Solar eclipse below the horizon 2019 – the World’s first webcam observation

My third remote observation of the solar eclipse was aimed at the fleeting lunar shadow when the Sun was already below the horizon. Previously I hadn’t had occasion to see a similar effect, as I watched the partial phases only. This observation was spectacular, because of the circumstances. I know only a few publications presenting some results of this kind of phenomenon. Because of this very little information, I decided to take a look at this celestial event. On top of that, my observation appears to be the first of the World like this, because as far as I know, observing the phenomena related to solar eclipses via webcams is a pioneering engagement.

1. INTRODUCTION

The phenomena occurring along with the solar eclipse, which takes place away from the Sun can be observed not only near the standard area of a solar eclipse, calculated by Fred Espenak between sunrise and sunset. Some of the atmospheric impacts are to be observed upon at least the nautical twilight because the upper layers of Earth’s atmosphere are sunlit much longer than a period of the astronomical day. In the theoretical sense, as the Sun plunges deeper below the horizon, these events tend to perform weaker and become confined to the solar azimuth only. The most effective progress of these phenomena is possible to see at the extension of the totality path. There is not enough information fully explaining this event, as most people are commonly concerned about the typical solar eclipse observation. As a result, all publications are concentrated on the changes in the atmosphere when the eclipsed Sun is above the horizon. On top of that, on average twice a year eclipse-related phenomena occur at the path of totality extension in different places in the World, which leads to the conclusion, that these events are as rare as solar eclipses. Despite increasing knowledge about solar eclipses possible to observe at some specified locations, people are still not aware of the solar eclipse’s impact on the twilight period. The best evidence of this is the utter lack of any footage and images on the web, that would prove any attention from anyone. This work shows probably the World’s first record of this experience, being carried out via webcams located at the extension of the total solar eclipse path. The aim of this article is to awaken interest and awareness about this kind of observation, which can be done by casual people living in the areas, where it happens.

2. LITERATURE REVIEW

As I said at the beginning, there are not many references describing this phenomenon. However, surprisingly these ones that exist apply to the places located very far from the eclipse phenomena itself. The most common expression, which came into the literature describes the “double dawn” or “double dusk” optical phenomenon (sometimes also “double sunrise” or “double sunset”). The oldest reports of the “double dawn” came from China, where for the first time this event was observed. The first person, who officially reported the observation of “double dawn” was King Yi from the Western Zhou dynasty, ruling the former Zheng region (today’s  Hua District in Shaanxi Province), who observed it accidentally on April 21^st, 899 BC (Cyiuan et al., 1999). That day, the greatest annular phase of the eclipse with magnitude 0.95-0.97 occurred around the civil dawn moment (Ansari  ) (Pic. 1). Because this observation is first reported in history, it’s often referred to the study of Earth’s past rotation (Zhang, Yanben, 2000). Nowadays, the full Earth rotation (the day period) lasts 86400 seconds in approximation. However, more than 2500 years ago, the Earth was rotating slightly quicker, doing its full circle within a 40ms shorter time (Stephenson, 2003). This is why all the solar eclipse measurements reaching ancient history are so important.
Unfortunately, this eclipse can be implausible, because it in question was only annular (Burkhardt, 1992).
From the former Zhang, the region is known for another similar “double sunrise” observation on April 4 368 AD, when the annular solar eclipse occurred at the solar elevation around 4 degrees below the horizon (Pic. 1).

Pic. 1 The local circumstances of the solar eclipse below the horizon, known as “double dawn” or “double sunrise” in Chinese literature and referring to two annular eclipses on April 21, 899BC and April 4 368 AD at the former Zhang location (Xjubier.free.fr).

According to some sources, the former Zhang region experienced twice annular eclipses with the greatest phase at the sunrise (Pic. 2). If I would be honest I trust more Fred Espenak and Xavier Jubier because their calculations are newer than the information provided in the Historical Atlas of China.

Pic. 2 The annular solar eclipses 21 April 899 BC and 4 April 368 AD in Eastern China (Herrmann, Ginsburg 1966).

