Beauty of sky

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

Umbra on the mountains and visual range during the totality

A visibility change during the solar eclipse is a very interesting, but not observed phenomenon during the solar eclipse. The visual range is an important thing for mountaineers, who desires to watch the mountains or different objects closely or further located and for people admiring a long-distance observations. The visual range changes during the solar eclipse is a very interesting issue, because the lack of direct solar illumination of part or all of the path between an observer and a distant object can result in that object will be visible or not.
A basic observation method of the visual range extension is pointing the camera towards presumed or barely visible remote mountain range. The observations carried out during the last totality were planned rather spontaneously. In order to continue these observations during forthcoming total solar eclipses the most helpful will be the Urlich Deuschle panorama generator or alternatively the PeakFinder website. Using these panorama generators you are able to set your obserwation place both against the umbral path and mountain ranges located at certain azimuth with a rough distance to them. Another helpful tool is the Google Earth path measure and estimated real-time shadow presence, that you can make on your own.
An important part of the observation method is also analyzing the footage, where you can see the details, snap the view and edit it giving the result beyond the ability of human vision. The observation of the extended visual range during the Great American Eclipse arose out of my observation of umbral movement across the mountain ranges, which I was aimed at. Analyzing later my images as well as supported footages from Carsten Jonas and mateusz Windak I found, that an visual range tends to increase significantly during the totality window. Aside for me some professors pointed out this phenomena as a first, making the publication, which is one of my reference (Vollmer, Shaw, 2018). I used the data from 2 observation places basically. One was mine, on the U.S. Hwy 26 between Riverton and Soshoni. There is a view towards Owl Creek Mts mainly and other remote mountains ocassionally. The Owl Creek Mts were at around 27 - 38 km distance (Arrowhead Ridge, Copper Mountain).
For another observation place considered I used Carsten's Jonas footage with camera headed south east. Carsten Jonas was observing the solar eclipse from Round Butte Overlook Park near Madras. There are some mountains on similar distance: The Grizzly Mountain - 31km and the Gray Butte - 25 km. Both observation places has an common denominator: they are located almost in the center of the totality path (first place with shadow deep 99,5% and 2m23s totality, second place was a little bit further with shadow deep 75,5% and 2m00s totality. Due to this all remote objects covered by camera frame experienced a shorter period of totality. The thing, that differ both these places is a direction. I was capturing a mountains at north west and north direction, whereas Carsten Jonas did it towards south east. There is also one another thing, which varies our places so far - this is a haze concentration. These aerosols were omnipresent, however Madras experienced extremely high concentration in eclipse day. It reflect in mountain view. The Grizzly Mountain was almost invisible throughout the partial phase, whilst the Owl Creek Mts were only a bit veiled by haze. Solar position was at 53 deg above SE horizon (azimuth 139deg) in my observation place and 42 deg above ESE horizon (azimuth 119 deg). In my observation I used Nikon D5300 camera with Nikkor 55-300mm lens and also Olympus E-510 camera with a kit lens 14-42mm. Carsten Jonas used Canon 7D2. In order to outreach my survey I used also a drone-perspective footage from Mateusz Windak. He was observing the totality also near Madras, but from the other side - on NE Quaale Rd, 16 km north east from Madras. During a whole totality the drone camera was headed towards NNE direction. Unlike to Carsten's Jonas footage haze density appeared to be much lower, possibly due to higher location the observation point plus additional drone altitude. Due to this the visibility range was much more extended.

There is a six different variations of radiance along the line of sight (Vollmer, Shaw, 2018), however I will write about them a bit later. During my amateur observation of shadow movement I could spot a basic 4 variations of radiance in terms of the particular mountain range (Owl Creek Mts) observed during the total solar eclipse:

Lunar shadow crossing above Grizzly Mountains around 2ndcontact (credits: Carsten Jonas).
Lunar shadow fleeting above Grizzly Mountains around mid-eclipse (credits: Carsten Jonas).

Lunar shadow receeding from Grizzly Mountains after 3rdcontact (credits: Carsten Jonas).
Lunar shadow fleeting above Grizzly Mountains around mid-eclipse (credits: Carsten Jonas).

Lunar shadow movement across the Owl Creek Mts.
Extended visual range during the totality: Casper Mts seen from Guersey, WY (credits: Petr Horalek).

