Zodiacal Light

Last updated on September 16, 2023

Zodiacal Light: A Celestial Phenomenon Connecting Planets and Dust Particles

Zodiacal light, a captivating celestial phenomenon, is not part of our atmosphere but rather originates between the planets. This ethereal cone of softly luminous white light is visible during the twilight hours, approximately an hour after sunset or before dawn. It extends outward and upward along the ecliptic, the path traced by the sun across the stars. Comparable in brightness to the Milky Way, the zodiacal light offers a mesmerizing spectacle for stargazers and astronomy enthusiasts alike.

To catch a glimpse of this enchanting display, it is best to observe when the ecliptic or zodiac makes a steep angle with the horizon. In the Northern Hemisphere, this occurs after sunset in Spring and before dawn in Autumn. The movement of the zodiacal light aligns with the stars, presenting a delicate balance between detecting its radiance against a darkening sky or waiting for optimal darkness when the tip of the cone is lower. Locations closer to the tropics or within low to middle latitudes offer better visibility as the cone stands more upright.

The zodiacal light emanates from the collective glow of dust particles orbiting the sun, stretching out to the orbit of Jupiter and possibly beyond. These particles, ranging from 1 to 300 microns (0.001 - 0.3 mm) in size, are spaced several miles apart from their neighboring particles. Some of this interplanetary dust is believed to originate from comets, while others stem from remnants of rare collisions between asteroids situated between the orbits of Mars and Jupiter. Additionally, orbital resonances with Earth could contribute to the formation of a dust ring at Earth's distance from the sun.

The scattering of light by these dust particles plays a pivotal role in creating the zodiacal light phenomenon. Due to their size relative to visible light wavelengths, the particles primarily scatter light forward, closely following the direction of the original sunlight. As a result, the brightest glow is observed near the sun. However, these particles also scatter light backward, albeit with significantly reduced intensity. In exceptionally dark skies, this faint backscattered light manifests as a delicate glow at the antisolar point, precisely opposite to the sun's position in the sky. Termed the "gegenschein" or "counter glow," it becomes more discernible with averted vision—looking away from the expected point of visibility to enhance sensitivity.

In rare instances of exceptionally dark and clear skies, the zodiacal band becomes visible. This band traverses the sky, connecting the cone of zodiacal light to the gegenschein. By harnessing the power of averted vision and allowing our eyes to adapt to darkness, we can immerse ourselves in these ethereal phenomena and witness their subtle beauty.

For further exploration of zodiacal light and to delve deeper into the captivating visuals it offers, additional viewing hints and stunning images can be found on OPOD (Optics Picture of the Day).

Please note that this article has been automatically converted from the old site and may not appear exactly as intended. The original article can be found here.

Pre Dawn Zodiacal Light imaged by Dominic Cantin (site) in September 2003. The ghostly white cone is sunlight scattered by interplanetary dust particles. The red glow near the horizon is an aurora and much closer. Image ©Dominic Cantin, shown with permission.

The zodiacal light is not part of our atmosphere, it is produced between the planets. Nonetheless it is directly linked to this site’s first topic - the scattering of sunlight by dust particles.

The zodiacal light is a softly luminous cone of white light visible from an hour or so after sunset or before dawn. It extends from where the sun is located beneath the horizon outwards and upwards along the ecliptic, the path of the sun across the stars. It is of similar brightness to the Milky Way.

It is best seen when the ecliptic or zodiac makes a steep angle with the horizon. In the Northern Hemisphere this is after sunset in Spring and before dawn in Autumn. The light moves with the stars and catching it is a balance between detecting its radiance early against a darkening sky or waiting until the sky is really dark when the tip of the cone is getting low. A location in the tropics, or low to middle latitudes, where the cone is more upright helps considerably.

Particles large compared to the wavelength of light (top) scatter light strongly forward with a lesser backward peak. Very small particles (bottom) scatter light more equally in all directions.	 The soft glow is the collective light of dust particles orbiting the sun out to the orbit of Jupiter and perhaps beyond. They are 1- 300 micron (0.001 - 0.3 mm) across and probably each several miles from its neighbours. Some dust is hypothesised to originate from comets, other from remnants of very rare collisions between asteroids between the orbits of Mars and Jupiter. Orbital resonances with Earth could also lead to a dust ring at Earth’s distance from the sun.

The particles are large compared with visible light wavelengths and scatter light strongly forward i.e. close to the original sunlight direction thus producing the brightest glow close to the sun.

They also scatter light backwards, although with much reduced intensity. The faint backscattered light is visible in very dark skies as a faint glow at the antisolar point, the position in the sky directly opposite that of the sun. The glow is the ‘gegenschein’ or ‘counter glow’. Exceptionally dark and clear skies reveal the 'zodiacal band' crossing the sky and connecting the cone of zodiacal light to the gegenschein.

These faint glows need 'averted vision'. With your eyes thoroughly dark adapted look away from the point where you expect to see the glow. It then becomes more easily visible because the eye's rod detectors away from our central vision are more sensitive.

More viewing hints and images on OPOD.

Note: this article has been automatically converted from the old site and may not appear as intended. You can find the original article here.

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