The “double dawn” is the same phenomenon as “double dusk”, which occurs in reverse sequence and is symmetrical. (Wang, Aki, 1995). Along with the terms “double dawn” and “double dusk” the terms “double sunrise” and “double sunset” are used. The “double sunrise” is an optical illusion produced by the central eclipse occurring just before sunrise. Likewise, such an eclipse right after sunset would be perceived as a “double sunset” (Andersen, 1997).
We have a piece of information about the solar eclipse below the horizon observation from recent times. June 30, 1954, a total solar eclipse made significant changes in the morning twilight sky at the Palomar Observatory, which was described in detail (Abell, Kearns, 1954). Good works are provided by Geyer H., Hoffman M., and Volland H., describing the influence of a solar eclipse on twilight from a location (Hoher List Observatory in Germany) far away from the central eclipse zone on July 22, 1990, and by Guliaev R.A. describing the possibility of using the solar eclipse below the horizon to observation the inner zodiacal light. In the most recent publications, the “sunrise at night” (Andersen, 1997) has been described, which refers to the “double sunset” phenomenon, as occurred on January 4, 1992, above California. Another one comes from the Yucatan peninsula, where the twilight contrast changes driven by an annular eclipse below the horizon were reported after sunset on June 10, 2002 (Aveni, 2017). See the map below showing how far from the central eclipse zone these observations were reported (Pic. 3).

Pic. 3 Previous reports of a solar eclipse below the horizon observations: 30.06.1954 – Palomar Observatory in California (USA), 22.07.1990 – Hoher List Observatory in Eifel Mts(Germany), 4.01.1992 – Californian shore (USA), 10.06.2002 – Yucatan Penninsula (Mexico)(Xjubier.free.fr).

3. METHOD AND LOCATION OF THE OBSERVATION

I was watching the 2019 total solar eclipse below the horizon via webcams located alongside the Atlantic coast in eastern Argentina and also at the mouth of the La Plata river in Montevideo. None of them was within the geometrical area of the solar eclipse and most of them were headed east in order to catch the lunar shadow receding towards the twilight wedge rising just after sunset. The webcams in Santa Teresita were located roughly at the extension of the totality path, whereas the webcams operated in Pinamar were recording this celestial event from about 25km distance to the southern limit of the umbra. The webcam in Montevideo was tailored for the distant type of observation, however, the poor weather conditions above Uruguay’s capital put the results from this camera in such disuse, as only faint darkening of clouds was noticed.

Pic. 4 The webcams location against the path of the Jul 2, 2019, total solar eclipse, where: 1 – Pinamar west-north-west (WNW); 2 – Pinamar east (two cameras covering similar direction); 3 – Santa Teresita (two cameras covering similar direction); 4 – Montevideo(Xjubier.free.fr).

The quality of the webcams was various, however, none of them reached the full HD level desired for the perfect-quality observation. The resolution of the cameras varies between 360p and 720p (HD). The lowest quality provides the LacostaDigital operating in Santa Teresita, while one of them was fixed about 2 hours before the eclipse. Another provider – CamerasVera managing within the Uruguay borders offers mostly 720p HD webcams. In Pinamar, the company maintaining the webcams is Telpin. Both from CamerasVera and Telpin I could get quite a good vista. From all providers, the autorefresh wasn’t 100% fluent, but fortunately, it didn’t impact the observation. Apart from these listed webcams, which were working at that time, I found another two webcams in Villa Gesell, located about 20km south of Pinamar. Unfortunately, one of them headed northeast towards the extended path of the totality was down on the July 2^nd evening, whilst another one covering the beach view towards the southeast wasn’t useful for observation purposes. Another highly desired, but disabled (a few hours before the event) webcam was in Mar de Ajó village, operated by LacostaDigital. All the most important webcams from the perspective of my observation were shown in the image above (Pic. 4).
The Telpin webcams were recorded for 10 minutes with the period of totality by the Screensatify recording plugin for the Google Chrome browser. The other views were regularly print-screened straight to PicPick, which is the best image-snipping tool available on the market.
The most important element is the schedule of the observation. I attended to this observation twice, exactly 24 hours before the totality occurred. The first attempt was a day before in order to record the sunset & twilight circumstances and compare them with the eclipse conditions that occurred the next day. At the finish, some of the snaps were processed in the Darktable in order to emphasize the optical phenomena that occurred that day on this part of the Atlantic coast.

