 The Atacama Desert in northern Chile is famed for its dark night skies, which can be enjoyed in their full glory thanks to the absence of light pollution. But even the darkest sky is not completely dark. Astronomers at ESO's observatories often encounter a natural light phenomenon above ESO's telescopes, known as the zodiacal light. This is the ESOcast, cutting edge science and life behind the scenes of ESO, the European Southern Observatory, exploring the ultimate frontier with our host Dr J, aka Dr Joe Liske. Hello and welcome to another episode of the ESOcast. Now ESO's observatories are such incredibly dark sites that normally the only thing that illuminates them on a moonless night is the faint light from the billions of stars in the Milky Way. But there are a number of other interesting phenomena that can be observed in the skies above the telescopes. These include for example the faint veil of air glow and the occasional appearance of red sprites above the odd distant thunderstorm. And then every night, especially in the hours just after dusk or before dawn, this faint fuzzy column of light appears in the sky, just above the horizon extending upwards. This ghostly glow is known as zodiacal light. Shortly after sunset, just as stars begin to appear in the sky, the first hints of zodiacal light also become visible. As darkness sweeps over the desert, this light becomes more prominent and can be seen as a bright column of light reaching up from the horizon. This luminous column follows the starry background across the sky, eventually disappearing below the horizon as the earth rotates on its axis. Even after the brightest part of the zodiacal light has dropped below the horizon, traces of it are still present, although it now resembles an extremely faint wispy bridge that brightens again in the early morning, just before daybreak. The origins of zodiacal light are to be found in the inner solar system. The sun is surrounded by tiny grains of ice and dust that are constantly being replenished by crumbling icy comets and colliding asteroids. These grains are distributed within the same flat disk of space inhabited by the planets. When viewed from earth, this disk appears as a narrow path across the sky, called the ecliptic, which the sun, moon and planets all appear to follow as they move in the sky. Now zodiacal light is created when light coming from the sun is scattered forwards off the particles in this disk in the direction of earth. Now when viewed from earth, this creates the appearance of a continuous band of light along the ecliptic that gets fainter as you look further away from the sun. Now the constellations of the zodiac of course also lie along the ecliptic and that's why we call this ghostly glow zodiacal light. Along the ecliptic, high in the sky, an oval patch of illumination can also appear known as Gagenschein or counter glow. Named by the German explorer Alexander von Humboldt, this phenomenon is created by sunlight that is scattered backwards off interplanetary dust particles. It can be seen at the point in the sky opposite the sun. A similar phenomenon can be experienced here on earth. When you turn your back to the sun in foggy weather, a halo of light called a glory sometimes appears around your shadow on the ground. The zodiacal light phenomenon seems to have been first investigated in the late 1600s by the Italian astronomer Giovanni Cassini and the Swiss mathematician Nicola Fattio de Duiier. They were absolutely fascinated with this light in the sky and of course back in the 17th century there was very little light pollution and so for them it was relatively easy to observe this phenomenon even from cities. Human observations have shown that the solar system might not be the only one to exhibit zodiacal light. Data from the very large telescope Interferometer at ESO's Paranal Observatory has revealed that numerous other planetary systems are also surrounded by interplanetary dust. Zodiacal light is a really photogenic feature and so it's no surprise that it's become a popular subject for nighttime photographers in the Atacama Desert. The fact that it's so prominent at ESO's observatories is a beautiful demonstration of how incredibly good the observing conditions at these sites actually are. This is Dr Jay signing off for the ESOcast. Join me again next time for another cosmic adventure.