965 the study of the astrophysics at the Imperial College in London. At this time he was already a member of the band "1984". In 1971 he investigated with colleagues by means of spectroscopic investigations on Tenerife the composition of the interplanetary dust and made this also the topic of his doctoral thesis. In 1973/1974, however, he broke it off in favor of his musical career. But he never lost the fundamental interest, because in 2006 he resumed the work, updated the data and finally graduated in astrophysics on August 23, 2007.
Fig. 3: Queen-Gitarrist Brian May am Paranal-Observatorium 2015; Source: Wikipedia
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