4. SOLAR ECLIPSE BELOW THE HORIZON – SIMPLIFIED PATTERN

At this point, I would like to explain, what is the most important in understanding the solar eclipse phenomenon occurrence in its extended sense. Our planet has a thick atmosphere, and as we have the astronomical day, which explanation is the presence of the Sun (or at least its peace) above the horizon, we can experience also the period, when only Earth’s atmosphere is illuminated, what is concluded as the part of the astronomical night.
If the lunar shadow affects the illumination level in the Earth’s atmosphere on its path, probably it happens over the entire length of the sunlit atmosphere, at least its upper parts. Thus the optical changes can be observed far away from the eclipse zone, with no presence of the Sun itself. The best impact obviously is to be noticed at the rough extension of the umbral or antumbral path, although the optic events tend to be appreciable also beyond the penumbral zone.
The best pattern showing the simplified situation of a solar eclipse under the horizon was sketched by R.A. Gulyaev (Pic. 5), who proposed the observation of internal zodiacal light roughly at this moment when the eclipsed Sun is shallow below the horizon.

Pic. 5 The pattern of solar eclipse below the horizon and different observer locations. If an observer moves from point A to point B, then he is located beneath the umbra, as the Sun is already below the horizon (Guliaev, 1992).

Basically, the diameter of the Moon’s lunar shadow is usually several times greater than the thickness of the lower layers of the atmosphere (about 80km), contributing to the twilight scattering of the solar light (Gyuliaev, 1992). Thence, the Moon’s shadow should reduce the brightness of the twilight atmosphere significantly. This is the moment when we can speak about such a double solar eclipse event when an observer is shaded by the Moon and Earth simultaneously (Gyuliaev, 1992). Let’s see below, how the double solar eclipse effect happens step by step (Pic. 6).

Pic. 6 Following Guilaev’s pattern above I prepared a more detailed circumstances diagram, applicable for the Pinamar position on the July 2, 2019 evening, when the totality occurred. The elements: 1 – lunar shadow; 2 – shadowed part of Earth’s atmosphere 3 – shadowed Earth’s surface (area of totality at the given moment); 4 – Sunlit Earth’s atmosphere; 5 – Sunlit Earth’s surface; 6 – Earth’s surface under the astronomical night conditions after sunset; 7 – Earth’s atmosphere shaded due to sunset; O – observer position. (reddish spot – Pinamar & Santa Teresita webcams). The red line shows the geometrical line of the sunset. Click to enlarge.

Let’s consider all 6 cases taken into account by this eclipse occurrence on July 2^nd, 2019 on the Argentinian Atlantic coast (Pic. 6).

The instance I – Before the totality – An observer is watching the waning crescent of the eclipsed Sun, which actually is going under the horizon. In Pinamar the maximum obscuration possible to observe (according to the Suncalc.org application) was 93,6% and for Santa Teresita accordingly 94,5%. Afterward, the crescent Sun went under the horizon. Referring to our pattern, an observer is watching the crescent sunset with both horns down. The atmosphere remains illuminated, analogically to non-eclipse conditions, only darker as the penumbra is progressing.
Instance II – Second contact at the line of sunset – what took place around 130km northwest of the Atlantic coast. For the observer located already in the shaded part of Earth, the atmosphere right above, and behind (antisolar direction) is still illuminated. The approaching umbra can be seen in the solar direction. The moment of second contact at the line of sunset means also the moment of II^nd contact in the upper atmosphere illuminated by the Sun. Unlike the higher altitude of the Sun, where the umbral shape is oval, at lower altitudes of the Sun it becomes far extended, often spreading from the solar direction to the opposite one. It is driven by changing the geometry of the shadow, which becomes more parallel near the Earth’s terminator. This is a wide topic for the future not for now. For our information, I would like just to say, that when this lunar shadow grows into a parallel position to the Earth’s surface, then its progress will be somewhat correlated to the Earth’s shadow. It means, that as the eclipse progresses, we should see that the atmosphere is shaded from the very bottom to the top.
Instance III –  between II^nd contact and mid-eclipse at the line of sunset – the situation, where the umbral zone progresses from the very bottom to the top of Earth’s atmosphere, lifting up the Earth’s shadow. Again, the phase of totality at the terminator line corresponds to the sunset in the upper part of Earth’s atmosphere. From the geometrical point of view, the pace of the darkening sky at the antisolar direction should be much higher, than in the zenith. Taking into account the lowest part of the atmosphere between the ground and the Ozone layer, which scatters the biggest amount of light, we shall need around the 30s to make it completely darker in zenith. In practice, it’s not really possible to see that way, as the sky near the horizon is brighter due to the thickness of the atmosphere. Even if it will be shaded at the very beginning of the eclipse, the amount of light scattered from outside of the umbra will be sufficient enough to make it brighter, than the zenith sky. Hence the moment of darkening will look the same for an observer.

