Zodiacal Light Over ESO’s La Silla Observatory

Moonlight and zodiacal light lights up the skies over ESO's La Silla observatory. (Credit: Alan Fitzsimmons/ESO)

We don’t put much stock in astrology or horoscopes here at Universe Today, but there’s one thing related to the zodiac that’s all science and no superstition: zodiacal light, captured here in a gorgeous photo by astronomer Alan Fitzsimmons above ESO’s La Silla Observatory.

Created by sunlight reflected off fine particles of dust concentrated inside the plane of the Solar System, zodiacal light appears as a diffuse, hazy band of light visible in dark skies stretching away from a recently-set Sun (or before the Sun is about to rise).

The Moon is located just outside the frame of this picture, bathing the observatory in an eerie light that is reflected off the clouds below.

The La Silla Observatory is located at the outskirts of the Chilean Atacama Desert at an altitude of 2400 meters (7,900 feet). Like other observatories in this area, La Silla is located far from sources of light pollution and, like ESO’s Paranal Observatory, it has some of the darkest night skies on the Earth.

The dome in the foreground, just to the right, is the Swiss 1.2-metre Leonhard Euler Telescope named in honor of the famous Swiss mathematician Leonhard Euler (1707–83).

Image credit: A. Fitzsimmons/ESO


Zodiacal light can be seen in the sky before sunrise or after sunset. Credit: Yuri Beletsky/ESO Paranal

Imagine you could see the position of the Sun, in the sky, relative to the stars (and galaxies, and quasars, and …). If you could, and if you plotted that position throughout the year you’d get a line; that line is called the ecliptic.

And why is it called the ecliptic? Because when the new or full Moon is very close to this, there will be an eclipse (of the Sun, and Moon, respectively).

The Earth goes round the Sun, in an orbit. That orbit defines a plane, which is an infinite two-dimensional sheet; the plane of the ecliptic.

The other planets in the solar system orbit the Sun in planes too, but those planes are slightly tilted with respect to the plane of the ecliptic … so transits of Venus (across the Sun) are quite rare (most times Venus passes either above or below the Sun, when it’s between Earth and the Sun). Mutual transits and occultations of planets are even rarer.

If you’re in a location relatively free of light pollution, on a clear, moonless night you may see zodiacal light. If you trace a line through the middle of it, you’re tracing the ecliptic (zodiacal light is due to reflection of sunlight off dust; dust in the solar system is concentrated in a plane close to the ecliptic plane).

Today astronomers use equatorial coordinates to give positions on the sky, right ascension (RA) and declination (Dec); these are like projections of longitude and latitude out into space (or onto the celestial sphere). However, in Europe ecliptic coordinates were used (up to the 17th century anyway). Here’s a curious fact: historically, Chinese astronomers used equatorial coordinates!

Universe Today stories: Plane of the Ecliptic, Vernal Equinox – Busting the Myth of Balancing Eggs, and Find the Zodiacal Light.

More: Astronomy Cast on Orbit of the Planets, and a Glow After Sunset.

Find the Zodiacal Light

Zodiacal Light

Look to the West, just after the Sun has set, and you might see a dimly glowing triangle rising up from the horizon. This is the zodiacal light, where sunlight reflects off dust particles in the Solar System. If you live in the mid-northern latitudes, look for the zodiacal light in the evening in Autumn and in the morning in Spring.

Now that the Moon has departed from the early evening sky, this is a good time in the northern hemisphere to watch the western skyline for the evening zodiacal lights. If you live in an area where light and air pollution isn’t heavy, you stand a very good chance of seeing the interplanetary dust in the plane of our own solar system lit by the setting Sun, and that is the zodiacal light.

In the Spring in the northern hemisphere, the ecliptic plane extends upright from the western horizon. (If you live in the southern hemisphere, you need to watch the eastern horizon before dawn after New Moon.) When the Sun is just below the horizon, we can see a ghostly glowing pyramid. But, what is the zodiacal light?

Sunlight is back-scattered off small interplanetary dust particles, perhaps some of them from the very formation of our solar system itself. However, a lot of these tiny, millimeter sized splinters are from asteroids – or debris ejected from comets. Some of these particles are initially distributed in the trails that cause meteor showers, but whole lot of the dust eventually gathers along the ecliptic plane.

For the ultra-tiny particles, the radiation and solar wind disperses them beyond the confines of our solar system. The larger particles spiral inwards, pulled towards the Sun by gravity and form a flattened disc – a very low density cloud of dust, coincident with the plane of the solar system. Sunlight absorbed by the particles is re-emitted as invisible infrared radiation. This re-radiation causes the particles to spiral slowly into Sun, thus requiring continuous regeneration of the dust particles composing this cloud. The reflective particle disc makes its home in the same path the planets take around the Sun – the ecliptic. This imaginary path across the sky is where we here on Earth see the Sun and Moon, and it’s also home to the constellations of the zodiac!

Using the same celestial mechanics that give us times of solstice, equinox, lunar and solar eclipses, it only stands to reason there comes a time when the ecliptic plane appears nearly vertical from a certain vantage point. For the northern hemisphere it’s west in the spring and east in the fall. For the southern hemisphere it’s just the opposite! When the plane is near vertical, the thick air near the horizon doesn’t block out relatively bright reflecting dust and we see the zodiacal light!

Head out to an open horizon area where you’re away from man-made light pollution. As the skies grow dark, look for a faint pyramid of light spread out over a very large area of the sky. The zodiacal light won’t be as dramatic as photos show it. Near its base at the horizon, it can measure as broad as 40 degrees (two handspans), and stretch up as high as 60-80 degrees under good conditions. The spectrum of the zodiacal light is the same as the solar spectrum, reinforcing the deduction that it is merely sunlight reflected by dust in the plane of the planets. If you think you see a ghostly glow, you’re probably right!

If you’ve seen the zodiacal light, let us know!