| MadSci Network: Astronomy |
hi Ed,
Good questions - I have often wondered the answer myself.
To answer the first part, nautical miles are used by convention in maritime travel and aviation. In other words, only for manned spaceflight (where the participants are aviators and in many cases, navy pilots). A nautical mile is defined and discussed at this Wikipedia link. It's a minute of an arc (1/60 of a degree) on the surface of the earth and so is a useful unit at sea (in the absence of landmarks) and when circumnavigating the earth above the surface. Unmanned space probes (at least in the United States) use statute miles (which I guess is the same as international miles - that's not a term that I've heard or that anyone outside of the US would use). Indeed, it's only US engineers that use miles in preference to kilometers and mixing up so-called "British" and SI units had disastrous consequences for one space probe.
As for the distance between bodies, I think this relates to the law of universal gravitation and in particular the equation:
F = G * m1 * m2 / r2where r is the distance between the 2 bodies and is always measured from centre to centre. Also the height of the surface varies - on top of Mount Everest you are almost 9 km closer to the Moon than at sea level, so a centre-centre measurement is more consistent. And finally, again in terms of human spaceflight, it would be less usual to talk about surface-surface distance and more usual to describe the height above the surface on arrival. thanks for the great questions,
Try the links in the MadSci Library for more information on Astronomy.