I knew where Japan and Korea were in relation to each other with reasonably accuracy, but it turns out that the bit of coast that runs north from North Korea is not China after all, it's Russia.
Which makes Vladivostock about 15 degrees further south than I thought it was, roughly level with Marseille when I had thought it was level with Stockholm. And also the northern tip of Japan is only 50km from a Russian island.
My Mum is going on a cruise to East Russia (flying into Seoul, then flying up to somewhere that I can't remember the name of and cruising down the east coast of Sakhalin island and then down the west coast of Japan and flying out of Sapporo) which is why I was looking at the map in the first place.
For some reason I had always though that Vladivostock was freezing and nearly arctic and that the border with China was much further north than it actually is.