Instance IV – between mid-eclipse and third contact – it’s the time when the Earth’s atmosphere is shaded at its whole thickness. The sky is to be divided into three major pieces, where the left and right are illuminated by the crescent Sun, and the middle one is shaded. It’s the best moment to see the stars or other celestial objects because the sky’s surface brightness is the lowest.

Instance V – just after the III^rd contact – in a geometrical sense the observer should see how the lowest parts of the atmosphere become brighter again, as the Moon’s shadow recedes. It should be another 30s on the zenithal sky, before the umbral edge reaches the Ozone layer, unlike the sky towards the antisolar direction, which the shadow cone leaves quickly.

Instance VI – shadow cone left the Earth’s atmosphere – it remains the circumstances of the partial solar eclipse somewhere on Earth, with no totality at all. In this event, we always have the place in the greatest magnitude of the eclipse. In our case, Santa Teresita was this place just tens of seconds after the totality below the horizon.

Another very important issue arises from the shadow cone geometry. It hits the terminator line not certainly at the centerline (Pic. 7, 8). The observer watching the shadow plunging into the atmosphere can feel like the shadow shifts from one side to another, which is noticeable in the reports below. This is a too complicated phenomenon to describe in one sentence. I am going to clarify it in the future.

The shadow calculated by Xavier Jubier is extended beyond the eclipse visibility, reaching down to 1.2 degrees below the horizon. That’s why Santa Teresita is covered (Pic. 7,8), where totality started at the solar altitude of -0,2 degrees and peaked at -1 degree. The rough range of visibility of the horizontal total solar eclipse can be seen in the screenshot below (Pic. 8), where we can see how the shadow looks at the 2^nd contact by the terminator line. It starts to extend further into the atmosphere.

Pic. 7 The moment of 2^nd contact at the terminator line. Beyond this line, the shadow starts to spread throughout the Earth’s atmosphere as it doesn’t have contact with the surface anymore (Xjubier.free.fr).

Pic. 8 Lunar shadow ending its route on the Earth’s surface near the Atlantic coast in Argentina (Xjubier.free.fr).

5. WEBCAM RECORDS FROM THE DAY OF TOTALITY

Because of the different video resolutions, the results vary. Another affliction of webcams, in general, is an automatic ISO change as the light level drops. It’s individual for each camera, but in a majority of cases excludes the comfort of this type of observation, unless the totality takes place, as here. The light drop, when the lunar shadow approaches is significant enough to beat up the webcam ISO settings. It didn’t happen for the Montevideo location, thereby the results of observation are poor. On the other hand, one webcam in Santa Teresita was a little intensified by the ISO changes, which brought excellent aftereffects because the sky during the greatest phase was completely dark. Now I would like to present how the 2019 solar eclipse below the horizon was recorded by a single camera.

A. Montevideo – the view from Playa Pocitas Panoramica was quite poor due to cloudiness. The best indicator of an occurring eclipse (except for streetlights) was the dimming light on the clouds. The cloud deck was thin enough to let some solar beams come through. At the maximum phase, they are not visible. Moreover, the cloudiness further toward the La Plata mouth appears to look darker. I believe, the umbra would be clearly visible under the clear sky there, as the webcam was located merely 85km from the northern limit of totality (Pic. 9).

Pic. 9 The Vera webcam image sequence from Montevideo with a view of the Playa Pocitas Panoramica in the south direction. Red text indicates the moment of the greatest eclipse, where clouds beyond tend to be darker (Cameras.vera.com.uy). Click to enlarge.

B. Santa Teresita I – the webcam with a smallish resolution (merely 360p) brought one of the best results not only particularly this observation but all of them since 2016. Due to lower intensification on the ISO sensitivity, this camera showed an exceptional umbral movement across the eastern sky with all optic elements related to this movement (Pic. 10).

Pic. 10 The webcam image sequence towards the antisolar direction on July 2, 2019, around sunset and afterward, when the greatest eclipse occurred (Lacostadigital.com).

Looking at the image sequence above we can spot the major moments of the eclipse, which is happening below the horizon. The view towards the antisolar direction shows the fleeting umbra behaviour. The top left image comes from around sunset moment. The greatest eclipse occurred about 5 min later. Because the shadow was approaching from the right side, it’s not really visible in the top right image. We can see it a bit further, along with extraordinary darkness, which swept through this part of the sky quickly. The last two images show the end of totality, where the reddish light is emphasized, which you can see also below (Pic. 12). The last image (bottom right) shows somewhat the umbral “meeting” with the Belt of Venus, just above the horizon.

