The curator of Kokura Historical Museum points at a map on the wall. It is an imaginary map, showing the city of Kokura overlaid with concentric rings and blobs of red and orange.
"This red zone is the area which would have suffered the worst of the damage," says Motoharu Nakagawa. "All the houses would have been destroyed, and most of the people would have been killed at once by the flash and the blast. On 9 August I was in my parents' house here, on the edge of it, and if things had been different that day, I would certainly have died.
''According to the official report it was cloudy, but I remember it as very clear, and I saw a bomber flying over that morning. Later we heard on the radio that 'a new type of bomb' had fallen on Nagasaki."
Today Kokura is a rather ordinary city, with a wide river, clean modern buildings of glass and steel and a feudal castle, reconstructed in 1960 in ferro-concrete. But Kokura is also a place of mind-boggling irony.
The B-29 bomber which the eight-year-old Mr Nakagawa saw that morning was called Bock's Car, and its cargo was a four-and-a-half ton bomb nicknamed "Fat Man" - in honour, so the story goes, of Winston Churchill. Fat Man contained a core of plutonium, more powerful than the uranium bomb which had destroyed Hiroshima three days earlier. By the slimmest of chances, Kokura escaped becoming the second city to suffer atomic bombing.
Climb to the top of the castle and you can still see what made Kokura such an ideal target. Between the town centre and the girdle of hills are the chimneys of the biggest industrial centre in western Japan.
Nippon Steel had, and still has, its principal works here. In 1945 there were several arsenals. Thousands of poison gas shells were manufactured and tested, along with 9,000 balloon bombs which were floated over the Pacific to land on the west coast of America. In the neighbouring town of Yahata, mobilised students and schoolchildren were churning out arms and munitions at factories operated by Mitsubishi Heavy Industries.
Nagasaki was an important port, with armaments and shipbuilding plants, but it was most famous as a city of culture: a hilly town of Buddhist temples, Shinto shrines and 19th-century houses overlooking a deep, beautiful harbour, the city of Madame Butterfly, the "Naples of the Orient". It had suffered little damage during the war.
In Kokura 2,000 people had been killed and 13,000 houses destroyed the day before, when B-29s rained conventional fire bombs on the Mitsubishi works. The fires were still burning and it was this attack, by another irony, which saved Kokura from total destruction.
The flight of Bock's Car, from the American-occupied island of Tinian, had been fraught with mishap from the start. The official photographer missed his flight after forgetting his parachute, and the aircraft lost its two escorts and circled for 45 minutes waiting for them. When the lone bomber finally made it to Kokura at 9.30am, the city was obscured by cloud and by smoke from the earlier fire bombs. After circling the city three times, with bomb doors open, the crew gave up and flew on to the secondary target, Nagasaki.
There was heavy cloud here too, and the bomb was released, not, as intended, over the shipyards and harbour, but over a residential area three kilometres north. Bock's Car did not make it back to Tinian, and virtually crash- landed on Okinawa, having run out of fuel.
Seventy-four thousand people died and 75,000 were injured, but if the bomb had fallen on its intended target, the damage might have been much worse. Masao Araki was working in a hospital near Nagasaki when the bomb exploded. Now he lives in Kokura, and he has published a book which reconstructs the effect the bomb would have had on the city.
"In Nagasaki, the river valley absorbed much of the blast," he says. "But Kokura is like Hiroshima, built on a flat plain." Kokura had a smaller population, but many more people would have been exposed. The arsenals would have exploded, the steel works would have melted, and 1,300 men would have been trapped down a coal mine.
In Nagasaki this morning, the Prime Minister and members of his Cabinet will give speeches to a crowd of thousands at a grand, solemn ceremony in the Peace Memorial Park. At the same time in Kokura, there will be a much smaller gathering in the city's little park.
"There are usually about 400 people there, nearly all of them elderly, and there are fewer and fewer of us every year," says Mr Nakagawa. "Plenty of young people don't know that Kokura was the target, but I go every year. Because, whatever anyone says I'm sure that it was fine and clear that morning. So I feel very grateful about that day."
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