After ruling out what didn't happen, MacGregor confronted the question of what might have.
Abandoning a ship in the open sea is the last thing a captain would order and a sailor would do. But is that what Captain Briggs ordered? If so, why?
His ship was seaworthy. "It wasn't flooded or horribly damaged," says Phil Richardson, a physical oceanographer at the Woods Hole Oceanographic Institution in Massachusetts and an expert in derelict vessels, whom MacGregor enlisted in her investigation. "The discovery crew sailed it, so it was in really good shape."
Briggs' life before the Mary Celeste offered no clues, says MacGregor, who visited the captain's hometown of Marion, Massachusetts, and interviewed descendants of Arthur Briggs, the 7-year-old son the Briggses had left behind so he could attend school. MacGregor learned that the captain was experienced and respected in shipping circles. "There was never a question that he would do something irrational," she says.
Did Briggs, then, have a rational reason to abandon ship? MacGregor figured that if she could determine the precise spot from which Briggs, his family and crew abandoned ship, she might be able to shed light on why. She knew from the transcriptions of the Mary Celeste's log slate—where notations were made before they were transcribed into the log—that the ship was six miles from, and within sight of, the Azores island of Santa Maria on November 25; she knew from the testimony of the Dei Gratia crew that ten days later, the ship was some 400 miles east of the island. MacGregor asked Richardson "to work backward and create a path between these two points."
Richardson said he would need water temperatures, wind speeds and wind directions at the time, data that MacGregor found in the International Comprehensive Ocean-Atmosphere Data Set (ICOADS), a database that stores global marine information from 1784 to 2007 and is used to study climate change. She, her yachtsman husband, Scott, and Richardson drew on the data to determine whether the Mary Celeste could have drifted from its recorded location on November 25 to where the Dei Gratia crew reported finding it on December 5. Their conclusion: yes, it could have, even without a crew to sail it. "We found out it basically just sailed itself," Richardson says.
At that point, MacGregor considered the fact that a captain would most likely order a ship abandoned within sight of land. Since Santa Maria was the last land for hundreds of miles, it seemed safe to assume that the Mary Celeste had been abandoned the morning of November 25, after the last log entry was written.
On this point, MacGregor says, Attorney General Solly-Flood's notes are crucial. He wrote that he saw nothing unusual about the voyage until the last five days, which is why he transcribed the ship's log starting five days from the end. The ship's log is believed to have been lost in 1885, so those transcriptions provided the only means for MacGregor and Richardson to plot the course and positions logged for the ship. The two then reconsidered those positions in light of ICOADS data and other information on sea conditions at the time. Their conclusion: Briggs was actually 120 miles west of where he thought he was, probably because of an inaccurate chronometer. By the captain's calculations, he should have sighted land three days earlier than he did.
Solly-Flood's notes yielded one other piece of information that MacGregor and Richardson consider significant: the day before he reached the Azores, Briggs changed course and headed north of Santa Maria Island, perhaps seeking haven.