Extended visual range seen in case of Mt. Hood from Madras, OR (credits: Mateusz Windak).
Extended visual range seen in case of Mt. Adams from Madras, OR (credits: Mateusz Windak).
Extended visual range seen in case of Mt. Adams from Madras, OR (credits: Mateusz Windak)..

A panorama of the Owl Creek Mountains comparison between partial and total phase of the eclipse. The range was seen from about 35-40km distance.
A panorama of eastern part of the Wind River Range as seen during the totality. The mountains were no longer visible. I have identified two the most prominent mountains: the Wind River Peak, partially hidden and the Roberts Mountain . Panorama description comes from the Urlich Deuschle webpage.

The visual range changes during the total solar eclipse can be visible once the shadow length will be longer than critical value, which depends on many factors like haze concentration, distance to the object, sun position on the sky, etc. The penumbra diameter is much larger than any practical visual range. Thus for these short distances any changes of illuminance and radiance may be approximated as a linear function for most of the eclipses. In the result the observed effect of visual range changes is pretty much the same when looking directly towards shadow-in and shadow-out sky. Situation can be more complicated when looking towards perpendicular direction. Then you can spot, that visual range varies according to logarithmic function behaviour reaching the maximum value in shadowed area. While the shadow of the Moon is racing above the observers head, the landscape looks like during the civil twilight. At this moment a visual range is the best and eventually restricted by the edge of umbra. Once the visual range under normal daytime conditions is shorter due to haze concentration the lack of direct solar radiation can extend it significantly up to the critical moment. This critical moment will occur, when far dark-looking object will be merged with shaded haze attenuating a bright skylight beyond. The extremely fast umbra makes a total solar eclipse a very dynamic phenomena. In the consequence the visibility circumstances are fleeting. The distance the observer to the edge of umbra determines the degree of the distant-sky airlight attenuation. This attenuation changes every second as the umbra moves. It can be seen on the footages, how remote objects slightly change the tint due to this. From practical point of view the smallest sky airlight attenuation is observed at the beginning or the end of totality at the distant object depends where we are looking. After the totality the radiance in the penumbra starts to initially increase linearly between the observer and object. The horizon radiance has an additional distant-sky radiance from the other penumbra observed above horizon seen in distance. The contrast between remote object observed and horizon sky evaluates throughout a whole eclipse event from the moment, when penumbra touches it until the 4th contact occurring at the observation place. To reaffirm the result of this observation is good to observe more objects at once, because one object, that contrast alone may not be sufficient. Next to the horizon and objects, mountains located far away a key role plays also the horizon sky brightness. Over the course of the eclipse phenomena the horizon sky brightness changes remains asymetrical, which has been raised here. Nevertheless here is an ocassion to explain again this issue in order to visibility conditions. The horizon sky brightness as well as a whole illuminance decreases by more than four orders of magnitude. Because this process is asymetrical on for example shadow-in side the eye not receive enough radiation at the beginning of the eclipse the mountains theoretically cannot be detected. Practically contrast between the dark horizon and bright sky airlight located behind opposite part of the shadow will allow on it. In this moment a shadow-out sky will offer faint, but getting better view of remote mountains.

Conclusions

I have shown fairly incidentally the observation od extended visual range during the Great American Eclipse. Most of things shown here has been analysed from photos and footages, because I didn't spot this phenomena directly during the eclipse. Nonetheless gathering the data from a few different observation circumstances under a different level of haze combined with probably the first scientific article about it allowed me to make this wide description. Due to overheating focus on solar corona only the visibility changes phenomena during the totality remains out of common interests. The umbra presence causes a visual range extension even on a tens kilometers during the totality under a hazy conditions. The hazy skies reduce the visual range outside the totality. These simple models covered in this article shows the examples of different situations, that are to be seen around the totality. Some situations are analogue also to the deep partial solar eclipses, where umbra location lies at least at the equal distance to visibility range or at least for obscuration higher than 86-88%, where penumbral affect on visibility towards the shadow-in sky is discernable. It would be interesting to conduct further experiments during future eclipses to enable further refinement and validation of the results of this observation, especially that every totality is different. The pivot of this kind observation is positioning a camera to view distant mountains, that are not visible in regular daylight (Vollmer, Shaw, 2018). For this purpose the Urlich Deuschle and PeakFinder mountain panorama generator will be helpful. Doing this observation during an annular eclipse would be an interesting extension of this issue, especially when we use a time-lapse or continuous video techniques in order to record a fascinating view of the changing contrast.
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