The unexpected contribution of medieval monks to volcanology

By observing the night sky, medieval monks unwittingly recorded some of history's largest volcanic eruptions. An international team of researchers, led by the University of Geneva (UNIGE), drew on readings of 12th and 13th century European and Middle Eastern chronicles, along with ice core and tree ring data, to accurately date some of the biggest volcanic eruptions the world has ever seen. Their results, reported in the journal Nature, uncover new information about one of the most volcanically active periods in Earth's history, which some think helped to trigger the Little Ice Age, a long interval of cooling that saw the advance of European glaciers.

It took the researchers almost five years to examine hundreds of annals and chronicles from across Europe and the Middle East, in search of references to total lunar eclipses and their colouration. Total lunar eclipses occur when the moon passes into the Earth's shadow. Typically, the moon remains visible as a reddish orb because it is still bathed in sunlight bent round the Earth by its atmosphere. But after a very large volcanic eruption, there can be so much dust in the stratosphere -- the middle part of the atmosphere starting roughly where commercial aircraft fly -- that the eclipsed moon almost disappears.

Medieval chroniclers recorded and described all kinds of historical events, including the deeds of kings and popes, important battles, and natural disasters and famines. Just as noteworthy were the celestial phenomena that might foretell such calamities. Mindful of the Book of Revelation, a vision of the end times that speaks of a blood-red moon, the monks were especially careful to take note of the moon's coloration. Of the 64 total lunar eclipses that occurred in Europe between 1100 and 1300, the chroniclers had faithfully documented 51. In five of these cases, they also reported that the moon was exceptionally dark.

The contribution of Japanese scribes

Asked what made him connect the monks' records of the brightness and colour of the eclipsed moon with volcanic gloom, the lead author of the work, Sébastien Guillet, senior research associate at the Institute for environmental sciences at the UNIGE, said: "I was listening to Pink Floyd's Dark Side of the Moon album when I realised that the darkest lunar eclipses all occurred within a year or so of major volcanic eruptions. Since we know the exact days of the eclipses, it opened the possibility of using the sightings to narrow down when the eruptions must have happened."

The researchers found that scribes in Japan took equal note of lunar eclipses. One of the best known, Fujiwara no Teika, wrote of an unprecedented dark eclipse observed on 2 December 1229: 'the old folk had never seen it like this time, with the location of the disk of the Moon not visible, just as if it had disappeared during the eclipse... It was truly something to fear.' The stratospheric dust from large volcanic eruptions was not only responsible for the vanishing moon. It also cooled summer temperatures by limiting the sunlight reaching the Earth's surface. This in turn could bring ruin to agricultural crops.

Cross-checking text and data

"We know from previous work that strong tropical eruptions can induce global cooling on the order of roughly 1°C over a few years," said Markus Stoffel, full professor at the Institute for environmental sciences at the UNIGE and last author of the study, a specialist in converting measurements of tree rings into climate data, who co-designed the study. "They can also lead to rainfall anomalies with droughts in one place and floods in another."

Despite these effects, people at the time could not have imagined that the poor harvests or the unusual lunar eclipses had anything to do with volcanoes -- the eruptions themselves were all but one undocumented. "We only knew about these eruptions because they left traces in the ice of Antarctica and Greenland," said co-author Clive Oppenheimer, professor at the Department of Geography at the University of Cambridge. "By putting together the information from ice cores and the descriptions from medieval texts we can now make better estimates of when and where some of the biggest eruptions of this period occurred."

Climate and society affected

To make the most of this integration, Sébastien Guillet worked with climate modellers to compute the most likely timing of the eruptions. "Knowing the season when the volcanoes erupted is essential, as it influences the spread of the volcanic dust and the cooling and other climate anomalies associated with these eruptions," he said.

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Stars disappear before our eyes

A startling analysis from Globe at Night -- a citizen science program run by NSF's NOIRLab -- concludes that stars are disappearing from human sight at an astonishing rate. The study finds that, to human eyes, artificial lighting has dulled the night sky more rapidly than indicated by satellite measurements. The study published in the journal Science showcases the unique contributions that citizen scientists can make in essential fields of research.

