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Image of the Day: Earth's Rare Zodiacal Light


One of the rarest of Earth's astronomical events is a ghostly glow called the zodiacal light that the ancient Greeks believed was caused by distant volcanic eruptions. Visible in the Northern Hemisphere for the next two weeks, the phenomenon -- caused by sunlight scattering off countless grains of microscopic interplanetary dust spread out to beyond the orbit of Mars -- will be visible above the western horizon as a faint cone of light that extends halfway up the sky for about an hour after sunset.

The vast majority of the interplanetary dust is concentrated within the plane of the inner solar system near the sun, making the dust grains combined with light appear along the ecliptic, the path in the sky each planet follows.

Image Credit: ESO




It's a lovely photograph!

I think this article is confused about where to see the zodiacal light these days, though. We should look in the *eastern* sky, before dawn -- which is also where Mars is at the moment -- rather than in the western evening sky.

In autumn, the zodiacal light is much harder to see in the evening, since the path of the ecliptic lies low in the southwest, almost parallel to the horizon and often lost in the murk. In the morning sky these days, the ecliptic rises very steeply above the horizon, so the zodiacal light is well above the horizon murk long before sunrise, while the sky is still dark.

The situation is reversed in the springtime -- then, the ecliptic rises steeply above the western evening horizon, so late winter/early spring is a good time to catch it in evening time, but it's hard to see in the morning sky. For the same reason, late winter/early spring is a good time to get a look at Mercury in the evening sky -- during its brief (week or two) excursions away from the sun's glare, it's easiest to see when high above the horizon. The steeply sloping ecliptic gives the best advantage.

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