Living on an airport I can tell you that this is nothing new; runway designations change over time because the magnetic poles slowly drift on the Earth's surface and the magnetic bearing will change. Depending on the airport location and how much drift takes place, it may be necessary over time to change the runway designation. As runways are designated with headings rounded to the nearest 10 degrees, this will affect some runways more than others. For example, if the magnetic heading of a runway is 233 degrees, it would be designated Runway 23. If the magnetic heading changed downwards by 5 degrees to 228, the Runway would still be Runway 23. If on the other hand the original magnetic heading was 226 (Runway 23), and the heading decreased by only 2 degrees to 224, the runway should become Runway 22. Because the drift itself is quite slow, runway designation changes are uncommon, and not welcomed, as they require an accompanying change in aeronautical charts and descriptive documents. When runway designations do change, especially at major airports, it is often changed overnight as taxiway signs need to be changed and the huge numbers at each end of the runway need to be repainted to the new runway designators. In July 2009 for example, London Stansted Airport in the United Kingdom changed its runway designations from 05/23 to 04/22 overnight.The magnetic north "pole" is always moving. MN is a result of the earth's core spinning. It spins somewhat independantly from the outer "shell" much like spinning a glass of water. The water never quite catches up so if you tilt the glass while spinning it the water takes a second to react and also tilt. Tilt the glass back the other way and the water once again has to play "catch up." Since the earth is constantly tilting back and forth the magnetic core is also playing catch up. It the earth didn't tilt over the seasons the magnetic core would eventually get caught up and be aligned with true north. That's why magnetic north changes over time.
Incidentally, topographic maps should have a date beside the listing for magnetic declination. If it's over a couple of years old get the correct number from the website below.
Here's a website to go to for up-to-date info on magnetic declination. NOAA's Geophysical Data Center - Geomagnetic Data
Put in your location and let it tell you what your magnetic declination is.
Alright, I'm a little confused now. My compass and their map differ by about 10°
According to the website your magnetic declination should be approximately 9 degrees, 11 minutes west of true north. (I used Pittsburg, PA for the zip code.) That means the compass needle is off by almost ten degrees from true north. (Ours is almost 15 degrees. ) In other words magnetic north is actually located west of true north when measured from PA.Alright, I'm a little confused now. My compass and their map differ by about 10°
I thought the earth always had the same tilt. And when it moves around the sun the hemisperes get more or less sunlight. Because the northern hemisphere is angled towards or away from the sun depending on where it is in its orbit.Since the earth is constantly tilting back and forth the magnetic core is also playing catch up. Put in your location and let it tell you what your magnetic declination is.
Good point Lex: I kind of set up the problem here by oversimplifying my explanation. The earth's tilt does change somewhat over time albeit, very slowly. See Axial tilt - Wikipedia, the free encyclopedia. It's a phenomenon the serious stargazers (astronomers) have to take into account when doing their calculations. I'm also working off knowledge gained from a book that's now almost 40 years old so there may be new or revised theories now.I thought the earth always had the same tilt. And when it moves around the sun the hemisperes get more or less sunlight. Because the northern hemisphere is angled towards or away from the sun depending on where it is in its orbit.