People not involved in map-related sports or professions probably don't think about "north" much. I think most people have learned that "true north" and "magnetic north" are not the same, but I worded the title of this post the way I did to cover the fact that magnetic north is also not the same as itself over time. The earth's magnetic field varies somewhat randomly on a daily basis and also steadily over time. It still amuses me when I come across concrete manifestations of the unconstant nature of magnetic north, and I saw one in a NOTAM recently.
110345 CYVR VANCOUVER INTL
YVR- VANCOUVER VOR 115.9 ROTATION, ADD 4 DEG TO ALL PUB APPROACH RDL ASSOCIATED WITH YVR. SPECIFIC RDL ISSUED BY ATC SHALL BE ADHERED TO AS PER THE RECEIVED AND ACKNOWLEDGED CLEARANCE
YVR is a VOR -- a navigational radio station -- near the CYVR airport and it "tells" a pilot that her airplane is on a particular radial, i.e. that it is located a particular magnetic direction from the VOR ground station. She uses that information to navigate in the vicinity of the airport, perhaps to do an instrument approach that follows a specified radial straight to a runway, or to stay out of a restricted area that she has determined from the chart to be west of a particular radial. A number of radials are published on charts as named routes, for example on the expired chart I have here, the 037 radial from the YVR VOR was designated as part of the V304 airway to Calgary, sort of like Portage Avenue in Winnipeg forms part of the Trans-Canada Highway. A pilot tracking a radial makes heading corrections as needed to compensate for any crosswind, but she expects that in zero wind she will be flying on the same magnetic heading as the radial while tracking away from a station, and on the reciprocal heading while tracking towards a station. (To get to the station on the 000 radial, you fly 180, or south). A pilot who filed a flight plan from Vancouver to Calgary along V304 would expect to be flying on a heading of 037, and because the published minimum obstacle clearance along that radial is 9000', she would also expect that by keeping the 037 radial selected and centred on her VOR at 9000' that she wouldn't fly through any clouds with crunchy centres.
A VOR does not use the orientation of the earth's magnetic field to function, but it is set up and calibrated so that its zero radial runs due magnetic north of the station. That means that when the earth's magnetic field shifts, the YVR 037 radial still runs along a safe path between YVR and the next VOR on the airway. The pilot might just have to fly a different heading to stay on the airway. But after a number of years the accumulated magnetic shift is enough that Nav Canada wants to recalibrate the VOR such that the 037R really does run in the 037 direction. So they rotate the actual VOR signal. I'm pretty sure they just go in there and electronically adjust the direction in which the station sends its signals, but it's more fun to imagine that they jacked the whole thing up (they're usually about the size of a garage, except round) and cranked it around four degrees counterclockwise. But once they've done that, the 037 radial no longer runs through that safe route towards Calgary. It might pass a lot closer to some pointy rocks. So they have to amend all the publications so that what once specified the 037 radial now specifies the 041 radial, and so on all the way around. There are about fourteen airways defined off the YVR VOR. And they do amend all the publications that show those airways, and pilots or their companies are required to buy new ones every fifty-six days, but they didn't rotate the VOR on the same day as the new publications come out. This NOTAM informed me that until the new chart became effective, on May 5th, that I should add four degrees to any published radial from YVR. The new chart labels the 041 radial as V304, and the 039 radial subsides into unpublished obscurity.
There was a Nav Canada Challenger at YVR while I was there, probably in town to check the alignment of the airways and approaches.