Pic. 11 Three major moments of the eclipse has been processed (the right row) in order to emphasize the colours of the sky (Darktable 2.6.2/Lacostadigital.com).

The contact of the Moon’s shadow with the Earth’s one results in the strong deterioration of the antitwilight arch appearance. As you can see, it emanates strong orange light on the right, which is completely shaded on the left and appear as a continuous blue colour, typical for the upper part of the sky. This view was a bit disturbed by the cumulus clouds producing the anticrepuscular rays in the middle of the image. The very end of the lunar shadow is to be spotted just above the Earth’s shadow line, what appears as a fuzzy tail, petering out when moving our sight up left. The cumulus cloud deck on the left remains still shaded.

Pic. 12 The lunar shadow fleeting about the Santa Teresita sky at the antisolar azimuth, where the left side (2^nd contact zone) appears to look bluish unlike the opposite side of the umbra (3^rd contact zone), which is reddish (Darktable 2.6.2/Lacostadigital.com).

Looking at the image above, we can spot a huge difference in the sky colouration between the two lunar shadow edges. From where does it come? There are basically 2 explanations for that. The first one explains the general shift towards red when the Sun is eclipsed by another body, which explains the Rayleigh scattering. The effect of the redder sky is in this event somewhat doubled, as the Sun is low above the horizon and the long light wavelengths are scattered with bigger intensity. However, if you take a detailed look, then you can see that the left edge of the umbra doesn’t look as reddish as the right one. It leads to the second explanation of this problem (Pic. 13).

Pic. 13 The typical sunset pattern with its shining through the thick Earth’s atmosphere compared to the local 2019 total solar eclipse circumstances (Baily’s Beads appearance) at the Atlantic coast.

Imagine, that you are watching the typical sunset. You can see, that when the Sun plunges into thicker Earth’s atmosphere, then its colour can slightly vary as per above. The upper part of the Sun can still look yellow, whereas the lower part may be reddish at the same time.
That would happen at the moment of totality, as it reaches its limit near the Atlantic coast in Argentina. The eclipsed Sun wasn’t visible from the ground, unlike the troposphere level, which causes around a 3-degree dip in the horizon.  If we provide 2 opposite circumstances on our pattern above and see Baily’s Bead’s appearance by the 2^nd and 3^rd contact, then we would spot a huge difference in the light quality. Because the umbra was moving not exactly along with the eclipse centerline, I placed Baily’s Beads slightly on the right and left unlike at the top and the bottom. Nevertheless, looking at this image (Pic. 13) you should understand this explanation. The 3^rd contact took place when the solar disk was shining through the thicker atmosphere, then by 2^nd contact at the same moment. Hence the sky near the umbral edge tends to scatter a bit more reddish light, which eventually faints, as more parts of the solar disk (while these ones shining upper above the horizon) are uncovered. A similar optic phenomenon is to be visible in the Pinamar webcam images, about which a bit later in this chapter (Pic. 17).

C. Santa Teresita II – fortunately, this webcam was working at the moment of totality. The fleeting Moon’s shadow has been captured perfectly.

Pic. 14 The webcam report of the 2019 total solar eclipse above the Santa Teresita coast in Argentina (LacostaDigital.com). Click to enlarge.

The image above (Pic. 14) shows the sequences from 1 to 6. The total solar eclipse has been captured between 2 and 5. Watch them in order to see, how the elusive umbra proceeded across the sky at the antisolar azimuth. Here I enhanced the major moments of the totality either (Pic. 15). A different element, which tells you about the sudden darkening is shown by the lamps on the pier, which become much brighter, as the camera tries to keep the sky brightness in fixed level.

Pic. 12 Total solar eclipse July 2, 2019, major moments captured by webcam in Santa Teresita with further processing on the right (Darktable 2.6.2/LacostaDigital.com).

The middle capture shows perfectly the Moon’s umbral range at the mid-eclipse, which vanishes the twilight wedge completely. It’s much better visible on the bottom snippet when the shadow is leaving the webcam frame. Both the twilight wedge and the Belt of Venus suddenly fade as the umbra moves (Pic. 13).

Pic. 13 The view is typical when a total solar eclipse happens below the horizon. Looking at the antisolar direction we can see how the umbra plunges into the Earth’s shadow. Eventually, it disappears, as the eclipse is gone from the ground position (LacostaDigital.com). Click to enlarge.

