Beauty of sky

Total solar eclipse, 21.08.2017, Riverton, WY (United States)

Shadow movements and color changes

Shadows movement observation - during the total solar eclipse is one of the uncommon observation possible to conduct during this celestial event. A vast majority of people is concentrated on Sun itself missing another phenomenas accompanying this rare occurrence. Only a vanishingly small amount of observers is able to spot a shadow movement across the sky or clouds, rather mountains. Nowadays this matter can be compensated by digital footages being developed rapidly both in resolution (4K, 8K) and the way of recording (wide-angle, spherical). These materials give us the option to multiple insight into the phenomena recorded and gather the interesting information.
I carried out the pioneer observation, which remains unsupported by scientific references. Despite of lack a decent literature I am bringing a sufficient observation results in this article. A basic method of the observation is undeniably a white surface, being able to reflect any light. It can be a typical sheet, which is commonly used to shadow band chasing around the totality. It's good to make this white surface rough, that enables it to reflect the light coming from different angles 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 phenomenas related to totality. Bearing it in mind our observation place should be thinked 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 with using a various modes, like high saturation or negative.
For my observation purposes I prepared a rough plain-coloured sheet, which behaviour has been recorded in 4K movie throughout the totality. In order to ascertain whether my result is reliable I used the Carsten's Jonas footage of shadow bands chasing, where in 4K movie a white, rough sheet was clearly visible. At the finish I compared these results raking 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.

Both mine and Carsten's Jonas footage shows an shadows movement throughout the totality, which is a bit merged with changing colours . Looking on the sheet roughness is easy to spot, that the bulks are situated at different angle. Their location against the solar position and umbral movement direction caused different view of the shaded areas and their movement throughout the totality period. We can easily notice, that in some cases the shadings remains almost unchanged. In another ones they appear to be seen on the opposite side of the bulge. General transition occurs in the darkest moment of the totality. This darkest moment of the totality is not during the mid-eclipse, what we can see on the sequence below the darkest moment during the totality occurs at 10-15sec after mid-eclipse. Looking on the distribution of shades between 2nd contact and mid-eclipse there is no difference except the illumination level. The brightest surfaces are much less illuminated at the mid-eclipse moment, which is understandable.

Shades movement across the totality. Refer to marked places.
Shades movement across the totality in negative.
Shades movement across the totality saturated.

The bulges oriented at E-W and also ENE-WSW directions doesn't show the local illumination changes throughout the totality. The biggest changes are visible for all bulges extended on N-S, SW-NE and SE-NW directions. For the first group the main role plays solar position and umbral way on the sky. As I remarked previously, that sky surface brightness before-after difference occurred more simetrically there was no reason to change the shades. Due to 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 forward scattering was bringing a weaker light. This backward scattered light coming onto white rough sheet was coming from illuminated sky and haze outside the totality.
A second group features a huge difference in local illumination. For bulges oriented in N-S and SE-NW we can spot opposite illumnance distribution between 2nd and 3rd contact. The shadings movement is amazingly significant. Another thing, that arises out of this situation is overall light level difference , which we can associate also with haze and light scattering on the sky outside the umbra. Whereas the backward light scattering makes eastern part of the sky brighter around 2nd contact the situation is quite opposite at 3rd contact, when Sun illuminates further section of 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 south-west direction at the moment just before 3rd contact. The reason seems to be straighforward; once angular distance between umbral edge and solar position was getting shorter the backward scattered light impact was higher, making the local surface brighter from south west direction. In general the view of the local shadings phenomena could be better when sheet would be more soft. 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 sheet was placed on flat surface Carsten Jonas set his sheet diagonally in 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 on a few interesting parts. The GoPro Hero 5 instrument made better quality 4K movie than Samsung Galaxy S5.

Shades movement across the totality. Refer to marked places.
Shades movement across the totality in negative.
Shades movement across the totality saturated.

The most significant changes are visible on the right upper part of the sheet, where 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 south-western sky remained still bright as well as south-eastern sky just after the 2nd contact. Just before 3rd contact some bulks were reflected the light scattered on south-western direction whereas other parts headed south-east wards were shaded. Moving our sight on the left we can spot another, extended bulge with surface headed towards 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 diagonal bulge headed west. It remains shaded after 2nd contact and reflect the faint light just before 3rd contact. Lower part of the sheet doesn't represent a clear illumination changes, because it reflects a dark surface combined with lower part of the SE sky. Hence yellowish hue performs there in places. Last part, which I would like do 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 south-eastern sky and just before 3rd contact it reflects the light from the opposite, western direction.