From the glowing arc of the Milky Way to dozens of intricate constellations, the unaided human eye should be able to perceive several thousand stars on a clear, dark night. Unfortunately, growing light pollution has robbed about 30% of people around the globe and approximately 80% of people in the United States of the nightly view of their home galaxy. A new paper published in the journal Science concludes that the problem is getting rapidly worse.

New citizen-science-based research sheds alarming light on the problem of 'skyglow' -- the diffuse illumination of the night sky that is a form of light pollution. The data for this study came from crowd-sourced observations collected from around the world as part of Globe at Night, a program run by NSF's NOIRLab and developed by NRAO astronomer Connie Walker. The research reveals that skyglow is increasing more rapidly than shown in satellite measurements of Earth's surface brightness at night.

"At this rate of change, a child born in a location where 250 stars were visible would be able to see only abound100 by the time they turned 18," said Christopher Kyba, a researcher at the German Research Centre for Geosciences and lead author of the paper detailing these results.

Light pollution is a familiar problem that has many detrimental effects, not only on the practice of astronomy. It also has an impact on human health and wildlife, since it disrupts the cyclical transition from sunlight to starlight that biological systems have evolved alongside. Furthermore, the loss of visible stars is a poignant loss of human cultural heritage. Until relatively recently, humans throughout history had an impressive view of the starry night sky, and the effect of this nightly spectacle is evident in ancient cultures, from the myths it inspired to the structures that were built in alignment with celestial bodies.

Despite being a well-recognized issue, however, the changes in sky brightness over time are not well documented, particularly on a global scale.

Globe at Night has been gathering data on stellar visibility every year since 2006.* Anyone can submit observations through the Globe at Night web application on a desktop or smartphone. After entering the relevant date, time and location, participants are shown a number of star maps. They then record which one best matches what they can see in the sky without any telescopes or other instruments.

This gives an estimate of what is called the naked eye limiting magnitude, which is a measure of how bright an object must be in order to be seen. This can be used to estimate the brightness of skyglow, because as the sky brightens, the fainter objects disappear from sight.

The authors of the paper analyzed more than 50,000 observations submitted to Globe at Night between 2011 and 2022, ensuring consistency by omitting entries that were affected by factors including cloud cover and moonlight. They focused on data from Europe and North America, since these regions had a sufficient distribution of observations across the land area as well as throughout the decade studied. The paper notes that the sky is likely brightening more quickly in developing countries, where satellite observations indicate the prevalence of artificial lighting is growing at a higher rate.

After devising a new method to convert these observations into estimates of the change in skyglow, the authors found that the loss of visible stars reported by Globe at Night indicates an increase in sky brightness of 9.6% per year over the past decade. This is much greater than the roughly 2% per year global increase in surface brightness measured by satellites.

"This shows that existing satellites aren't sufficient to study how Earth's night is changing," said Kyba. "We've developed a way to 'translate' Globe at Night observations of star visibility made at different locations from year to year into continent-wide trends of sky brightness change. That shows that Globe at Night isn't just an interesting outreach activity, it's an essential measurement of one of Earth's environmental variables."

Existing satellites are not well suited to measuring skyglow as it appears to humans, because there are no current instruments monitoring the whole Earth that can detect wavelengths shorter than 500 nanometers, which corresponds to the color cyan, or greenish blue. Shorter wavelengths, however, contribute disproportionately to skyglow, because they scatter more effectively in the atmosphere. White LEDs, now increasingly commonly used in high-efficiency outdoor lighting, have a peak in emission between 400 and 500 nanometers.

"Since human eyes are more sensitive to these shorter wavelengths at nighttime, LED lights have a strong effect on our perception of sky brightness," said Kyba. "This could be one of the reasons behind the discrepancy between satellite measurements and the sky conditions reported by Globe at Night participants."

Beyond wavelength differences, space-based instruments do not measure light emitted horizontally very well, such as from illuminated signs or windows, but these sources are significant contributors to skyglow as seen from the ground. Crowd-sourced observations will therefore always be invaluable for investigating the direct human effects of sky brightness.

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