The image above shows a formidable view, which is the same rare, as the total solar eclipse. The Sun at that moment was deep enough under the horizon to let the Belt of Venus be visible to the observer.  As a result, you can now see how the lunar shadow erases all colours related to this phenomenon. On the other hand, the sky appears to look uniform down to the horizon.

D. Pinamar I – the best webcam quality, which unfortunately wasn’t translated into better results, due to weather conditions, which were much worse than in Santa Teresita, 25km towards the north. The Pinamar town was close to the low-pressure area, heading from the Atlantic to the mainland. In turn, a lot of cumulus and a few vertically developed clouds with virga were observed within the webcam field of view. It complicated my observation because I couldn’t see clearly the Moon’s shadow edges (Pic. 14).

Pic. 13 The solar eclipse circumstances in Pinamar Riviera, located 25km outside of the totality area (Telpin.com.ar). Click to enlarge.

Likewise previously (Pic. 11), I numbered these images in order. The Moon’s shadow can be seen from 3 to 10. The most intriguing thing is the various cloud’s illumination. Unfortunately, the cumulus on the left was shaded by another cloud instead of the Moon. Moreover, this cloud was still too far from the southern limit of totality to be inside the umbra. Looking at every screenshot one by one you can see how the sky colour changes during the umbral movement. In the beginning, it’s dark blue, which turns blue-reddish afterward. There is no clear shadow edge on the late part of the eclipse, as it disappears uniformly in the atmosphere, which was discussed previously (Pic. 6 pattern V). I am not sure about the very end of this marvelous celestial event, as the near-horizon sky is almost completely overcast. The fleeting umbra is better expressed in the processed images (Pic. 14).

Pic. 14 Processed images (on the right) of the three major moments of this eclipse captured by webcams in Pinamnar (Darktable 2.6.2/Telpin.com.ar). Click to enlarge.

It’s worth looking at the bottom screenshot, where the umbral borders are fuzzy. The umbral edges don’t really change throughout the whole eclipse, as it disappears finally into the atmosphere (Pic. 15).

Pic. 17 The borders of the umbra (blue dotted lines), seen via webcam in Pinamar Rivera on the Atlantic coast in Argentina. The colour difference is noticeable, where on the right-hand side at the 3^rd contact zone, the reddish appearance is observed (red dotted line). The aim of this image is to show real umbra presence with a similar position on the sky throughout the entire totality near the terminator line and its aftermath observed in the early civil twilight zone (Telpin.com.ar). Click to enlarge.

The image stack above shows the umbra shape and position throughout the entire totality, seen in the sky from Pinamar. Unlike the solar eclipse observed in normal conditions within the astronomical day, where the umbra moves, here it looks like this movement has been frozen. This is obviously an optical illusion, which can be regarded as the aforementioned solar eclipse below the horizon pattern (Pic. 6), where the Moon’s shadow appears to be parallel to the Earth’s shadow. For the observer watching it in Pinamar, the total solar eclipse occurred near the terminator line, around 130km northwest of the Atlantic coast. Once the totality finished there, the umbra was not visible anymore, leaving behind the reddish appearance of the sky, as explained above (Pic. 13). The red dotted line showed somewhat the “border” of this red loanword, which tends to be wider, as the eclipse is about to be finished. This is fully understandable because, after the mid-eclipse at the terminator line, the Moon’s shadow is more extended and thinner, whereas its position in the sky remains similar.

E. Pinamar II – it was the only webcam headed toward the solar direction. As a result, I saw how the glow was cut from the right (Pic. 18).

Pic. 18 View towards the northwest on July 2, 2019, where the solar eclipse took place under the horizon in Pinamar (Telpin.com.ar). Click to enlarge.

The umbra can be noticed from screenshot 3 to screenshot 10. Clouds were not affected by the umbra. They were reflecting red light, which means, that it was nearly sunset time from their altitudes. Below I gathered also some processed images, displaying the eclipse local circumstances for three major moments.

Pic. 19 The solar eclipse below the horizon seen from Pinamar. Processed images are plotted on the right-hand side. You can see roughly where the umbra emerged and disappeared after the greatest eclipse (Darktable 2.6.2/Pinamar.com.ar). Click to enlarge.

