Hybrid Solar Eclipse of 1986 October 3
from an aircraft over the north Atlantic Ocean near Greenland
On 1986 October 3, a remarkable hybrid solar eclipse occured over the north Atlantic Ocean to the east of Greenland. The eclipse path, on the surface of the Earth, was a narrow, tapered, horse-shoe, and visible only from a thin strip between Greenland and Iceland. What made it remarkable was its uncorrected maximum duration —predicted by Jean Meeus in 1966 in his Canon of Solar Eclipses as "0m00s", as the mean lunar and solar seen semi-diameters were topocentrically virtually identical—. Of course, correcting for the true lunar limb profile, then one might expect it to be annular at sea level. By intercepting the umbral shadow cone from an aircraft at high altitude you’d get closer to the Moon relative to a sea level site, and thus "growing" the Moon’s angular diameter with respect to the Sun which with such a marginal eclipse could make the whole difference. That being said to witness a "true" total solar eclipse with complete photospheric extinction would have required flying much higher than what was posible.
An Icelandic astronomer, Þorsteinn Sæmundsson, and his brother Stefán who was the pilot, chartered a Cessna Citation II aircraft to observe the eclipse from an altitude of 40,000 feet (12,192 meters). To cover the costs they were later joined by a small group of intrepid eclipse chasers, lead by John Beattie and Glenn Schneider. Unfortunately for them, with the tools and knowledge available at the time, their computations of the eclipse path were slightly off and the aircraft trajectory difficult to assess in this pre-GPS era. Nevertheless this eclipse flight was a success. On a similar scale of planning and navigational nightmare, I had my share with HSE 2013 and the events that went wrong.
Due to the low elevation of the Sun the eclipse path at the altitude of the aircraft (12,192 meters or 40,000 feet) was shifted towards the west by about 105 kilometers (65 miles). The light gray curve on the right side is shown at sea level and the point of greatest eclipse is also marked. The path is also slightly wider as you get closer to the Moon.
Thanks to new advancements in my Solar Eclipse Maestro application we will see below that the aircraft most likely flew about 8 kilometers (5 miles) west of the planned route (drawn in brown on the map below) as the Baily’s beads simulations show. During many years I was puzzled by the fact that actual observations didn’t match my computations and the only possible explanation I could come up with to explain this mystery was that the position of the aircraft provided by the participants at the time was wrong. Moreover one of their computed eclipse path was also quite far off and would have made their observations even worse if it had been flown to. Moreover from the pictures taken one can notice the asymmetric evolution of the observed beads with the top part remaining clear, hinting at the Moon being to high relatively to the Sun which means an aircraft too far west.
The following screenshot retraces well the unfolding of events at the time. The aircraft flew too far to the west mainly because the eclipse computations and the positionning were not accurate enough. It should be pointed out that back then with the means available it wasn’t easy to do any better.
As one can see below there is a good match between the eclipse simulation and the frames. However the position of the aircraft wasn’t very accurately known, the Global Positionning System wasn’t yet available, which can explain the small discrepancies. Moreover the whole simulation is made from a fixed position, this will need to be improved in a future revision. From the look of the Baily’s beads the aircraft was more than likely located a few kilometers west of the optimal location. Further simulations will one day show this when I get more time to work on it. It is also clear that the eclipse could never be total even at that altitude, the tip of the shadow cone being much higher up, and the simulation clearly demonstrates it. From the position determined by the simulation it appears that what the passengers of the aircraft saw was in fact an extrememy deep partial as they were off track, west of the shadow cone axis.
Simulation using Solar Eclipse Maestro against image sequence (Glenn Schneider) every second
"diamond tiara" photographed from a Cessna Citation II aircraft at an altitude of 40,000 feet over the north Atlantic Ocean near Greenland
[note that the flight was executed at 40,000 feet and not the 44,000 feet advertized, and at slightly adjusted coordinates]
The above montage of 25 exposures starting at 19:05:14 U.T. spans only eight seconds of time (uniform exposure spacing at a rate of three frames per second). The unfiltered 1/500-second images were captured on Kodacolor 100 (GA 5095) film with a 400mm EFL lens stopped down to f/22 on a Nikon FM2 camera. The solar corona was easily visible to the unaided eye, though this photographic sequence was designed to capture the chromosphere and circum-lunar bead phenomena. Four frames, 19:05:19.33 to 19:05:20.33 (in penultimate row above) were, unfortunately, affected by vibration in the aircraft.
Baily’s beads simulation from the eclipse flight at 40,000 feet over the north Atlantic Ocean near Greenland
(created with Solar Eclipse Maestro)
The "gang of 9" (from left to right: Páll Reynisson, Þorsteinn Sæmundsson, John Beattie, Glenn Schneider, John Goodman, Ragnar Axelsson, Mary Goodman, Stefán Sæmundsson and Ágúst Arnbjörnsson) stand before their Cessna Citation II chase plane, with the celebratory eclipse flag (graciously on loan from Craig Small) unfurled after a successful venture
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