The Longest Eclipse Observation
from Gerhard Holtkamp, 04. July 2009, 21:27
36 years ago some astronomers managed to observe a total eclipse of the Sun for a duration of 75 minutes - a record that has not been broken. How was this possible?
This month there will be a total eclipse of the Sun on 22 July 2009 visible in South Asia. Not just any eclipse - with a maximum totality of 6 minutes 39 seconds (somewhere out over the Pacific Ocean) it will be the longest total eclipse this century. But there have been other long eclipses in the past.
Every 18 years, 11 1/3 days (or 10 1/3 days or 12 1/3 days, depending on intervening leap years) the relative positions of the Earth, Moon and Sun are very similar and you can expect a similar lunar or solar eclipse. This is called a Saros cycle and was already known to Babylonians.
So 18 years and 36 years ago there should have been long total eclipses of the Sun. But due to the 1/3 day the Earth has turned one third and the venue of the eclipse would have shifted by one continent. Indeed there had been a 6 minute 53 second eclipse on 11 July 1991 in Mexico and a 7 minute 4 second eclipse on 30 June 1973 with maximum over the Sahara desert.
The eclipse of 11 July 1991 first caught my attention while I was part of a team planning for a Spacelab mission. An originally proposed launch date had been July 1991 and due to an astronomy experiment on board we would try to launch a few days before New Moon. On checking this particular New Moon we found that there would be this long solar eclipse connected with it. I then did some quick calculations trying to find out whether our Space Shuttle would fly through the Moon's shadow.
As it turned out the way our orbit and launch window was set would have seen us with some 15 seconds of totality for the astronauts to watch and for the public relations department to exploit. A fairly minor change in the orbit would have made the Shuttle fly through the umbra in an optimal way and given us about 40 seconds of totality - a lot less than the almost 7 minutes on the ground. The Shuttle simply flies much too fast for the Moon's shadow.
Shortly after I had done my small study our launch date shifted (and it would continue to shift until we finally launched in Spring 1993). Not having to sit in a control room at the time I was now free to travel to Mexico and see the real thing (having done this study made me feel that this was my very personal eclipse!). So in July 1991 I found myself in Puerto Vallarta just below the zone of totality and close to the point of maximum eclipse.
You normally would expect hotels to be totally booked out and charging extra rates for an event like this but - surprise - hotels in Puerto Vallarta were quite empty and hotel rooms were half price! There was a reason for this: July is off season for that part of Mexico because it is the rainy season with dark clouds often blocking the Sun. The vast majority of eclipse watchers had gone to Baja California or Hawaii which offered much better weather prospects.
Nevertheless, on 11 July 1991 four large busses and a small one all packed with tourists left Puerto Vallarta early in the morning to arrive at a gorgeous hotel deep inside the zone of totality on a large bay overlooking the Pacific Ocean and steep mountains at the back. Total eclipse would happen exactly at noon to be followed by some nice buffet lunch.
First Contact (the moment when the Moon touches the Sun and begins to take a bite out of the Sun's disk) was greeted by loud cheers but a look back showed ominous clouds building up over the mountains. About half an hour into the partial eclipse phase and still one hour to go until totality the clouds had swallowed up the Sun and you could hear some thunder. We were too close to the mountains.
Only the distant tip of the bay still seemed to be in sunshine. A few of us managed to convince the driver of the small bus (the other drivers refused and missed their chance for a really big tip) to drive us to this spot. It became a race against time. With just five minutes left before start of totality we arrived at a small place called San Blas. There were some cirrus clouds but the Sun's corona is bright enough to shine through and the prominences are brighter still.
We didn't have the time to set up tripods etc. but this didn't matter because at that site the Sun was directly in the zenith during totality.
So you simply put your camera flat on the back (but it was quite a strain on your neck). And what a sight it was! The prominences in particular were just spectacular. Shortly after the crescent of the Sun reappeared again the dark clouds caught up with us but it didn't matter anymore (nor did it matter that the buffet lunch was finished by the time we returned to the hotel)! The duration of totality at our site had been 6½ minutes. This was my longest eclipse observation.
Impressive as that may have been one Saros period further back on 30 June 1973 a few astronomers set out to observe a total eclipse for a much longer time. This was only possible due to a combination of lucky circumstances.
During totality the shadow of the Moon typically moves across the surface of the Earth with a speed of some Mach 3 or more (three times the speed of sound). While this was too slow for the Shuttle it is too fast for airplanes to follow. But having the particularly long eclipse on 30 June 1973 near noon near the equator (where the Earth rotation would show its greates effect) the relative speed slowed to less than Mach 2 - within range for a Concorde supersonic plane to follow.
There remained one problem: The Sun would be high overhead and airplane windows go to the side. Now the final lucky circumstance came into play. The very first Concorde ever build - Prototype 001 - had finished its flight test program and was about to be retired. So engineers were free to cut holes into the roof and put windows with cameras there.
So it came that on 30 June 1973 veteran test pilot André Turcat took Concorde F-WTSS (Prototype 001) out of Las Palmas, intercepted the Moon's umbra and stayed with it for a continuous 75 minutes, finally landing in Fort-Lamy (N'Djamena, Chad) with hardly any fuel left.
If you are ever in Paris don't miss visiting the Musée de l'Air et de l'Espace in Le Bourget.
This very Concorde is on display there. You can enter it and see where the holes were cut into the roof and there are also some of the large cameras on display which were used for the eclipse observations.
More details (in French) of the Concorde on display at the Musée de l'Air et de l'Espace.
This month there will be a total eclipse of the Sun on 22 July 2009 visible in South Asia. Not just any eclipse - with a maximum totality of 6 minutes 39 seconds (somewhere out over the Pacific Ocean) it will be the longest total eclipse this century. But there have been other long eclipses in the past.
