OAKLAND — For nearly a century, pilots flying into Oakland knew three things for sure: Runways 27L, 27R and 29.

That ended in October, when the Oakland International Airport’s runway numbers changed for the first time. The reason is buried deep below the airport, inside Earth’s mysterious molten core.

“Amelia Earhart landed at Runway 27 when she flew from Hawaii to Oakland in the 1930s, but now it’s called Runway 28,” said Mel Woolcock, a retired Air Force pilot living in Hayward. “I don’t think anybody snuck in there at night and jacked up the runway. So, what happened?”

What happened is the Earth’s magnetic field, on which the Federal Aviation Administration bases all airport runway number assignments, has slowly changed its shape.

As a result, Oakland’s runway numbers “are changing to match Earth’s geological shift,” said Scott Wintner, an Oakland airport spokesman. Pilots use the numbers to navigate, so it’s critical that they be accurate.

Magnetic field measurements are tied to the Earth’s core, which naturally creates electrical currents that generate the magnetic field that is measured by a compass, said Jeffrey Love, a research geophysicist at the U.S. Geological Survey. The explanation may sound simple, but like a daredevil pilot, the Earth’s core and magnetic field can be quite unpredictable.

“This complicated dynamo that exists between the Earth is a subject of continued research,” Love said. “People are still trying to understand it.”

Studies show that the slowly changing shape of the magnetic field in Oakland is causing the compass arrow to move west at a rate of one-tenth of 1 degree every year, Love said. “The magnetic field is changing everywhere, all the time, and it’s doing something different in every part of the Earth,” he said. “In Oakland, it’s doing something different from in Denver, which is different from New York, where it is doing something different from what it’s doing in London.”

That unpredictable nature prompts scientists to continually study its movements and publish updated maps based on their findings. But using those findings to predict the field’s future behavior is nearly impossible, as it could change directions at any time, Love said.

The National Oceanic and Atmospheric Administration and the National Geodetic Survey study shifts in the “magnetic variation” and publish its changing values every five years, FAA spokesman Ian Gregor said.

“The FAA advises airports to change their runway markings when the magnetic heading shifts more than 5 degrees from the existing runway marking,” Gregor said.

That’s what happened last month in Oakland, officials said.

For 86 years, compasses measured the magnetic field near two Oakland runways around 90 degrees in one direction and about 270 degrees in another. So, those runways there were called 9R-27L and 9L-27R (“L” for left and “R” for right). When the area’s magnetic field recently was found to have shifted more than 5 degrees, they were renamed 10R-28L and 10L-28R to match the new measurement, Wintner said.

Likewise, Runway 11-29 is now known as Runway 12-30, reflecting the shift in its magnetic field measurements.

Similar changes are occurring at airports in Orange County and Tampa, Fla., officials said.

The FAA regularly produces aviation charts that includes changes, such as new runway numbers, and pilots are expected to check often for the latest updates, Wintner said. Outside of airline personnel, only aviation buffs would care to know about the new runway numbers, he said.

Woolcock, 89, said he noticed the runway changes while listening to air traffic communication on a multifrequency radio he bought 30 years ago. He was an Oakland toddler in 1927 when his hometown airport opened and quickly made a name for itself. Famed aviator Charles Lindbergh oversaw the dedication of the airport, which was barely 2 years old when Howard Hughes filmed daredevil airplane scenes near there while making his 1930 movie, “Hell’s Angels.” And Earhart put Oakland Airport in international headlines in the ’30s as she departed and arrived there for some of her historic flights.

Woolcock said he once rode his bicycle near the Alameda air base Earhart called home. He saw her in person in 1937 before she flew out of Oakland for the last time. Months later, Earhart was flying over the Pacific Ocean when, infamously, she disappeared.

If she somehow ever returns to Oakland, fans like Woolcock can only hope she finds the right runway.

Contact Chris De Benedetti at 510-353-7011. Follow him at Twitter.com/cdebenedetti.