Going to Greenwich, England? You might have a magnetic experience that hasn't occurred there in about 360 years, the Guardian reports. Compasses in the London borough will finally point true north as Earth's magnetic field—which is changing all the time—lines up with what's geographically north. The Royal Observatory Greenwich was opened there in 1676 and hosted a special magnetic observatory from 1840 to 1926 to measure what's called declination, per the Mirror. That's the angle on a compass needle between magnetic north and true north, which varies depending on the moment and location. The "zero declination" line, known as the agonic, has been moving west at roughly 12 miles per year.
"At some point in September, the agonic will meet zero longitude at Greenwich," says geomagnetism scientist Ciaran Beggan. "This marks the first time since the observatory's creation that the geographic and geomagnetic coordinate systems have coincided at this location." Beggan adds that "the agonic will continue to pass across the UK over the next 15 to 20 years. By 2040, all compasses will probably point eastwards of true north." Yet no one can predict exactly how the magnetic field will change. It's been moving faster in recent years and even managed to throw off navigation systems worldwide, per Popular Mechanics. (Scientists had to update the World Magnetic Model sooner than expected.)