A solar eclipse is the most spectacular natural phenomenon, as we can see on Earth. It induces at least a dozen or so other events, which occurrence can be observed during this time. In this article, I would like to list 10 of the most intriguing and uncommon phenomena, which an observer is able to see during a solar eclipse event.
- Changes of illumination – an understandable thing is, that the light level and radiation change throughout the solar eclipse. People don’t really realize the light level changes. The basic reason is that the human eye responds logarithmically to stimuli according to Fechner’s law (Fechner, 1860). Secondly, the decrease in the radiance of the blue sky and our surroundings is the same as that of the primary light source, so that contrast remains unaffected (Koonen, Hinz, 2009). Moreover, the human eye adapts effectively to changes in the overall illumination, likewise, most camcorders and cameras do. Until the observer won’t looks at some fixed light (like remote street lights, which gradually gain brightness) at once the illumination difference may not be noticed by him. The skylight may be considered as attenuated sunlight up to at least 99,8% obscuration (Sharp, Silvermann, Lloyd, 1971) or 99,7% obscuration (Konnen, Hinz, 2008) when a decrease in sky-light radiance starts to stagnate with respect to that of the Sun. It’s also worth mentioning, that more or less till 98,7% obscuration (depending on the lunar radius and light wavelength) the brightness of the Sun and sky decreases with respect to non-eclipse conditions (Konnen, Hinz, 2009). The illumination change is not linear progress. Because the light level changes correspond with a logarithmic function, at the very beginning of the eclipse the attenuation won’t be noticeable. As the Moon covers more and more the Sun this effect is more noticeable, especially when the eclipse magnitude exceeds 0.9 (Konnen, Hinz, 2009). Moreover, the illuminance level changes also throughout the total phase. The skylight consists of multiple scattered lights from outside the umbral region (Sharp, Silverman, Lloyd, 1971). Usually, a moment around the mid-eclipse is the darkest, but not always. The details of my observation and measurements are available in this article.
- Sky brightness and color changes – along with the light level a sky brightness surface changes. These changes can be observed mostly when the Sun is obscured by 50% or more. The sky becomes darker and also changes color a bit. A reddish tint appearance is to be seen for solar obscuration bigger than 50%. I wrote about this nearly 2 years ago. At the deepest phase of partiality, a sky’s surface brightness and color changes are considerable. During the totality, the sky is similar to the blue hour period of twilight. A zenith sky is a navy blue, whereas the horizon around, illuminated by airlight has a yellowish appearance likewise during civil twilight looking towards a solar point. Sometimes we are able to spot reddish clouds. As the totality progresses, the sky brightness and color also change as per the description of the previous article.
- Solar corona outside the totality – an interesting phenomenon to be seen even up to 5 minutes outside the totality when sunlight is weak enough. The solar corona has a brightness nearly equal to the full Moon, thus it could be easily seen during a deep partial phase when the lunar disk almost fully overlaps with the solar one. Unfortunately, the solar corona, which appears on the opposite side of the crescent is literally washed out by its much stronger gleam and remains invisible to the observer. There are solutions, that can help you to spot the solar corona outside the totality. If you don’t have a binocular just block the light coming from a thin solar crescent and try to spot a fuzzy arc next to your thumb. It should work at least 2 min before or after totality. When you have a binocular do analogically, placing the sunlight outside your view (otherwise it can be dangerous for your eyes!) enables you to see a discernable fuzzy arc even up to 5 min before or after totality. During the 2017 eclipse observation, I used a long lens Nikkor f3.5-5.6 55-300mm for the solar corona observation outside the totality. In order to capture the solar corona next to the crescent sun, I used the light-fall feature of the Nikkor 55-300mm lens, which could save my sight and camera a bit diminishing a major light at once. At 300mm the fuzzy arc was looking big likewise you could watch it through a good binocular. Finally, I made it, and discovered, that solar corona can be visible upon 93% obscuration, which is amazing! Interesting can be also the point of the lunar and solar disk cross, where some prominences are to be visible! For deep annular eclipses, it can be noticed just before 2nd and after 3rd contact, which has been roughly described in this article.
