Zodiacal light

The zodiacal light ( gr ζῴδιον Zodion " zodiac signs " ) is an extremely weak permanent luminous phenomenon in the sky, which belongs to the astronomical phenomena. The luminous phenomenon is limited to the vicinity of the ecliptic, the zodiac. The zodiacal light is caused by reflection and scattering of sunlight from the constituents of the gas and dust cloud surrounding the sun as a thin disk in the planetary level.

The zodiacal light spans the sky along the ecliptic. Close to the Sun to a distance of a few degrees is called the F- corona. Your joins the zodiacal light. It goes over into the light bridge - exactly opposite to the Sun - leads to Gegenschein.

Observation

In absolutely clear nights you can see a faint streak of light on a flat plane or sea over the horizon shortly after astronomical twilight in the west or just before the east.

In principle, one can observe the zodiacal light throughout the year, if you are far enough away from artificial light sources and the light pollution caused by them. In temperate latitudes, it is best in spring after the evening twilight, in the fall before the dawn.

Formation

The F- corona, zodiacal light, light bridge and the Gegenschein caused by reflection and scattering of sunlight by dust particles surrounding the sun as a thin disk in the planetary level. This so-called interplanetary dust is produced constantly by new collisions of small rocks as meteoroids and asteroids.

The density of the dust particles from 0.001 to 0.1 mm in size is 10 to 14 particles per cm ³. Due to the Poynting - Robertson effect, there is a size-selection of the particles with the effect that particles larger than 0.001 mm braked by the sunlight, spiral drive to the sun and eventually evaporate. Smaller particles are subject to the Poynting - Robertson effect is limited and will be driven by the majority here, solar radiation pressure from the solar system.

The zodiacal light is particularly easy to see when the sun is at a steep angle as possible up or goes down, which is given for example in Central Europe Spring and early autumn. During this time, the dawn is shortened and the glow rises very high above the horizon. In the equatorial latitudes, this condition is met throughout the year.

The brightness of the zodiacal light is directly related to the angular dependence of light scattering. With the parameters particle size and density, refractive index and reflectivity, the brightness distribution of good play with a Mie scattering and classical scattering. The forward scattering is too large, see the pronounced F- corona and the relatively bright zodiacal light near the sun. At a distance of about 90 degrees to the sun, the scattering is small, corresponding to a very faint light bridge. The backward scattering 180 degrees rises somewhat, which is why the Gegenschein stands out as a light spot. The brightness of the light bridge and the backlight with background correction is 100 to 150 S10, ie in the same order as the absolute clear night sky (not visible to the eye single stars ). The sum of the substrate and Gegenschein is plotted on the graph against the distance to the sun.

History

Probably already knew the Egyptians several thousand years ago this light phenomenon. The first detailed description of the zodiacal light, however, was made ​​only in the years 1682-1683 by the Italian mathematician and astronomer Giovanni Domenico Cassini and his Swiss counterpart Nicolas Fatio de Duillier.

The Gegenschein of the zodiacal light was, as far as known, first observed in 1730 by the French Jesuit priest and professor Esprit Pezenas ( 1692-1776 ). Further observations were made by Alexander von Humboldt during his 1799-1803 trip to South America. In it, the term " Gegenschein " goes back.

The Danish astronomer Theodor Brorsen published in 1854 the first systematic studies on the Gegenschein him and could already interpret them correctly. At the same time Brorsen observed here as the first, that the zodiacal light is a phenomenon that spans the entire sky, which can be seen as a weak bridge of light from zodiacal light up to Gegenschein under favorable conditions. The causing dust clouds finally discovered the astronomer Walter Grotrian ( 1890-1954 ).