Posted on by David Dickinson

The September Equinox: ‘Tis the Season to Spy the Zodiacal Light

The zodiacal light in the Nevada dawn. The plane of the ecliptic can be traced by Jupiter in Gemini & Mars in the Beehive cluster just below center. (Credit: Cory Schmitz, used with permission).

This week leading up to the September equinox offers you a fine chance to catch an elusive phenomenon in the pre-dawn sky.

We’re talking about the zodiacal light, the ghostly pyramid-shaped luminescence that heralds the approach of dawn. Zodiacal light can also be seen in the post-dusk sky, extending from the western horizon along the ecliptic.

September is a great time for northern hemisphere observers to try and sight this glow in the early dawn. This is because the ecliptic is currently at a high and favorable angle, pitching the zodiacal band out of the atmospheric murk low to the horizon. For southern hemisphere observers, September provides the best time to hunt for the zodiacal light after dusk. In March, the situation is reversed, with dusk being the best for northern hemisphere observers and dawn providing the best opportunity to catch this elusive phenomenon for southern observers.

The clash of the zodiacal light and the plane of our galaxy. (Credit: Cory Schmitz, used with permission).
The clash of the zodiacal light and the plane of our galaxy. (Credit: Cory Schmitz, used with permission).

Cory Schmitz’s recent outstanding photos taken from the Nevada desert brought to mind just how ephemeral a glimpse of the zodiacal light can be. The glow was a frequent sight for us from dark sky sites just outside of Tucson, Arizona—but a rarity now that we reside on the light-polluted east coast of the U.S.

In order to see the zodiacal light, you’ll need to start watching before astronomical twilight—the start of which is defined as when the rising Sun reaches 18 degrees below the local horizon—and observe from as dark a site as possible under a moonless sky.

The Bortle dark sky scale lists the zodiacal light as glimpse-able under Class 4 suburban-to-rural transition skies. Under a Class 3 rural sky, the zodiacal light may extend up to 60 degrees above the horizon, and under truly dark—and these days, almost mythical—Class 1 and 2 skies, the true nature of the zodiacal band extending across the ecliptic can become apparent.  The appearance and extent of the zodiacal light makes a great gauge of the sky conditions at that favorite secret dark sky site.

The source of the zodiacal light is tiny dust particles about 10 to 300 micrometres in size scattered across the plane of the solar system. The source of the material has long been debated, with the usual suspects cited as micrometeoroid collisions and cometary dust. A 2010 paper by Peter Jenniskens and David Nesvorny in the Astrophysical Journal cites the fragmentation of Jupiter-class comets. Their model satisfactorily explains the source of about 85% of the material. Dust in the zodiacal cloud must be periodically replenished, as the material is slowly spiraling inward via what is known as the Poynting-Robertson effect. None other than Brian May of the rock group Queen wrote his PhD thesis on Radial Velocities in the Zodiacal Dust Cloud.

But even if you can’t see the zodiacal light, you still just might be able to catch it. Photographing the zodiacal light is similar to catching the band of the Milky Way. In fact, you can see the two crossing paths in Cory’s images, as the bright winter lanes of the Orion Spur are visible piercing the constellation of the same name. Cory used a 14mm lens at f/3.2 for the darker image with a 20 second exposure at ISO 6400 and a 24mm lens at f/2.8 with a 15 second exposure at ISO 3200 for the brighter shot.

The orientation of the ecliptic & the zodiacal band as seen from latitude 30 deg north in September, about 1 hour before sunrise. (Created by the author in Stellarium).
The orientation of the ecliptic & the zodiacal band as seen from latitude 30 deg north in September, about 1 hour before sunrise. (Created by the author in Stellarium).

Under a truly dark site, the zodiacal light can compete with the Milky Way in brightness. The early Arab astronomers referred to it as the false dawn. In recent times, we’ve heard tales of urbanites mistaking the Milky Way for the glow of a fire on the horizon during blackouts, and we wouldn’t be surprised if the zodiacal light could evoke the same. We’ve often heard our friends who’ve deployed to Afghanistan remark how truly dark the skies are there, as military bases must often operate with night vision goggles in total darkness to avoid drawing sniper fire.

Another even tougher but related phenomenon to spot is known as the gegenschein. This counter glow sits at the anti-sunward point where said particles are approaching 100% illumination. This time of year, this point lies off in the constellation Pisces, well away from the star-cluttered galactic plane. OK, we’ve never seen it, either. A quick search of the web reveals more blurry pics of guys in ape suits purporting to be Bigfoot than good pictures of the gegenschein. Spotting this elusive glow is the hallmark of truly dark skies. The anti-sunward point and the gegenschein rides highest near local midnight.

And speaking of which, the September equinox occurs this weekend on the 22nd at 4:44 PM EDT/20:44 Universal Time. This marks the beginning of Fall for the northern hemisphere and the start of summer for the southern.

The Full Harvest Moon also occurs later this week, being the closest Full Moon to the equinox occurring on September 19th at 7:13AM EDT/11:13 UT. Said Moon will rise only ~30 minutes apart on successive evenings for mid-northern latitude observers, owing to the shallow angle of the ecliptic. Unfortunately, the Moon will then move into the morning sky, drowning out those attempts to spy the zodiacal light until late September.

Be sure to get out there on these coming mornings and check out the zodiacal light, and send in those pics in to Universe Today!

Posted on by Jason Major

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

Posted on by Jean Tate

Ecliptic

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

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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.

Posted on by Tammy Plotner

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!