- Fleeting shadow – when close to the totality is easy to spot a shadow moving fast across the sky. The speed of the lunar shadow depends on a few factors, where the most important is the solar altitude above the horizon. Due to this, the umbral speed can vary from around 600m/s, even to 55km/s (Carton, 2011). This enormous speed is to be seen especially in the sky and high-level clouds just before and after the totality. The sky color and lunar shadow appearance change significantly every second, especially when crossing the zenith sky. Sometimes this phenomenon is also possible to see in remote mountains in some rough areas. Read more about it here.
- Prominences movement against the lunar disk – very interesting event, quite easy to spot when looking at the Sun through binoculars or long lenses. Prominences and solar chromosphere appear in pinkish color. It is really hard to spot it by the naked eye, so better equip it with at least a small binocular. The longest prominences are quite rare, but when occur we can see them even for the entire totality. The moment after beginning and before finishing the total eclipse gives us the opportunity to see a lot of prominences surrounding the solar limb. Most of them are relatively small and can be seen as an additional, pinkish layer of the solar atmosphere. More about the prominences appearance during the totality has been mentioned in this article.
- Ground darkening – the event associated with the Moon’s shadow approaching. The umbra’s presence is visible on the ground as such on the sky or clouds. It looks much better from a drone perspective or some elevated observation venue (Pic. 8) Basically the surface radiation changes throughout a whole partial phase of the solar eclipse as the solar irradiation drops. The same situation is with the irradiation level of both a primary light source and surroundings, which decreases equally we cannot spot it when the eclipse phase is not advanced enough. The ground darkening is to be seen shortly before the totality and just after. It is better to observe it from some elevated place. The ground darkening is to be seen for a dozen or so seconds only.
- Local shadows movement – a very interesting occurrence, which takes place during each totality. To watch it we need a white sheet with at least a few rough shapes. Gnomon can be helpful also. As a result, you will be able to see how the local shadows change throughout the totality. In the beginning, the shadows will head almost in the opposite direction against the end of the eclipse. The details of this observation have been described in this article.
- Surface color changes – another very interesting observation, that can be carried out by the way of shadow band chasing. Likewise in previous observation, the plain-colored sheet will be required. In effect of the shadow movement and light scattering from the different parts of the sky throughout the totality, different colors of light scattered from outside of the umbral region will be visible. The details of this observation I have described in this article.
- Significant changes in the visual range – are haze-dependent and will be the best visible under the high density of atmospheric aerosols. The objects, that are normally invisible outside the total solar eclipse conditions suddenly emerge on the horizon as the umbra approaches. Read more about it in this article.
- Dark shadow cone – which is to be visible mainly towards a near-solar direction, where a forward scattering takes place. A shadow cone is less pronounced far away from the Sun due to backward scattering. I can occur both shortly before and just after the totality depending on the position of the Sun in terms of the umbral path across the sky. For example when an eclipse takes place in the late afternoon being in the shadow-in sky at once. Then the observer will see the dark shadow cone just before the totality. During the 2017 total solar eclipse, which I was watching around noon the dark shadow cone was observed on the shadow-out sky, being located closer to the Sun. In general, the dark shadow cone can be clearly visible up to 40-50 seconds after or before the total phase, which corresponds to 99% obscuration. I found it during my private light level measurements, which have been described before.
1. Carton W.H.C., 2011, The speed of the lunar shadow on Earth during solar eclipses, (in:) Journal of the British Astronomical Association, vol. 121, no 2 p.105-108.
2. Fechner G. T., 1889, Elemente der Psychophysik, Teil 2, Breitkopf & Hartel, Leipzig
2. Können G.P., Hinz C., 2009, Visibility of stars, halos and rainbows during solar eclipses, (in:) Applied Optics, vol. 47, issue 34, p. H14-H24
3. Sharp W.E., Silverman S.M., Lloyd J.W., 1971, Summary of sky brightness measurements during eclipse of the Sun, (in:) Applied Optics, vol. 10(6), p. 1207-10