The light scattering issue in the Earth’s atmosphere has been described recently for typical daylight, evening, and twilight conditions. In this article, I would like to explain a light scattering phenomenon under total solar eclipse conditions, which has been observed accidentally during the 2017 Great American Eclipse. Initially, the aim of my observation was a record the shadow bands on the white sheet. Besides I was going to chase the lunar shadow movement on this white sheet, which was prepared as much rough as possible. Finally, I didn’t spot the shadow bands at all, unfortunately.
1. INTRODUCTION AND OBSERVATION METHODS
The shadow movement observation during the total solar eclipse is one of the uncommon observations possible to conduct during this celestial event. A vast majority of people are concentrated on the Sun itself, missing other phenomena accompanying this rare occurrence. Only a vanishingly small amount of observers are able to spot a shadow moving across the sky or clouds, rather mountains. Nowadays this matter can be compensated by digital footage being developed rapidly both in resolution (4K, 8K) and the way of recording (wide-angle, spherical). These materials give us an option to multiple insights into the phenomena recorded and gather interesting information about them.
The light scattering phenomena during the totality appear to have not been discussed significantly in the literature before. I carried out the pioneering observation, which remains very little supported by scientific references. Despite the lack of decent literature, I am bringing sufficient observation results to this article.
A basic method of observation is undeniably a white surface, which is able to reflect the full amount of light. It can be a typical sheet, which is commonly used to shadow bands chasing around the totality. It’s good to make this white surface rough, which enables it to reflect the light coming from different parts of the sky and shade some parts of this surface at once.
According to rapid light scattering condition changes throughout the totality, these shadows caused by local surface roughness will move. Basically, our attention during the totality is distracted by many other interesting phenomena related to totality. Bearing it in mind our observation place should be thought over and prepared earlier. Moreover to proper explanation the mechanism of the light scattering phenomena in the atmosphere some high-quality footage is necessary. Having the footage material we are able to analyze our results by using various modes like high saturation or negative.
For my observation purposes, I prepared a rough plain-colored sheet, which behavior has been recorded in the 4K movie throughout the totality (Pic. 1). In order to ascertain whether my result is reliable, I used Carsten’s Jonas footage of shadow bands chasing, where in the 4K movie a white, rough sheet was clearly visible (Pic. 2). At the finish, I compared these results taking into account 3 the most important moments of the totality: 1 sec after 2nd contact, mid-eclipse moment and 1 sec before 3rd contact. Other moments were also shown when necessary.
Another thing, which I would like to explain is the sky division during a total solar eclipse. It refers to the place, where a total (or annular) solar eclipse is observed. The plot below (Pic. 3) clearly shows how the sky can be divided. Basically, there are 2 parts of the sky: one part, from where the umbra approaches, is said to be a shadow-in sky (or shadow-in direction). Another opposite part is a shadow-out sky (or shadow-out direction). Speaking about a shadow-in or shadow-out sky we mean the estimated directions, where the sky tends to change illumination during a deep partial phase of the eclipse.
2. SHADOW MOVEMENTS THROUGHOUT THE TOTALITY
In order to better understand the light scattering during the total solar eclipse phenomenon, it’s good to take a glance at the plots below (Pic. 4 – 9), which show the directions of scattered light at the major stages of the totality. I have prepared charts, showing what direction is the most illuminated during the totality. I also considered this phenomenon in 2 cases: looking from the north and looking from the east. In terms of my observations, I took into account not only the level of illumination, which is given by light scattered in the atmosphere but also the color of this light, which is to be received on the white surface during the eclipse.
First of all, I would like to show you how light scattering changes throughout the totality. We can consider this phenomenon in 3 major moments of the totality: at the beginning (Pic. 4, 5), around the mid-eclipse (Pic. 5 – 7), and just before the totality reaches completion (Pic. 8, 9). I have analyzed the circumstances for the last total solar eclipse, which occurred in August 2017. I was watching this phenomenon in Wyoming, where Sun was at above 53 deg altitude. It was more zenithal rather than the horizontal type of eclipse. All cases shown will correspond to the zenithal type of the eclipse then. Another statement is placing an observer exactly in the middle of the eclipse path (shadow depth 100%).
Looking at the image above we can refer to the moment when Sun was already covered by Moon. In this event, a shadow-out sky remains still bright for the first seconds of totality. In the case of the 2017 Great American Eclipse, the shadow-out sky was the eastern and south-eastern parts of the sky, as is shown in the plot above. A scattered light coming from the shadow-out part of the sky prevails. On top of that, the small angular distance to Sun causes a forward scattering, which enhances an illumination level.
