La
Paloma, Uruguay
General: Time 11h00m, Alt. 8 degrees, Saros 146,
path 215 km, duration 03m03s (Max 05m20s)
Uruguay in the year 1992
Lunar eclipse on 15th June
On the night of 14th to 15th June a partial eclipse of the moon took place. In Uruguay, where I was staying at that time, this phenomenon was expected to provide optimal viewing, but the weather was not so great. As the country is situated at a latitude of 35° south and it was winter at the time, one could not hope too much. The lunar eclipse was completely spoilt by rain and consequently I didn't see anything.
The solar eclipse on 30th June
By coincidence, a total eclipse of the sun was visible during the same holiday and in the same country. In Uruguay totality would be seen at sunrise, before the shadow would move on to the ocean. Expectations weren't too high. The most eastern spot had a solar height of max. 4 degrees. The weather forecast for that period was not too good. There was a 5 % chance of seeing "something".
My round trip in Uruguay,
Paraguay, Brazil and Argentina didn't provide much good weather. Only at a
latitude of 25° south, 10 degrees closer to the equator, I enjoyed a beautiful
blue sky and admired the magnificent star-spangled expanse. The surrounding
area of the rivers Parana, Iguasu and Uruguay were completely submerged. The
flood victims had all been evacuated. The water level in some places was
sometimes more than 6 meters higher than usual. That didn't seem promising for
the solar eclipse, but it didn't bother me too much. Among my equipment, I
carried a sophisticated illuminance meter. As I had experienced the solar
eclipse in Finland in 1990 under a layer of clouds, it would now again grow
dark anyway. So I would always be able to carry out measurements, whether it
was cloudy or not. I intended to do my observations at the coast of Valizas.
The family Machado-Minne lived some 20 km to the east of Montevideo and took
care of my instruments during my round trip, and also put me into contact with
a local. In the winter no vehicles can reach Valizas because of the bad state
of the road system, but we had found a solution. With a radio transmitter I
would call up this person who would pick me up at a certain place in his land
rover. He would drive me to a pile dwelling where I would be able to observe
the seals during totality.
It seemed great to observe seals for two days and to be cut off from the outside world. I had to be back on the spot the morning before the big event though because I had to compare the light measurements of the solar eclipse with those of the morning before, in exactly the same circumstances.
But, as you can guess, a Uruguayan fly had to fall in my chunky soup. On Sunday, there was no public transport at all. The next bus was only due on Monday morning, and by then I already had to be measuring as the solar eclipse would take place on Tuesday morning.
So I would not be able to carry out comparative measurements. Because this was quite a disappointment and I didn't want to run the risk of being stuck there afterwards, I decided to stay at the coast of La Paloma. Moreover, totality would last a couple of seconds longer where I was, because I was closer to the central line. On this very spot I would have a totality of 3 minutes and 3 seconds at a solar height of 4 degrees. I also had the advantage of being able to carry out light measurements on Sunday and Monday morning. I measured from ... 5 o'clock in the morning until 9.10 a.m., so until after the fourth contact. I started that early because I wanted to begin two hours before sunrise.
The first morning, 28th June, the situation was already desperate. The sky was completely overcast. I completed my programme anyway. Pinched with cold at the beach, I walked back to the hotel like a robot. In the village, not the least bit of an eclipse fever was apparent.
The following day was completely different. It was a clear morning, so that I could finally watch the splendid star-spangled sky. I was amused to see Orion getting out of bed with its legs in the air. At the horizon, where the sun would rise, the sight was not 100 %, although it was not too bad. Now it could not go wrong anymore. I had observations of a cloudy and a bright morning. It didn't matter what the weather would be like the next eclipse morning, I had a comparative measurement. La Paloma was now coming under the spell of the eclipse fever. More and more tourists came flocking in. Not only foreigners (most of them gringo’s), but also Uruguayans and even Argentines came pouring in. On the evening before the event, there was a ruckus of the first order. The morning of the eclipse some 20 buses coming from Montevideo were expected. That meant there would be at least 1000 people on that little beach, all gathered close to a lighthouse. And I had to be on the very same spot ... because of my previous measurements. At 3.20 a.m. on 30th June, the sound of a bus woke me up. Oh no, it was not possible! Shit! Like a Saturn V rocket I jumped out of my bed. I dressed quickly and headed immediately for the beach where I saw a bus and a car. I set up my C90, 1000 mm. The illuminance meter was set up in the same way as the previous days. The whole area was marked out with a rope and flags of ... toilet paper.
