Francois Hugo asked:
How is north determined for planets that don't have a magnetic field? And would there serious cartographical stress if the earthís polarity flips around?
Andrew - North would generally be determined, I would say, in terms of the rotation of a planet. So when we talk about North here on Earth, what we really mean is that you can look at the way that the Earthís rotating. There are two fixed points. There are two points on the Earth which never actually move at all and thatís what defines where the poles are on earth. Now which oneís North and which oneís South is going to be determined by say, you look from the top and you see it going around clockwise, then that tells you which one youíd call North and which one you'd call South. So thatís going to apply to any planet regardless of whether thereís a magnetic field. Now of course here on Earth, we do have a magnetic field. Itís actually not perfectly aligned with the rotation of the planet and as I think you're hinting, sometimes it even flips around. So the naming of the North and South poles in terms of magnetic fields of course, does come about historically because it was roughly lined up with the way that the Earth rotated, but itís not a direct physical link, if you like.
Chris - And this is slightly more speculative on his part, ďWould there serious cartographical stress if the earthís polarity flips around?Ē I guess not really, would it?
Andrew - Well, not for cartographers exactly, but it would have really serious consequences. I'm by no means an expert, but from my geology courses way back when, I remember that you can find certainly records of these flips and they correlate very well with mass extinctions. One of the reasons people think this might be is that the Earthís magnetic field protects us from an awful lot of nasty particles streaming through space. At the point at which it reverses, [the magnetic field] probably shrinks down to more or less nothing, and then all these particles can bombard the organisms living on the earth and...
Chris - This is solar radiation. Itís that solar wind, isnít it?
Andrew - Itís the solar wind and other cosmic rays in fact from other sources, and clearly causes major problems for Earth, so we really don't want that to happen and we don't know how much warning weíd get either if it was about to happen.
Chris - Historically, looking at the records written geologically in rocks, itís every 100,000 years or so. But it hasnít happened for 800,000 years, leading people to suggest that weíre overdue. There is an anomaly over the Southern Ocean or the southern Atlantic isnít there, and the satellite ironically was put up to study the earthís magnetic field and it ended up floating across this anomaly, and got bombarded and it actually got messed up by the very solar radiation, that weíre worried about basting the earth, so yes, a point well made. Thank you, Andrew.