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This is not possible. And this has nothing to do with the actual brightness of explosion. A similar explosion on the Moon will probably be visible from the earth if the weather conditions are good and if you know when and where to look.

According to the Wikipedia, the Nagasaki bomb was exploded at the height of 503 m, about 1/2 km. To be directly visible from 810 km distance it had to be exploded at the height more than 60 km.

(Calculation: 810 km=437 nautical miles=7.289 degrees. The radius of Earth is 6370 km; 6370/cos 7.289=6423. Subtract 6370, we get 63 km. This is the height at which the bomb had to be exploded to be visible at 810 km. I neglected refraction but the difference between 60 km and 0.5 km is so large that refraction can be neglected.)

Of course there are described cases of strong anomalous refraction, but they usually occur in polar regions http://en.wikipedia.org/wiki/Novaya_Zemlya_effect, and one does not expect it on the latitude of Japan.

EDIT. A suggestion in a comment was that a reflection from a cloud could be seen. Again, the information found in Wikipedia shows that clouds do not reach such heights, except in polar regions (stratospheric clouds). In addition to this, a reflection would be much weaker than the original flash, and perhaps cannot be seen at 800 km.

EDIT2. I am surprised by the number of comments to this answer. Nobody would discuss the question whether the Great Fire of London could be visible from Switzerland (about the same distance), because of some super-unusual refraction or some super-high clouds, or whatever. The only difference is that a nuclear explosion involves a very bright (and very short) flash. But this difference is irrelevant.

This is not possible. And this has nothing to do with the actual brightness of explosion. A similar explosion on the Moon will probably be visible from the earth if the weather conditions are good and if you know when and where to look.

According to the Wikipedia, the Nagasaki bomb was exploded at the height of 503 m, about 1/2 km. To be directly visible from 810 km distance it had to be exploded at the height more than 60 km.

(Calculation: 810 km=437 nautical miles=7.289 degrees. The radius of Earth is 6370 km; 6370/cos 7.289=6423. Subtract 6370, we get 63 km. This is the height at which the bomb had to be exploded to be visible at 810 km. I neglected refraction but the difference between 60 km and 0.5 km is so large that refraction can be neglected.)

Of course there are described cases of strong anomalous refraction, but they usually occur in polar regions http://en.wikipedia.org/wiki/Novaya_Zemlya_effect, and one does not expect it on the latitude of Japan.

EDIT. A suggestion in a comment was that a reflection from a cloud could be seen. Again, the information found in Wikipedia shows that clouds do not reach such heights, except in polar regions (stratospheric clouds). In addition to this, a reflection would be much weaker than the original flash, and perhaps cannot be seen at 800 km.

This is not possible. And this has nothing to do with the actual brightness of explosion. A similar explosion on the Moon will probably be visible from the earth if the weather conditions are good and if you know when and where to look.

According to the Wikipedia, the Nagasaki bomb was exploded at the height of 503 m, about 1/2 km. To be directly visible from 810 km distance it had to be exploded at the height more than 60 km.

(Calculation: 810 km=437 nautical miles=7.289 degrees. The radius of Earth is 6370 km; 6370/cos 7.289=6423. Subtract 6370, we get 63 km. This is the height at which the bomb had to be exploded to be visible at 810 km. I neglected refraction but the difference between 60 km and 0.5 km is so large that refraction can be neglected.)

Of course there are described cases of strong anomalous refraction, but they usually occur in polar regions http://en.wikipedia.org/wiki/Novaya_Zemlya_effect, and one does not expect it on the latitude of Japan.

EDIT. A suggestion in a comment was that a reflection from a cloud could be seen. Again, the information found in Wikipedia shows that clouds do not reach such heights, except in polar regions (stratospheric clouds). In addition to this, a reflection would be much weaker than the original flash, and perhaps cannot be seen at 800 km.

EDIT2. I am surprised by the number of comments to this answer. Nobody would discuss the question whether the Great Fire of London could be visible from Switzerland (about the same distance), because of some super-unusual refraction or some super-high clouds, or whatever. The only difference is that a nuclear explosion involves a very bright (and very short) flash. But this difference is irrelevant.

3 added 340 characters in body
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This is not possible. And this has nothing to do with the actual brightness of explosion. A similar explosion on the Moon will probably be visible from the earth if the weather conditions are good and if you know when and where to look.

According to the Wikipedia, the Nagasaki bomb was exploded at the height of 503 m, about 1/2 km. To be directly visible from 810 km distance it had to be exploded at the height more than 60 km.

(Calculation: 810 km=437 nautical miles=7.289 degrees. The radius of Earth is 6370 km; 6370/cos 7.289=6423. Subtract 6370, we get 63 km. This is the height at which the bomb had to be exploded to be visible at 810 km. I neglected refraction but the difference between 60 km and 0.5 km is so large that refraction can be neglected.)

Of course there are described cases of strong anomalous refraction, but they usually occur in polar regions http://en.wikipedia.org/wiki/Novaya_Zemlya_effect, and one does not expect it on the latitude of Japan.