The same as above, the middle screenshot shows the conditions that occurred near the greatest eclipse.  The lack of direct sunlight led the Moon’s shadow to be clearly visible. In other conditions, where the Sun would illuminate the atmosphere boundary layer, this shadow wouldn’t appear so clearly. It applies to both directions.

F. Villa Gessel – the webcam headed southeast didn’t provide the results at all. As it worst, the greatest eclipse has been missed there, because I was focused on the webcams located within the path of totality. Only the artificial lights were turned on in the span of a few minutes, which is quite normal when the Sun goes down (Pic. 20).

Pic. 20 Poor stuff from the webcam located further south in the villa Gessel village, where the moment of the greatest eclipse was missed (Gesell.com.ar).

The image below summarizes this section. There are all screenshots presenting the same moment – the greatest phase of the eclipse, as seen via webcams on July 2, 2019, after sunset (Pic. 20).

Pic. 21 The summary of the 2019 total solar eclipse, which happened when the Sun was already under the horizon on the Atlantic coast in Uruguay and Argentina. The webcam enumeration corresponds to Pic. 4 above. Camera.vera.com.uy/LacostaDigital.com/Telpin.com.ar/Eclipse.gfsc.nasa.gov). Click to enlarge.

6. ECLIPSE IMPACT ON TWILIGHT

Each eclipse, which touches the twilight period, changes the illuminance conditions. These circumstances are various, depending on the eclipse timing against the sunset and consequently the twilight period. Basically, the issue can be analyzed in three ways:

• when the greatest eclipse occurs during the astronomical day,
• when it falls within the twilight zone from the observer’s point of view (as per the Atlantic coast in Argentina),
• when it occurs completely outside of the twilight zone, but penumbra enters the atmosphere during twilight.

The last case is the most simple, as we can only observe relatively the quicker twilight progression.

More curious in my point of view are the first two, while the second one applies perfectly to this article. I will explain shortly this phenomenon for now, although I would like to clarify it more in the future. The greatest eclipse with the total phase falls after sunset so in the period of twilight. In this event, the standard process of the vista diminishing was distracted. The observer in Pinamar could watch two moments with the same level of brightness throughout the twilight on July 2, 2019. This is shown the best in the image below (Pic. 22), where the illumination level as seen during the greatest eclipse was observed again about 30 mins later when the Sun was around 7 degrees below the horizon, so just after the nautical twilight began.

Pic. 22 The impact of the 2019 solar eclipse on twilight in Pinamar on the Argentinian Atlantic coast (Telpin.com.ar). Click to enlarge.

The image stack above shows a similar level of light at the greatest phase of the eclipse and the beginning of nautical twilight about 30 minutes later, where the Sun was still eclipsed with 50%. The brightest moment fell a few minutes after the greatest phase, and next the twilight processed as normal remaining slightly darker than usual. The yellow dotted lines show the visual range of illumination from the streetlights seen on the surface. Because the last image shows the lowest level of illuminance, we can guess, that the moment with a similar light level occurred around the end of civil twilight, about 5-10 minutes before. It’s typical for most of the total solar eclipses, which conditions inside of the umbra remain this period of twilight. Here however the location was just outside of the umbra, so it was much brighter than in the very middle of the totality. If then a similar brightness level was observed at the end of civil twilight, then in Santa Teresita totality conditions would remain about -8 degrees nautical twilight, as the camera was showing black sky. It’s only an assumption, which requires further observations. The grey dotted lines compare this range with the previous (upper) image.  This is simple and the best way to visualize the illuminance level changes, as happened throughout the second period of this eclipse.

Seeing, what happened in Pinamar, we can conclude, that in the last case, when a solar eclipse occurs before sunset, the brightest moment should occur a while after. An observer will still be able to notice two periods of similar illumination level between the greatest eclipse and some moment during the twilight.

7. ECLIPSE DAY AND DAY BEFORE COMPARISON

In the last chapter, I would like to show you the comparison between the moment of the eclipse and non-eclipse conditions. As I said at the beginning, and what I would advise you for future observations such as this, the one day before the observation is very important in order to check, that everything is alright.
The major element, which shows the eclipse conditions, even with pretty much similar illumination of the sky, as gained by different webcam ISO sensitivity is the presence of street lamps and other artificial lighting, which was turned on at the time of the eclipse.
See the webcams below, as they recorded the difference between the 1^st and 2^nd of July 2019 on the Argentinian and Uruguayan Atlantic coast.

Pic. 23 The comparison between the day before and the eclipse day in Montevideo (Cameras.vera.com.uy).

