Scientists interested in the spread of volcanic dust now have a detector - the lunar eclipse. During a total eclipse, the atmosphere acts like a giant lens refracting sunlight into Earth's shadow. This is the ''earth-light'' which illumines the eclipsed moon. If the atmosphere is dusty, the lighting is dimmed.
Richard A. Keen, an environmental scientist at the University of Colorado, has learned how to use such dimming to gauge the average dustiness of the global atmosphere. And when an eclipse occurs within a few months or years of an eruption, scientists can use it to estimate the intensity of the volcanic dust veil. Dr. Keen, himself, has studied 21 total lunar eclipses that occurred between 1960 and 1982. However, in publishing this study in Science, he notes that it may be possible to backtrack eclipses at least to the year 1600 and build up an historical series of dust estimates.
He says that most dark eclipses since 1600 can be linked to volcanic eruptions. The great 1893 explosion of Krakatau, in particular, was followed by a series of such dimmed eclipses.
In his own study, the 1982 eruption of El Chichon stands out. It had the strongest dimming effect since that of the Mt. Agung eruption in 1963. Agung dust gave colorful sunsets for several years.
The trick in making these dust measurements is to make a fair estimate of the theoretical undimmed earthlighting and to compare this with the observed brightness. The theory takes account of such factors as the moon's distance, the influence of global cloudiness on the earthlight effect, and the response of the dark-adapted human eye to the lighting.
Keen acknowledges that this introduces some arbitrariness into the technique. However, any bias due to theoretical assumptions will affect all the estimates so that meaningful distinctions can be made between eclipses dimmed by volcanic dust and unobscured eclipses. In his study, eclipses that occur within a few months or years of a volcanic eruption are significantly dimmer than those not associated with volcanism.
One subtle effect in the measurements is related to how the moon passes through the planet's shadow. If the moon crosses the shadow centerline, the earthlighting is due, more or less equally, to the Earth's two hemispheres. But if the moon passes north or south of the shadow centerline, the influence of the northern or southern hemisphere is enhanced.
Also, if eclipses occur at propitious times, the spread of a dust veil can be detected. An eclipse in July 1982, three months after El Chichon's eruption, showed uneven earthlighting. The northern part of the moon was darker than the southern part. Keen attributes this to the incomplete dispersion of the dust.
A second eclipse in December 1982, nine months after the eruption, had much more uniform earthlighting. This is the eclipse that was the darkest seen since Mt. Agung's eruption in December 1963.
Keen notes that the technique has drawbacks. Eclipses don't occur to order, when scientists might want them. But, in compensation, scientists do have a way of directly estimating the dimming effect of volcanic dust veils. This is an effect of high interest to atmospheric scientists. If dust dims earthlight on the moon, it also dims sunshine at Earth's surface. And that could affect the climate.