It's all too easy to get carried away, sometimes.
The lay of the land is one thing. I don't think I can over-emphasise what a fantastic tool Google Maps is; being able to zoom in on a satellite map of Japan, right down to the level of individual buildings, is amazingly handy for getting geographic details right. (I'll have to revisit chapter 10 sometime, with its excursion to the countryside, to make sure I've got the details at least half-way credible.)
But, as I said, you can get carried away. For example: if one is out and about by night, on July 23rd, 4200, it's tempting to just throw in a reference to navigating by moonlight, to make things a little easier. But, dammit, will there be moonlight on that date?
Well, thanks to a rather nifty program called Starry Night—which, incidentally, I can recommend as a terrific program even just to play with; it's an astromonical simulator, capable of showing you the sky from any location in the solar system (or, I suspect, the galaxy, though I haven't tried that yet), at any moment in time you choose, and even of running in real-time or faster so that you can see things change—thanks to that, as I said, I now know that (a) the moon will be below the horizon in Tokyo by 5pm on that date; and (b) even it it wasn't it'll still be a new moon so there won't be any moonlight anyway.
If you're thinking that that's getting a little anal, you're not the only one.
On the other hand, it was kind of fun working it all out…