The slowly drifting location of Earth's magnetic north pole means the Hillsboro Airport's main runway underwent a name change early this week - from 12/30 to 13/31.

The new numbers were painted at one end of the runway early this week as part of general maintenance, said a spokeswoman for the Port of Portland, which owns and operates the airport.

Pilots and clients of the airport were mailed notices to be on the lookout for the runway work, but the airport was never shut down, as was rumored, said spokeswoman Kama Simonds.

Runways are designated according to the points on a compass, and the changing location of true magnetic north meant the runway sometimes is renamed.

Experts say magnetic north is slipping slowly from above the Arctic Ocean in a north-northwestern direction toward Siberia, at about 34.2 miles per year. When the magnetic points were recalibrated, Hillsboro, like some other airports, had to renumber its main runway.

"Over time, every 20 years or so, the magnetic headings change, and the runways have to be renamed," Simonds said

Every five years, federal agencies tabulate magnetic variation, which varies by location. It correlates true direction to the magnetic compass readings needed for navigation.

While modern navigation is moving toward more satellite-based systems, most U.S. aviation still relies on ground radar - calibrated to the local magnetic variation used in navigational aids such as instrument landing systems and beacons, she said.

The adjustment is the result of a natural, ongoing process. The Earth has an iron core, and movement within its outer part is believed to be responsible for sustaining the magnetic field used to measure the Earth's surface.

This makes the Earth like a giant magnet, but with the location of its north and south poles always shifting around. As a result, true magnetic north, on which the workings of a compass needle are focused, doesn't always match up with the permanently drawn lines on the map. Thus, the location of the airport in relation to true north has to be adjusted.

Scientists believe the activity of the Earth's core could cause massive shifts in polarity, including the north and south poles reversing themselves.

It's believed this last happened 780,000 years ago, and took about 10,000 years to happen.

Pilots and clients at the airport were warned to be cautious Sunday and Monday as the new numbers were painted, in addition to the paving and milling and at the end of the landing strip.

But the timing, on the 10th anniversary of the 9/11 World Trade Center attacks, was mere coincidence, Simonds said.

"It just so happened to be scheduled for the second week of September, and the weather was still nice," Simonds said.