Coronality vs Totality: Total Solar Eclipses from the edge

Descriptions of total solar eclipses observed from the very edge of the path of totality are few and far between. So we would like to share our experience and what we gathered from observing TSE2017 from just a couple of hundred meters within the southern limit of the umbral shadow path, south of Vale (Oregon, USA), as the following figure shows.

We are very familiar with the word totality. It is the defining moment of a total solar eclipse, full of awe and exhilaration, charged with ancestral deep feelings. We would like to highlight something that will seem so trivial to many eclipse chasers but which will illuminate the understanding of the experience of a total eclipse observed from the edge of the umbral shadow path. We know that the solar corona is always there, at all times, but that the brightness of the sky is almost always so intense to hide it away from view. When the Moon completely occults the photosphere, the brightness of the sky becomes lower than the brightness of the solar corona and the corona becomes visible.

We usually define the period when the photosphere is completely out of view (photospheric extinction) as totality and we approximately identify it with the period when the solar corona is fully visible. Near the centreline this is basically the case as the solar corona only fully emerges into view a couple of seconds before second contact and almost entirely disappears out of view few seconds after third contact. The photosphere is too intense at all other times to allow a clear view of the corona. We can try to artificially cover the re-emerging photosphere just after third contact and still see some traces of corona but that is dangerous and unsafe.

On the contrary, very near the edges of the umbral shadow path, the approximate identification of totality with the period of visibility of the solar corona is not really true anymore. By observing from just a few hundreds meters within the umbral path limit, despite the period of photospheric extinction being very brief, only 10- 20 s long or less, the solar corona (including the faint outer corona) is fully visible for much longer, for about 50-60 s. That means that the brightness of the sky drops below the brightness of the outer corona for far longer than the mere period of totality.

We feel that we should coin a new term, coronality, alongside totality. Coronality is the period when the brightness of the sky falls below the brightness of the outer corona. Totality is the period when the Moon completely occults the photosphere. On the centreline (and almost anywhere else within the umbral shadow path), the two terms are almost synonyms. Very close to the edge, the two terms part ways and coronality becomes substantially longer than totality.

Coronality is a superset totality and provides some mesmerising and awe-striking views hardly seen anywhere else within the umbral shadow path.We saw slowly evolving Baily’s beads over the back drop of a perfectly developed inner and outer corona: a vision of rare and deep beauty. These Baily’s beads were not blinding bright, their intensity looks more like the one of Venus and Jupiter, and they were very long lasting. There was no acceleration of transient phenomena, like near the centreline, but the exact opposite happened, a general slowing down. Everything was subtle and gentle, the ambient lighting fell organically, no banging diamond rings. Our experience was of understated tremendous beauty.

You can get an idea of this type of slowly evolving dynamic in the following video. Imagine adding the full solar corona as a backdrop and you can appreciate how weirdly strange the eclipse looks.

The dynamics of an eclipse observed so close to the edge of the umbral shadow path are of course a simple consequence of the relative movements of the solar and lunar limbs, the displacement of which is almost tangential instead of being almost orthogonal, as on the centreline. If your objective is to maximise the duration of totality, then of course you should head to the centreline (or thereabout). But if you end up at the edge, do not discount the experience: you will be treated by a lot of weird understated beauty!!!

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