At the mid-eclipse, in the very middle of the umbra, a light scattering should be equal due to geometric conditions. However, an atmospherical issue causes an uneven level of light scattering in different directions. This is driven by haze concentration and atmospheric molecules, which scatter light more effectively on the same part of the sky, where the Sun is located. Because in Wyoming, the totality occurred, when Sun was above the southeast horizon, then the shadow-out eastern sky remained still brighter, than the shadow-in western sky. The moment, when the light scattering was equal from each direction, occurred about 10-15 sec later, which I wrote below.
At the end of totality, the umbra position covers usually completely different parts of the sky. However, this is not the opposite part of the sky due to the solar position. When the shadow comes from the west or northwest (as it happened during the Great American Eclipse) and Sun shines at the southeast or south horizon, then an umbral position between the 2nd and 3rd contact is different. It happens in the majority of eclipses. The exceptions are totality occurrence when Sun is roughly in zenith or when totality occurs during sunrise or sunset.
Anyway to understand the umbra’s position in the sky between the most extremal moments of the eclipse and a light scattering pattern at once we must compare plots between the 2nd and 3rd contact. As you can see, just after 2nd contact a zenith sky is shadowed, unlike to moment before the end of totality. If Sun shines about 50 degrees above the south-eastern horizon, like 21.08.2017 in Wyoming or Oregon, then a zenith sky becomes illuminated a few seconds before the end of the eclipse. In this case, a whole shadow-in sky radiates diffused light. In spite of the opposite side from the Sun, the light coming from the whole western side of the sky is much stronger than light scattered by the eastern (shadow-out) sky.
At the finish of this description, I must tell you about another very important thing, which is cloudiness. Clouds can enhance or reduce light scattering in a particular direction. As you may have noticed in the pictures above (Pic. 5, 7, 9) during the 2017 total solar eclipse in Wyoming the high-level and mid-level clouds were visible, which modified a bit the direction and level of light radiation.
Both mine and Carsten’s Jonas footage show the shadow’s movement throughout the totality, which is a bit merged with changing colours. Looking at the sheet roughness is easy to spot, as the bulks are situated at different angles. Their location against the solar position and umbral movement direction caused a different view of the shaded areas and their movement throughout the totality period. We can easily notice, that in some cases the shadings remain almost unchanged. In other ones, they appear to be seen on the opposite side of the bulge. General changes occur in the darkest moment of the totality. This darkest moment of the totality is not during the mid-eclipse, what we can see in the sequence below the darkest moment during the totality occurs 10-15 sec after mid-eclipse (Pic. 11, 12). It was also mentioned when describing light level changes during the total eclipse.
Looking at the distribution of shades between 2nd contact and mid-eclipse there is no difference except for the illumination level. The brightest surfaces are much less illuminated at the mid-eclipse moment, which is understandable.
The bulges oriented at E-W and also ENE-WSW directions doesn’t show the local illumination changes throughout the totality. The best changes are visible for all bulges extended in N-S, SW-NE and SE-NW directions. For the first group, the main role plays a solar position and umbral way in the sky. As I remarked previously, that sky surface brightness before-after difference occurred more symmetrically there was no reason to change the shades. Due to the high haze presence forward light scattering was prevalent. In the result, the surface was receiving much stronger scattered light from the south than from the north, where backward scattering was bringing a weaker light. This forward scattered light coming onto a white rough sheet was coming from the illuminated sky and haze outside the totality.
The second group features a huge difference in local illumination. For bulges oriented in N-S and SE-NW, we can spot opposite illuminance distribution between the 2nd and 3rd contact. The shadings movement is amazingly significant. Another thing, that arises out of this situation is an overall light level difference, which we can associate also with haze and light scattering on the sky outside the umbra. Whereas the forward light scattering makes the eastern part of the sky brighter around the 2nd contact the situation is quite opposite at the 3rd contact when the Sun illuminates a further section of the sky. Thus the overall scattered light reflection is weaker than at the beginning of the totality making the darker appearance of this white surface. Another roughness-oriented SE – NW directions show slightly less discernable illumination difference, however, we can spot a strong impact of light from the southwest direction at the moment just before 3rd contact. The reason seems to be straightforward; once the angular distance between the umbral edge and the solar position was getting shorter the forward scattered light impact was higher, making the local surface brighter from the southwest direction.
