Olson figured out that the marines' assault had taken place on one of two days that year when the Moon was farthest from Earth and also in its quarter phase, creating an unusually low tide. "When people have asked me who was to blame, I can tell them with confidence that the answer is no one," says Olson.
World War II war correspondent Robert Sherrod, who had been at the battle, presented Olson's conclusions at a reunion of Tarawa survivors and told Olson that the marines were gratified that there was a scientific explanation. And a military historian included the findings in his account of the battle.
While researching Tarawa, Olson came across another amphibious-landing mystery. In Julius Caesar's account of his invasion of Britain in 55 B.C., the general wrote that after seeing the enemy atop high cliffs (of Dover), he ordered his fleet to put ashore several miles away. But the exact location of where it landed has been debated by historians for centuries: historical descriptions seem to match a spot to the northeast, but currents that day would have taken the ships southwest.
Since coming across the puzzle 20 years ago, Olson has been collecting information about it, including a firsthand account of the battle by a Roman soldier. In 2007, Olson even sailed to the site himself to test conditions. That's when he figured out that Caesar had to have put in at a point to the northeast, near the town of Deal, as historical descriptions suggest. The date given in Caesar's history—or in the transcribed copies that remain—had to be wrong, and the landing must have taken place on August 22 or 23, not the 26th, as Caesar has it.
Olson published his Caesarean findings—one of the most recent of about 50 puzzles he has solved—in the August 2008 issue of Sky & Telescope magazine, whose readers (professional and amateur astronomers), he expects, check his calculations. (So far, he's not been found in error.)
Olson also investigated the sinking of the USS Indianapolis at the end of World War II. Two torpedoes from a Japanese submarine struck the heavy cruiser around midnight on July 30, 1945. The resulting explosion killed 300 sailors, and the ship sank in the Philippine Sea, casting 900 men into shark-infested waters. Only 317 survived to be rescued four days later. The Navy charged the ship's captain, Charles B. McVay, with negligence for failing to evade enemy fire. He was court-martialed and demoted, despite protests from men under his command that he was following standard procedures and testimony from the Japanese sub commander that McVay could not have escaped the attack. McVay committed suicide in 1968. He was exonerated in 2000 by a Congressional resolution that declared "the American people should now recognize Captain McVay's lack of culpability for the tragic loss of the USS Indianapolis and the lives of the men who died."
The case piqued Olson's interest. After reading survivors' accounts, researching weather conditions and analyzing astronomical data at the time of the attack, Olson concluded that the submarine had surfaced just when the Indianapolis was in the glittering path of the Moon's reflection, allowing the Japanese to see it silhouetted from ten miles away but obscuring the submarine from the Americans' view. And once it was spotted, "the ship was doomed," he says.
In addition to puzzles in history, literature and art, Olson also likes photographic puzzles, such as the one he posed to students in an Astronomy in Art History and Literature course he taught at Texas State: When did Ansel Adams capture his celebrated Yosemite National Park image Moon and Half Dome? Although Adams kept careful technical records—noting shutter speeds, f-stops, lenses and film—he rarely dated his negatives, to the frustration of art historians. In the case of Moon and Half Dome, Adams noted only that he had taken it in 1960.
After a field trip to view Yosemite's cliffs and using clues from the photograph—the amount of snow on the ground, the phase of the Moon and the depth of the shadows on the granite dome—Olson and his students concluded that the photograph had been taken at 4:14 p.m. on December 28, 1960. And since they also determined that the Moon and Sun would be in nearly identical places at 4:05 p.m. on December 13, 1994, dozens of Adams fans and even a couple of the photographer's relatives, including his daughter-in-law and grandson, went to the park on that day to shoot their own versions of the iconic photograph.
Olson and another group of students took on Adams' Autumn Moon, a panorama of Yosemite Valley that had been dated in various books to either 1944 or 1948. A series of photographs of the valley taken by a park ranger in 2004 helped them pinpoint where Adams likely took the picture, while weather records and the angle of the Moon helped narrow down the day. Shadows in a color photograph of the scene that Adams took two and a half minutes before he made the black-and-white exposure (based on the position of the Moon) gave clues to the Sun's location and the time of the shot. Olson determined that it had been taken at 7:03 p.m. on September 15, 1948.