The last time Greenwich, England, had zero declination, King Charles II was on the throne. But in a couple of weeks, true north and magnetic north will align at the prime meridian for the first time in 300 years.
For the next few weeks, compass-using Londoners won’t have to pull out those pesky magnetic declination charts, because true and magnetic north will be exactly the same. If you didn’t know already, there’s more than one north: magnetic north is determined by our planet’s magnetic field lines, and true north refers to a geographic line along a meridian.
We all know the Earth is constantly moving, but so is its magnetic field. “The line of zero declination, called the agonic, is moving westward at a present rate of around 20 km [about 12.5 miles] per year,” said the British Geological Survey (BGS).
The Significance of Greenwich
This year, that westward movement is closing the gap between true and magnetic north just over Greenwich, England. This is noteworthy because Greenwich is home of the Royal Greenwich Observatory, and, more importantly, the point of zero degrees longitude. Also known as the prime meridian, it’s the birthplace of universal time.
“The observatory pav[ed] the way for a global reference system for maps and navigation that we know today as the Greenwich Meridian and with it, Greenwich Mean Time (GMT),” said the BGS. Basically everyone, from 17th-century astronomers to modern-day outdoor recreators, uses the Greenwich Meridian. It’s the reference point for all other longitudinal meridians, many charts and maps, and world time zones. And for the next 2 weeks, compass reading just got easier.
Don’t worry, the geographic and geomagnetic alignment won’t have any wonky effects on compasses, world satellites, or, more importantly, your phone’s GPS. And sadly, the world — and compass needles — keep turning, so this convenient moment of synchronicity won’t last long.