In general, the view of the local shadings phenomena could be better when the sheet would be softer. Because the sheet surface was quite coarse the total effect is less performed.
Taking into account the sheet surface itself better effect has been spotted in Carsten’s Jonas footage. Unlike to my planned observation, where the sheet was placed on flat surface Carsten Jonas set his sheet diagonally in the solar direction. At this stage, the directions from west to east has been ruled out. The light scattering and next reflection from the white surface could be noticed mainly from SE, S and SW directions. Because the sheet was lying on bush twigs, it featured far higher roughness than mine. In the effect some light from W, E and zenith directions could be reflected eventually. To simplify my description I divided this sheet into a few interesting parts. The GoPro Hero 5 instrument made better quality 4K movies than Samsung Galaxy S5.
The most significant changes are visible on the right upper part of the sheet, where the rough surface was able to reflect the light from SE and SW directions. There we can see a yellowish bright area just after 2nd contact. Because the lunar shadow approached from WNW direction the southwestern sky remained still bright as well as the south-eastern sky just after the 2nd contact. Just before 3rd contact, some bulks reflected the light scattered on southwestern direction whereas other parts headed southeastwards were shaded. Moving our sight to the left we can spot another, extended bulge with a surface headed towards the zenith and upper western sky. This area represents two opposite situations reflecting the light just before the end of the totality and being shaded at the totality commencement. A very similar situation features the leftmost part of the sheet with a diagonal bulge headed west. It remains shaded after 2nd contact and reflects the faint light just before 3rd contact. The lower part of the sheet doesn’t represent clear illumination changes, because it reflects a dark surface combined with the lower part of the SE sky. Hence yellowish hue performs there in places. Last part, which I would like to describe is situated in the middle. This area, headed SSW doesn’t feature the scattered light reflected difference (except the colour), but there is a small bulge extended throughout this area, which after 2nd contact reflects the light scattered on the south-eastern sky and just before 3rd contact it reflects the light from the opposite, western direction. There are a lot of things to say about the colour of light scattered and reflected, which I would like to describe in the following section.
3. SURFACE COLOUR AND VISUAL ILLUMINATION CHANGES THROUGHOUT THE TOTALITY
Aside from changing the illumination and shade movement on the white surface, another intriguing thing happened. It was a colour change during the totality.
Before I show the pictures I would like to explain how the surface colour changes occur during the totality. This phenomenon is caused by shifting light scattering, which was discussed in the previous section. Because the direction of scattered light changes during the totality, the scattered light changes the colour too. Based on the 2017 Great American Eclipse example the colour of diffused sky radiation changed from yellowish, just after 2nd contact to more bluish before 3rd contact. Interesting is also a mid-eclipse moment, when secondary-scattered light prevails, making a strong dark blue hue on the white surface. As the non-umbral region of the sky goes further, then the colour of scattered light turns from yellowish to reddish (Pic. 11, 12).
Firstly I would like to describe the colours if scattered light at the beginning of the totality (Pic. 15).
At the beginning of the eclipse, when Sun is located in the southeast section of the sky and the umbra is coming from the west, the eastern (shadow-out) sky emits a mainly blue and faint blue hint of diffused light. Aside from it, an observer can spot a big contribution of the yellow colour of light, which is related to forward scattering on haze particles. It takes place on solar azimuth and similar, where the sky looks much brighter than in other regions. As I discussed in this article, when the angular distance to the Sun is less than 90 degrees, then forward scattering takes place, and makes the sky brighter. Once Sun shines, this effect appears as a white hue of the sky. Because the moment of totality moves the primary illumination source further from the observer, this scattered light is being attenuated by haze within the umbral region, which results in shifting to the red spectrum. It is related to a long way in the atmosphere, which this attenuated beam must travel to an observer’s eye. That’s why this scattered light is turning yellow as the total solar eclipse progress.
At the opposite side of the sky, from where the umbra approached (shadow-in) there is no significant scattered light, that reaches an observer’s eye. It is beaten up by a much brighter shadow-out section of the sky, however, if we would cut out a whole eastern section of the sky we would receive predominantly secondary-scattered bluish light, coming from the shaded, dark blue sky.
