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Igor S, thank you for pointing this out. David Pike, thanks for the link to the color video. 

The Wikipedia article on these tests says that the Grapple 1 test was an airbust at 10:38 "local time" over a mile above Malden Island (4.0°S, 154.9°W). When I simulate the sky for May 15, 1957 at 20:44 UT (*), I find the Sun would have been about 58° high on an azimuth of 44°. The observers with the sextants are looking in a direction nearly opposite the Sun. Shadows in the image are also consistent with that altitude. This implies that the observation vessel was located toward the northeast from the island. That makes sense. They would have wanted the Sun at their backs for better observation of the fireball and mushroom cloud. We can also see that the observers sitting on the ship's deck are facing the Sun, and when they stand to look at the explosion 15 seconds after the detonation, they are then facing almost directly away from the Sun.

Using a marine sextant to measure the angular height of the mushroom cloud was of course "overkill", but that's fitting for an atomic bomb test. A kid with a protractor and a washer on a string could have produced sufficiently useful angular altitudes. 

It's interesting that this first British hydrogen bomb test was a failure though this was kept secret at the time. That wasn't unusual with early h-bomb tests, and they still put on a good show since most were mixed fission-fusion bombs anyway.

Frank Reed

* For the time in the simulation, attached below, I used 20:44:24. The seconds are just random. I selected 44 minutes after the hour under the assumption that the sextant observations are some minutes after the detonation time and also that the "local time" is local zone time, ten hours from Greenwich.

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