Time went by much faster now. Moreover, I had a visitor now and then. At the beginning, the sky was very bright. I couldn't believe my eyes. This would be too beautiful to be true. At 5 o'clock a gem of a sparkling fireball appeared, but was it the forerunner of upcoming clouds?
As I feared, more and more clouds were coming from the inland. It even became so bad that it began to rain a little and at 5.30 a.m. the sky was overcast. But totality would only take place at 8 o'clock, about a quarter after sunrise.
More and more buses arrived now. It was a real feast. Some people brought drums and blowers with them. These were their instruments. Was this a football game or a solar eclipse? All secondary schools from Montevideo had planned a bus trip for this event. But no problem, it was fun. They were very friendly people.
The sky seemed to open up again. At about 7 a.m. (always local time), the first contact, far below the horizon, must have taken place. The sun only rose at 7.46 a.m. And as the sun was rising, there was more and more light. Only a string of clouds were hanging just above the horizon. After 7 o'clock light measurements were carried out every 5 minutes. Before 7 a.m. it was done every 10 minutes and after 7.30 a.m. it would be done every minute.
And indeed, at 7.46 a.m. a splendid sunrise took place. Only a beautiful red-orange coloured solar crescent emerged. The eclipse had already progressed a lot and the sky showed off a beautiful pink-blue tint. Some birds were flying over the ocean at low altitude. Some annoying clouds, however, were hanging just above the horizon, sometimes blocking the sun. It was exciting.
It grew remarkably darker. The crescent was narrowing and filters were not necessary. The low position of the sun decreased light intensity sufficiently.
One minute before totality, at 7.59 a.m., a bird was flying low over the water, twittering loudly. The crescent was becoming narrower and weaker. The Baily's Beads were vaguely visible ... but they looked magnificent in the C90. Very fast the sunlight perished. And then ... nothing was left.
We couldn't see a thing. Many were flabbergasted. It was dark, it was night. I didn't stop measuring. Where no clouds were hanging low, the horizon was coloured red. Half a minute before the end of totality, the corona emerged. Not entirely, but over some 200 degrees. Very soon afterwards the red glow of the chromosphere was visible with prominence as well and it looked just magnificent. It all happened very fast, it was very dark, it really seemed to be night. And then, ... flash! the diamond ring, violent and sensational. That was fantastic! Hundreds of people all began to shout spontaneously and they cheered and clapped their hands. The sun appeared again and dawn progressed rapidly. But the light was still not too strong so that I could watch the solar crescent without filters. It was over ... A splendid solar eclipse! I was delighted. It had been another wonderful experience. I continued measuring until the last contact at about 9.10. a.m. The results of the light measurements will be published in one of the next editions of Heelal.
Just as the orbits of sun and moon cross each other at a total eclipse of the sun, the orbits of eclipse freaks seem to cross each other at the same time. I met Wasil again, our VVS-colleague from Brussels and we both agreed: witnessing this is a must, this eclipse was a gem. Kristina, Wilson, Rafael, Annahi, Yannick, Maite, Dolores, Enrique, Rodrigo, Ross, Claudia and Pablo : muchas gracias para todo. Without your help, but above all your southern friendship, my trip wouldn't have been such a success. It's something you can't buy, and from which, we Westerners, can still learn a lot.
Luminosity during a total solar eclipse
In the previous edition of Heelal you could read the story of my expedition in Uruguay to study the total solar eclipse of 30th June 1992. Before, during and after the eclipse I carried out light measurements, which I compared with the measurements of the two previous days in exactly the same circumstances.
The illuminance meter
For these measurements an illuminance meter from Minolta was used. Because this instrument weighs only 220 gram and has a size of 170x72x33 mm, it was not so difficult to carry around.
The illuminance meter is equipped with an extremely sensitive photocell and a very accurate digital display. The instrument is used to measure illuminated road signs and display windows but has other applications.
A battery lasts 100 hours in the so-called FAST mode and 30 hours in SLOW mode. This may sound a bit illogical, but in the first mode the measurement is carried out continuously and that seems to require less power than instantaneous exposures whereby results must be preserved. The response time of the meter is 1 ms (one thousandth of a second); the result is expressed in Lux (lx).
Before you start measuring, you have to fulfil a calibration procedure which takes some 3 seconds after which light intensities between 0 lx and 99 900 lx can be measured, with an accuracy of at least 3 decimal places.
