Light shed on Moon’s eclipse
IWAS fascinated to read a recent research paper that wonderfully tied together a total lunar eclipse with a substance found inside an ice core from Greenland. The association between the two has helped to pin down the date of a volcanic event.
The sulphate deposit in question dates from the first few years of the 12th century.
For quite some time, it seemed rather “obvious” that there was a connection between it and an eruption of a volcano in Iceland.
Indeed, Iceland is well known for its volcanic nature. I found it fascinating to climb a small volcano on the Icelandic island of Heimaey last year, and to examine the many museum displays there and in Reykjavik on the subject of volcanism.
It was the most well known of Iceland’s volcanoes, Mt Hekla, that was thought to be responsible for the sulphate deposit, following its eruption in the year 1104. However, a group of researchers led by Sébastien Guillet, of the University of Geneva’s Institute for
Environmental Sciences, has obtained evidence that the deposit was formed several years later, and support for this later date has come from records of observations of a total eclipse of the Moon in the year 1110.
On the night of May 5, 1110, the Moon passed through Earth’s shadow in space, being completely immersed in the shadow for more than an hour and a half. It’s not an unfamiliar sight; indeed, we have had a few of these visible from Tasmania in recent years. In these events, the Moon typically takes on a dull coppery-red colour, because some sunlight passes through Earth’s atmosphere and falls onto the Moon.
The colour arises because more light from the red end, rather than the blue end, of the spectrum passes through.
The Moon is far less bright than normal, but it can normally still be seen with the unaided eye.
The eclipse of May 1110, however, was different. The Moon did not become the familiar reddish disc that some people today call a “blood moon”. Instead, according to accounts by witnesses to the event, the Moon completely disappeared when it entered the depths of Earth’s shadow.
It is well known that the degree of darkening of the Moon during a total eclipse is not the same for each event.
Observers rate this on a scale called the Danjon Scale, in which a value between 0 and 4 is assigned, with 0 being the very darkest. It is clear that the May 1110 eclipse, had the scale been in use then, would have been easily rated as zero!
It may seem odd that there is a connection between this eclipse and the deposit in Greenland, but it is based on a major reason for the difference in the Moon’s darkness during eclipses: the state of the Earth’s atmosphere.
Following a major volcanic eruption, much material is injected into the atmosphere.
This can sometimes result in interesting effects in the sky, including colourful sunsets, as we witnessed after the eruption of Mt Pinatubo in the Philippines in 1992.
During those times, another effect is that the amount of sunlight reaching the Moon during a total lunar eclipse can be much less than normal. Indeed, it seems fairly clear that that is what happened in 1110.
Even though it was a very long, “deep” eclipse, with the Moon passing almost through the centre of Earth’s shadow, that would not normally have been enough to render the Moon invisible.
However, this was several years after Mt Hekla’s 1104 eruption, so it seems that Hekla was not to blame. The researchers think it likely that a different eruption occurred sometime in 1108 or 1109.
It is possible that Mt Asama in Japan, which erupted in 1108, may have been the one.
However, as the researchers point out, the material from that eruption may not have found its way to distant Greenland. So perhaps another eruption around that time may have been responsible.
It is great that two or more different topics in science can be used together to improve our knowledge!