As the totality progress, the yellowish light, coming from the shadow-out sky is turning more reddish, because the distance of the shaded area is getting longer, hence this light must travel a long way to the observer. At the mid-eclipse, the strongest scattered light is to be received still from east and southeast (shadow-out) directions. Despite the equal level of illuminance from a geometrical point of view the sky section being closer to the Sun emits still stronger light, due to forward scattering. As the plot above (Pic. 16) shows the strongest light comes from the east and southeast. The colour of this light is still yellowish but turns orange and red as the umbra covers more of this part of the sky. An opposite side of the sky (shadow-in) becomes brighter, especially near mid-eclipse, when the umbra moves upwards and the non-umbral region of the sky increases. It results in bluish scattered light emission, due to backward scattering. The sky in the antisolar direction is usually intensively blue, then diffuse sky radiation has also a blue colour. However, there is also a reddish tint of light, that becomes stronger around the mid-eclipse. This is an analogous situation to the shadow-out sky. The region of sky directly illuminated by the Sun is still far away, but as the totality is closer to finish, this tint will become more yellowish.
At the end of the total phase, an observer can face another interesting situation. When the umbra is confined only to the eastern part of the sky (shadow-out), what usually happens when Sun is located at some particular altitude (obviously much lower than near-zenith altitude) before astronomical midday, then a whole western side of the sky (shadow-in) is illuminated. Diffuse sky radiation, which comes to an observer’s eye emits mainly blue and bluish colours. A yellow tint plays a minor role, the same as a reddish light scattered on the opposite side of the umbra. However, this reddish light is better expressed on the white surface, because it remains still the brightest light, which comes from that direction. Comparing to the same situation on the opposite side of the sky at 2nd contact, this scattered light is far better expressed due to forward scattering. This redness is clearly visible then, on every part of the surface, which is shaded from the western part of the sky.
After this theory, we can look at the pictures below and confirm this phenomena’s occurrence.
As you may have noticed in the pictures in the previous chapter (Pic. 11, 12) the general hue of the observation areas is different. At the moment of 2nd contact, a big influence of yellowish and orange-reddish tints can be visible both on mine and Carsten’s Jonas example. On the contrary to the beginning of the totality, just before 3rd contact, a bluish tint prevails. What is the reason for this? We know, that the solar position at the totality was around 53 deg above the SE horizon in my observation place. At the Carsten’s Jonas observation place, near Madras, the Sun was even lower, 42 deg above the ESE horizon. Once 2nd contact occurred lunar shadow was exactly at the solar position. Due to these circumstances the sky section being upper the Sun was shaded by Moon already, whereas the lower part of the sky was still illuminated by the direct Sun (Pic. 4). As we know from the previous section and my previous articles the lower part of the sky has always fainter hue due to the thickness of the atmosphere. On top of that when we add up a dense haze presence to this factor we will see the sky with blue colour almost totally washed out. Moreover, the short angular distance to the illumination source, which obviously is Sun, causes a forward scattering, making the sky brighter than normal. Combining these factors in one, being a result of rural continental aerosols we should get a bright yellow appearance of the sky, which will slightly transform towards orange and red near the horizon due to scattered light absorption by aerosols successively less illuminated and shaded by umbra eventually. The redness was also caused by nitrous oxide appearance in the atmosphere because of wildfire smoke in the air.
The opposite situation takes place before 3rd contact, where the illuminated horizon changes from reddish through yellowish to faint blue as the umbra leaves the sky and the scattered light is getting less absorbed by shaded and next-illuminated aerosols. Both on mine and Carsten’s Jonas sheet surfaces the yellowish hue presence is strong just after 2nd contact and as totality progress is turns more reddish around mid-eclipse.
At the moment of mid-eclipse, we can see a mix of navy blue and reddish tints on the sheets. This situation arises out of mathematically the longest distance to illuminated areas of the sky and the aerosols at once. As a consequence, a lot of backward scattered light mentioned earlier is absorbed by haze particles being shaded by Umbra. It results in an analogous situation to the twilight period, when near the horizon long light wavelengths scattering plays the main role. This reddish scattered light coming to the white sheet surface remains still stronger than scattered light coming from any other direction. The reason for it is forward scattering. A dark bluish appearance of any other sheet surfaces arises out of the zenith skylight scattering reflection. Even under the densest hazy conditions, the zenith sky keeps a blue hue, because the zenith is the shortest way through the atmospheric boundary layer and atmosphere itself. Threading its way the zenith sky is the most reliant on the Rayleigh scattering conditions. A little bit of light presented during totality arises from secondary molecular scattering of sunlight at high altitudes (near or above 10 km), whereas the light from lower altitudes experiences high extinction (Konnen, Hinz, 2008). Even during the totality, when high haze occurs the zenith sky has a blue appearance, which is far darker due to low surface brightness. The zenith sky during the totality is very similar to the sky observed under the blue hour conditions. Hence the bluish appearance on every plain-coloured surface dominates. Conversely, the colour of the brighter sky below the Sun shifts towards red (Gedzelman, 1975).