Light level changes throughout the totality, as recorded by Samsung Galaxy S5 with an automatic white balance.1 frame = 5 sec interval.

Light level changes throughout the totality, as recorded by Go Pro Hero 5 by Carsten Jonas with a fixed white balance. 1 frame = 5 sec interval.

Surface color changes - has been spotted aside for change the illumination and shades movement on the withe surface another intriguing thing happened by. It was a colour changes during the totality. As you may have noticed in the pictures at the previous chapter a general hue of the observation areas is different. At the moment of 2nd contact a big influence of yellowish and orange tints can be visible both on mine and Carsten's Jonas example. On the contrary of the beginning of the totality, just before 3rd contact a bluish tint prevails. What is the reason of this? We know, that solar position at the totality was around 53 deg above SE horizon in my observation place. At the Carsten's Jonas observation place, near Madras the Sun was even lower, 42deg above 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 arleady, whereas the lower part of the sky was still illuminated by direct Sun. As we know from previous section and previous my articles the lower part of the sky has always fainter hue due to 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 backward scattering, making the sky brighter than normal. Combining these factors in one, being a result of rural continental aerosols we should got 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 reddiness was also caused by nitrous oxide appearance in the atmosphere because of wildfire smokes in the air .


Different tints seen on the white sheet surface throughout the totality, recorded by Samsung Galaxy S5. From the left: after 2ndcontact, mid-eclipse and before 3contact.

Different tints seen on the white sheet surface throughout the totality, recorded by Go Pro Hero 5 by Carsten Jonas. From the left: after 2ndcontact, mid-eclipse and before 3contact.


Opposite situation takes place before 3rd contact, where the illuminated horizon changes from reddish through yellowish to faint blue as 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 colour presence is strong just after 2nd contact and as totality progress is turning 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. In the consequence a lot of forward scattered light mentioned earlier is absorbed by haze particles being shaded by umbra. It results analogous situation to the twilight period , when near the horizon a long light wavelengths scattering plays a 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 of it is forward scattering. A dark bluish appearance of any other sheet surfaces arises out of the zenith skylight scattering reflection. Even under the most dense hazy conditions the zenith sky keeps a blue hue, because zenith is the shortest way through the atmospheric boundary layer and atmosphere itself. Threading it's 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 altitude (near or above 10km), whereas the light from lower altitudes experiences high extinction (Konnen, Hinz, 1987). 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 surfaces dominates. Conversely, the color of the brighter sky below the Sun shifts towards red (Gedzelman, 1975).

As remarked earlier, at least in case of Great American Eclipse for my observation point the darkest moment of the totality occurred 10-15sec after the mid-eclipse. For Carsten's Jonas point this moment occurred under different circumstances, because the Sun was lower above horizon on slightly different azimuth and totality was shorter itself. Thus the surface darkening during the totality is slightly less pronounced. Anyhow each the darkest moment throughout the totality leads to significant changes of surface coloration. Within the minutes of totality reaches completion we can observe a brightening of the surface and surroundings. It clearly indicates, that lunar shadow is leaving the sky. The shadow-in sky glow become brighter and the light scattering increases. In the consequence more scattered light is reflected on the white surface making it obviously brighter. Is worth to take a look and deduce, that the colour of this plain-coloured surface is different. A rapid increment of bluish hue is observed. Yellowish and orange tint is still noticeable, but less pronounced. The scattered light become 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 upper part of the sky is illuminated. So conversely to the 2nd contact the part of sky with stronger blue color 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 arleady illuminated giving a faint blue and yellowish light. Until 3rd contact occur this light scattered by lower part of the shadow-in sky is still absorbed by shaded haze, turning its colour to more orange.

Conclusions

The light scattering in the Earth's atmosphere during the total solar eclipse is one of the most interesting phenomenas, that can be observed in 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 the light scattering during the totality changes rapidly. The symetrical moments of total solar eclipse feature a completely different coloration and local surface roughness shading. Just after 2nd contact a yellowish and orange tints played key role unlike to moment before 3rd contact, when bluish hue prevailed. At the mid eclipse main colour seen on the surface were reddish and navy blue due to 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 Great American Eclipse, when Sun was at SE around 40-50 deg above 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.

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