Every 18 years, 11 1/3 days (or 10 1/3 days or 12 1/3 days, depending on intervening leap years) the relative positions of the Earth, Moon and Sun are very similar and you can expect a similar lunar or solar eclipse. This is called a Saros cycle and was already known to Babylonians.
So 18 years and 36 years ago there should have been long total eclipses of the Sun. But due to the 1/3 day the Earth has turned one third and the venue of the eclipse would have shifted by one continent. Indeed there had been a 6 minute 53 second eclipse on 11 July 1991 in Mexico and a 7 minute 4 second eclipse on 30 June 1973 with maximum over the Sahara desert.The eclipse of 11 July 1991 first caught my attention while I was part of a team planning for a Spacelab mission. An originally proposed launch date had been July 1991 and due to an astronomy experiment on board we would try to launch a few days before New Moon. On checking this particular New Moon we found that there would be this long solar eclipse connected with it. I then did some quick calculations trying to find out whether our Space Shuttle would fly through the Moon's shadow.
As it turned out the way our orbit and launch window was set would have seen us with some 15 seconds of totality for the astronauts to watch and for the public relations department to exploit. A fairly minor change in the orbit would have made the Shuttle fly through the umbra in an optimal way and given us about 40 seconds of totality - a lot less than the almost 7 minutes on the ground. The Shuttle simply flies much too fast for the Moon's shadow.
Shortly after I had done my small study our launch date shifted (and it would continue to shift until we finally launched in Spring 1993). Not having to sit in a control room at the time I was now free to travel to Mexico and see the real thing (having done this study made me feel that this was my very personal eclipse!). So in July 1991 I found myself in Puerto Vallarta just below the zone of totality and close to the point of maximum eclipse.
You normally would expect hotels to be totally booked out and charging extra rates for an event like this but - surprise - hotels in Puerto Vallarta were quite empty and hotel rooms were half price! There was a reason for this: July is off season for that part of Mexico because it is the rainy season with dark clouds often blocking the Sun. The vast majority of eclipse watchers had gone to Baja California or Hawaii which offered much better weather prospects.
Nevertheless, on 11 July 1991 four large busses and a small one all packed with tourists left Puerto Vallarta early in the morning to arrive at a gorgeous hotel deep inside the zone of totality on a large bay overlooking the Pacific Ocean and steep mountains at the back. Total eclipse would happen exactly at noon to be followed by some nice buffet lunch.
First Contact (the moment when the Moon touches the Sun and begins to take a bite out of the Sun's disk) was greeted by loud cheers but a look back showed ominous clouds building up over the mountains. About half an hour into the partial eclipse phase and still one hour to go until totality the clouds had swallowed up the Sun and you could hear some thunder. We were too close to the mountains. Only the distant tip of the bay still seemed to be in sunshine. A few of us managed to convince the driver of the small bus (the other drivers refused and missed their chance for a really big tip) to drive us to this spot. It became a race against time. With just five minutes left before start of totality we arrived at a small place called San Blas. There were some cirrus clouds but the Sun's corona is bright enough to shine through and the prominences are brighter still.
We didn't have the time to set up tripods etc. but this didn't matter because at that site the Sun was directly in the zenith during totality.
So you simply put your camera flat on the back (but it was quite a strain on your neck). And what a sight it was! The prominences in particular were just spectacular. Shortly after the crescent of the Sun reappeared again the dark clouds caught up with us but it didn't matter anymore (nor did it matter that the buffet lunch was finished by the time we returned to the hotel)! The duration of totality at our site had been 6½ minutes. This was my longest eclipse observation.Impressive as that may have been one Saros period further back on 30 June 1973 a few astronomers set out to observe a total eclipse for a much longer time. This was only possible due to a combination of lucky circumstances.
During totality the shadow of the Moon typically moves across the surface of the Earth with a speed of some Mach 3 or more (three times the speed of sound). While this was too slow for the Shuttle it is too fast for airplanes to follow. But having the particularly long eclipse on 30 June 1973 near noon near the equator (where the Earth rotation would show its greates effect) the relative speed slowed to less than Mach 2 - within range for a Concorde supersonic plane to follow.
There remained one problem: The Sun would be high overhead and airplane windows go to the side. Now the final lucky circumstance came into play. The very first Concorde ever build - Prototype 001 - had finished its flight test program and was about to be retired. So engineers were free to cut holes into the roof and put windows with cameras there.
So it came that on 30 June 1973 veteran test pilot André Turcat took Concorde F-WTSS (Prototype 001) out of Las Palmas, intercepted the Moon's umbra and stayed with it for a continuous 75 minutes, finally landing in Fort-Lamy (N'Djamena, Chad) with hardly any fuel left.If you are ever in Paris don't miss visiting the Musée de l'Air et de l'Espace in Le Bourget.
This very Concorde is on display there. You can enter it and see where the holes were cut into the roof and there are also some of the large cameras on display which were used for the eclipse observations.More details (in French) of the Concorde on display at the Musée de l'Air et de l'Espace.
A fascinating personal account of your 1991 eclipse experience - which stands in striking contrast to my 1999 experience, where I saw nothing, due to cloud cover and rain. So I envy you, even though you missed your buffet lunch.
Incidentally, just as an aside, I would like to add that the 1999 eclipse was followed by not one, but three Concordes: Two operated by British Airways and one by Air France. The Air France one was chartered by the French government, the British Airways flights were filled with paying passengers.
These got nowhere close the record held by Turcat, as far as I know they managed up to 11 minutes of eclipse observation, because the 1999 eclipse, passing at higher latitudes, led to a higher apparent speed of the shadow.
But on the upside, I heard that a very nice buffet was served to all of the passengers on these flights.