As remarked earlier, at least in the case of 2017 Great American Eclipse for my observation point the darkest moment of totality occurred 10-15sec after the mid-eclipse. The first and last 20-25 seconds of the totality shows usually a much brighter scene than in another time of totality (Pic. 11, 12). This is because a huge part of the sky is directly illuminated by sunlight and scatters the light towards the observer (Pic. 4, 8, 15, 17). My proven observation refers to a particular location and the 2017 solar eclipse ephemera. For Carsten’s Jonas point this moment occurred under different circumstances because the Sun was lower above the horizon on a slightly different azimuth and the totality was shorter itself. Thus the surface darkening during the totality is slightly less pronounced. Anyhow each darkest moment throughout the totality leads to significant changes in surface colouration. Within the minutes of totality reaching completion, we can observe a brightening of the surface and surroundings. It clearly indicates, that lunar shadow is leaving the sky. The shadow-in section of the sky glow becomes brighter and the light scattering increases. As the consequence, more scattered light is reflected on the white surface making it obviously brighter. Is worth taking a look and deduce, that the colour of this plain-coloured surface is different. A rapid increment of bluish hue is observed. The yellowish and orange tint is still noticeable, but less pronounced. The scattered light becomes brighter above the shadow-in horizon, but the angular distance from the Sun is too big to make it bright as well as shadow-out at the 2nd contact. Here the forward scattering plays a key role. Just before the end of the totality, the blueness of the white surface is considerable. This is because at the 3rd contact the umbral edge is located again at the solar position. But on the contrary to 2nd contact now a lower part of the sky, being under the Sun is shaded and the upper part of the sky is illuminated. So conversely to the 2nd contact, the part of the sky with stronger blue colour is reflected from the white surface. This is because on the upper part of the sky a Rayleigh scattering plays a key role even under the hazy weather. When the lower part of the shadow-out sky remains shaded, the opposite lower part of the shadow-in sky is already illuminated giving a faint blue and yellowish light. Until 3rd contact occur this light scattered by the lower part of the shadow-in sky is still absorbed by shaded haze, turning its colour to more orange.
The light scattering in the Earth’s atmosphere during the total solar eclipse is one of the most interesting phenomena, that can be observed at this time. A white surface is the best to see and record how this scattered light is next reflected by this surface. Thanks to my observation and Carsten’s Jonas support material I was found, that light scattering during the totality changes rapidly. The symmetric moments of a total solar eclipse feature a completely different colouration and local surface roughness shading. Just after 2nd contact, yellowish and orange tints played a key role unlike to moment before 3rd contact, when a bluish hue prevailed. At the mid-eclipse main colour seen on the surface were reddish and navy blue due to the big absorption of the scattered light coming from horizon and Rayleigh scattered zenith skylight under low surface brightness conditions. The moment of mid-eclipse remains the moment of blue hour, which we can observe daily before sunrise or after sunset.
This type of observation has been carried out during the 2017 Great American Eclipse when Sun was at SE around 40-50 deg above the horizon. The observation results brought in this article cannot be applicable to every total solar eclipse, because every totality is different. It only shows how significantly things change during a very short period of totality.
- Gedzelman S.D., 1975, Sky color near the horizon during a total solar eclipse, (in:) Applied Optics, vol. 14, p. 2382 – 2387
- Koonen G.P., Hinz C., 2009, Visibility of stars, halos and rainbows during solar eclipses, (in:) Applied Optics, vol. 47(34), p.H14-24
- Shaw G.E., 1978, Sky radiance during a total solar eclipse: A theoretical model, (in:) Applied Optics vol 17 (2), p. 272-276.
1. Light scattering in the Earth’s atmosphere, part 1 – scattering and related phenomena
2. Light scattering in the Earth’s atmosphere, part 2 – why is the sky blue and how the sky colour change?
3. Why the haze is an important weather factor, part 2 – impact on visibility and light scattering