The instrument offers a number of other advantages. For instance, the measuring unit and microprocessor can be separated by a long cable of up to 10 meters. This enables the user to carry out measurements in areas which are not easily accessible and yet to read the results in a more comfortable place. As the whole is operated by means of a microprocessor, it is obvious that several measurement functions are available, such as differential or integral functions. In the latter case, the results are expressed in Lux hours. The integration time may vary from 36 seconds to 999 hours. The instrument is quite accurate. The spectral response remains within 2 % of the ideal response curve, as is defined by the CIE (Commission Internationale de l'Eclairage). A second diagram shows to what extent the reception of light varies with its angle of incidence. Notice also that this curve deviates little from the standard.
The measurements
As described in my travel report, I had carried out comparative measurements two days before the eclipse. The first day was cloudy, but the weather was clear the following day, except for a few clouds at the horizon. The circumstances on the day of the eclipse were similar. I started measuring at 5 o'clock in the morning and stopped at 9.10 a.m. local time. Before 6 o'clock, however, I measured less than 0.01 lx so I started entering the data in diagram after 6 o'clock.
At approx. 7.00 a.m. the first contact took place under the horizon. On the morning of the eclipse, the sun rose only at 7.46 a.m. The second contact took place some 4° above the horizon at 8.00 a.m. Three minutes later the third contact, which marked the end of totality, took place and at 9.10 a.m., at the fourth contact, I stopped the measurements.
During these three days, the instrument was always set up in the same place and in the same direction. The photocell was directed towards the zenith in order to measure the entire sky. The reception of slanting light, however, quickly decreases with the angle of incidence, but it did not interfere with my goal: I wanted to know how dark it would get during the eclipse.
The readings were done visually, in the beginning I recorded the measurements every 10 minutes and at the first measurable sign of twilight every 5 minutes. Finally, between 7.35 a.m. and 8.35 a.m., I even carried out measurements every minute.
The results
The results are excellent, as the diagram on the left shows. The scale of luminosity is logarithmic because of the considerable disparities (from 0.00 lx to 35 000 lx).
Time is represented in hours and decimals.
The arrow on the diagram indicates the moment of sunrise on the day of the total solar eclipse, which corresponds also to the two previous days. Until some 10 minutes before sunrise, measuring results on the day of the eclipse and on the cloudy day run parallel. The partially eclipsed sun under the horizon had almost the same effect as a layer of clouds.
From this moment on, however, the effect of the eclipse became more and more obvious, but until 3 minutes before sunrise, daylight was still increasing. The diagram shows that night falls again during totality.
After the third contact it became light again and 10 to 15 minutes after totality I obtained values of the same order as on the cloudy morning. Only at the fourth contact I recorded a light intensity equal to the clear morning.
In the table some other measurements are mentioned which I carried out with the same instrument which may help to put them in the right perspective.
The concept of "Lux"
Minnaert's book, "De natuurkunde van 't vrije veld - deel 1" describes the concept lux in a plain and common way. Lux is what used to be called meter candle. Nowadays it is defined as: the quantity of light produced by a light flux of one lumen, falling on a surface of one square meter: 1 lx = 1 lm m-2. It is the SI-unit of light intensity. A lumen (lm) is the SI-unit of luminous flux (light flux), equal to the quantity of light emitted through a pointed source of light from one candela in a solid angle of one star radian (1 lm = 1 cd sr). The candela is the SI-basic unit of light intensity. Minnaert states that if we would compare the light intensity of a candle with the star Capella (magnitude 0.3), we would have to move away 900 meter from the candle in order to obtain the same luminosity as the star. Thus we acquire a light intensity of 1/9002 = 1/810 000 lux.
The light intensity of the Full Moon according to Minnaert's experiments is 0,13 lx, which corresponds to my own measurements.
The book also mentions that the sun is 300 000 times brighter than a blue sky and a white cloud 10 times. On a clear day, 80 % of the light emanates directly from the sun, 20 % from the sky. At noon, the light shed by the sun on a horizontal surface at close range of a cloudless sky is 80 000 lx in the summer and 20 000 lx in the winter. After sunset, the light intensity quickly decreases from 400 lx (0°) to 40 lx (-3°) and 1 lx (-6°) to 0.1 lx (-8°). The values that I obtained during the total solar eclipse correspond to a sun altitude of -6°. This corresponds with the end of the civil twilight.
Picture and Graph with measurements by Patrick Poitevin
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