EDIT. A suggestion in a comment was that a reflection from a cloud could be seen. Again, the information found in Wikipedia shows that clouds do not reach such heights, except in polar regions (stratospheric clouds). In addition to this, a reflection would be much weaker than the original flash, and perhaps cannot be seen at 800 km.

This is not possible. And this has nothing to do with the actual brightness of explosion. A similar explosion on the Moon will probably be visible from the earth if the weather conditions are good and if you know when and where to look.

According to the Wikipedia, the Nagasaki bomb was exploded at the height of 503 m, about 1/2 km. To be directly visible from 810 km distance it had to be exploded at the height more than 60 km.

(Calculation: 810 km=437 nautical miles=7.289 degrees. The radius of Earth is 6370 km; 6370/cos 7.289=6423. Subtract 6370, we get 63 km. This is the height at which the bomb had to be exploded to be visible at 810 km. I neglected refraction but the difference between 60 km and 0.5 km is so large that refraction can be neglected.)

Of course there are described cases of strong anomalous refraction, but they usually occur in polar regions http://en.wikipedia.org/wiki/Novaya_Zemlya_effect, and one does not expect it on the latitude of Japan.

This is not possible. And this has nothing to do with the actual brightness of explosion. A similar explosion on the Moon will probably be visible from the earth if the weather conditions are good and if you know when and where to look.

According to the Wikipedia, the Nagasaki bomb was exploded at the height of 503 m, about 1/2 km. To be directly visible from 810 km distance it had to be exploded at the height more than 60 km.

(Calculation: 810 km=437 nautical miles=7.289 degrees. The radius of Earth is 6370 km; 6370/cos 7.289=6423. Subtract 6370, we get 63 km. This is the height at which the bomb had to be exploded to be visible at 810 km. I neglected refraction but the difference between 60 km and 0.5 km is so large that refraction can be neglected.)

Of course there are described cases of strong anomalous refraction, but they usually occur in polar regions http://en.wikipedia.org/wiki/Novaya_Zemlya_effect, and one does not expect it on the latitude of Japan.

EDIT. A suggestion in a comment was that a reflection from a cloud could be seen. Again, the information found in Wikipedia shows that clouds do not reach such heights, except in polar regions (stratospheric clouds). In addition to this, a reflection would be much weaker than the original flash, and perhaps cannot be seen at 800 km.

2 added 9 characters in body
source | link

This is not possible. And this has nothing to do with the actual brightness of explosion. A similar explosion on the Moon will probably be visible from the earth if the weather conditions are good and if you know when and where to look.

According to the Wikipedia, the Nagasaki bomb was exploded at the height of 503 m, about 1/2 km. To be directly visible from 810 km distance it had to be exploded at the height more than 60 km.

(Calculation: 810 km=437 nautical miles=7.289 degrees. The radius of Earth is 6370 km; 6370/cos 7.289=6423. Subtract 6370, we get 63 km. This is the height at which the bomb had to be exploded to be visible at 810 km. I neglected refraction but the difference between 60 km and 0.5 km is so large that refraction can be neglected.)

Of course there are described cases of strong anomalous refraction, but they usually occur in polar regions http://en.wikipedia.org/wiki/Novaya_Zemlya_effect, and one does not expect it on the latitude of Japan.

This is not possible. And this has nothing to do with the actual brightness of explosion. A similar explosion on the Moon will probably be visible from the earth if the weather conditions are good and if you know where to look.

According to the Wikipedia, the Nagasaki bomb was exploded at the height of 503 m, about 1/2 km. To be directly visible from 810 km distance it had to be exploded at the height more than 60 km.

(Calculation: 810 km=437 nautical miles=7.289 degrees. The radius of Earth is 6370 km; 6370/cos 7.289=6423. Subtract 6370, we get 63 km. This is the height at which the bomb had to be exploded to be visible at 810 km. I neglected refraction but the difference between 60 km and 0.5 km is so large that refraction can be neglected.)

Of course there are described cases of strong anomalous refraction, but they usually occur in polar regions http://en.wikipedia.org/wiki/Novaya_Zemlya_effect, and one does not expect it on the latitude of Japan.

This is not possible. And this has nothing to do with the actual brightness of explosion. A similar explosion on the Moon will probably be visible from the earth if the weather conditions are good and if you know when and where to look.

According to the Wikipedia, the Nagasaki bomb was exploded at the height of 503 m, about 1/2 km. To be directly visible from 810 km distance it had to be exploded at the height more than 60 km.

(Calculation: 810 km=437 nautical miles=7.289 degrees. The radius of Earth is 6370 km; 6370/cos 7.289=6423. Subtract 6370, we get 63 km. This is the height at which the bomb had to be exploded to be visible at 810 km. I neglected refraction but the difference between 60 km and 0.5 km is so large that refraction can be neglected.)

Of course there are described cases of strong anomalous refraction, but they usually occur in polar regions http://en.wikipedia.org/wiki/Novaya_Zemlya_effect, and one does not expect it on the latitude of Japan.

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