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Message: <blockquote data-quote="gban007" data-source="post: 9812711" data-attributes="member: 56488">In my limited understanding,  it feels you are missing Umbrans point.  Everything is relative, so there is no absolute point of reference.  Thus you can choose any frame of reference you want and work from there.So you could say that the landing point on the Moon or Mars is point zero / the centre of frame of reference and thus unmoving.  But then relative to the moon, the earth, sun etc are moving, so you still need to map out a path to get there noting that the earth and space shuttle will be moving relative to the moon so can't go in a straight line.Or can take Earth as fixed point, and then have to deal with the moon moving relative to the earth, but would still end up with same path required.Or use Sun as point of reference, and still need to work out path working out how relative movements work, and end up with same path required.Ultimately I think for simplicity sake, wr often take centre of galaxy as point of reference.  Other galaxies are moving further away or in some cases closer, the sun is orbiting centre of galaxy, we are orbiting sun etc.  That all allows newtonian physics to work well and things go from there.We could take Earth as centre, but in that case the Sun doesn't orbit earth, but instead sketches a strange pattern in relation to an unmoving Earth.  Thus we take a frame of reference that makes sense of orbits easier.  But by relativity, we dont have to, we just need to be aware that just choosing Earth as centre, doesn't make the universe orbit Earth, just all our describing of the movements of bodies not on Earth would be described relative to Earth.Your example earlier of moving 100 feet above Earth is an interesting one on this basis.  Is that relative to Earth, in which case stay 100 feet above a rotating Earth.  Or is it relative to the Sun, in which case the Earth may rapidly disappear and return in 365 days.  Or is it relative to the centre of the Galaxy, in which case the whole solar system may disappear to return eons later.So if setting a time travel device, and Paris is its centre of revenue, it doesn't need to take into account Paris is moving a lot relative to every thing else, as it isnt moving relative to itself.It does though that challeng for time travel, is ensuring the device is enforcing a frame of reference as such, and maybe time travel hasn't worked well so far as they arent getting whatever msths or science required right, and so they do appear in open space.</blockquote>

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