The difference in Montevideo is quite considerable, despite the lack of totality at that location. Both the deep umbra and cloud deck caused a significant light drop, which can be seen at the greatest phase of the eclipse.

Pic. 24 The day before and the eclipse day captured by webcam in Santa Teresita (LacostaDigital.com).

The view in Santa Teresita covers the moment about 10 min after totality (20:54 UTC), where the difference in illumination can be easily noticed. The sky also looks dark blue with greenish and reddish elements, which is typical for deeper partial eclipses.

Pic. 25 The situation in Pinamar about 5 minutes before the 2019 total solar eclipse below the horizon and the day before (Telpin.com.ar).

In Pinamar basically, the eclipse conditions were displayed by shining street lamps, which were off at the same time the day before. The image comes from the moment around 5 min before the greatest eclipse.

Pic. 26 The view towards the northwest in Pinamar was captured both 5 minutes before (top) and at the greatest phase of the eclipse (bottom) with the day before comparison (Telpin.com.ar). Click to enlarge.

The biggest difference can be seen in the screens provided by the only webcam headed in the solar direction. The difference in vista between the moment of the eclipse and the day before is significant.  Look also at the car lights shown on the left screenshots, presenting the view about 5 min before the eclipse and the day before. Both streetlamps and car lights are stronger.

8. CONCLUSIONS

This observation displayed the situation of the total solar eclipse continuation below the horizon, which can be seen mainly by the light level drop and fleeting umbra. It was observed, that the darkest moment of the eclipse corresponds to the beginning of the nautical twilight. Another result coming from this observation shows the light colour difference, accompanying the edges of the shadow. The 2^nd contact zone features mostly bright light, whereas the opposite – 3^rd contact zone appears to look more reddish. Moreover, this reddish appearance tends to linger, as the umbra is disappearing in the atmosphere near the end of the totality.
The umbral movement across the sky, which is very fast under astronomical day conditions, tends to stop, as encounters the Earth’s shadow. Afterward, its shape tends to blur in the atmosphere, remaining reddish sky straight after. This is driven by the different positions of the shadow against the Earth, which now lies parallelly instead of perpendicularly around midday. The observation also caught the moment, when the umbra meets the Earth’s shadow washing out the Belt of Venus appearance.
The last thing is the brightest moment occurring after the greatest eclipse, which fell shortly after the umbra was gone. It was driven by the shallow position of the Sun under the horizon, which even eclipsed still illuminated the atmosphere significantly. Seeing from Pinamar the clarity of shadow, whilst the view was about 25km outside of the totality, I can guess that some phenomena related to the total solar eclipse (i.e. visual range changes, dark cone) might have a completely different character. There is still a lot of stuff to observe and explain next time, especially with the overall eclipse impact into the whole twilight period. It would be brilliant to measure precisely the light level changes throughout the whole twilight period, in which the eclipse takes place as well as the sky surface brightness in all directions. Moreover, the nautical twilight-looking situation leads to more stars being visible within the shaded section of the sky. Following Guilaev’s work possibly also the inner zodiacal light would be visible. The scarcity of this type of observation places most of these phenomena into assumptions. Another thing is the people’s interest, especially those who live in the area, where this type of celestial event occurs. If we would raise their awareness about it, possibly it would result in a bit more reports showing how this kind of eclipse looks like from different locations at the same time. The best occasion to check it this way will happen in August 2026 in the Mediterranean when the umbra extends the path of totality throughout the atmosphere from Balearic to the Aegean Sea.

Mariusz Krukar

References:

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9. Razaullah AnsariS.M., 2002, History of Oriental Astronomy, Springer Science + Business Media, Dordrecht
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Links:

1. Famous Solar Eclipses in History – ‘Double Dawn’ Eclipse in China
2. LacostaDigital.com – Santa Teresita webcams
3. Vera cameras Uruguay
4. Telpin webcams Pinamar
5. Villa Gessel Hotel Zanzibar webcam
6. https://www.screencastify.com/
7. https://picpick.app/en/
8. https://www.darktable.org/

Forums:

1. https://forum.cosmoquest.org/showthread.php?2304-Solar-eclipses-at-dusk-dawn

Wiki:

1. Belt_of_Venus
2. Solar_eclipse_of_July_2,_2019

Youtube:

Read also:

1. How the lunar shadow looks like during the solar eclipse?
2. Top 10 intriguing thing to see during the solar eclipse.