gravity and earth's position in space

Hi, I an very curious about something! Hope I can articulate my
question.
The question is relating to gravity and earth's position in space.
Imagine you are traveling towards earth. If you were to travel towards
the bottom part of the sphere, how does one's orientation change so
that gravity is still below? I am thinking it would not be such that
earth would be above you and therefore gravity above you. Or is that
the case? I guess I am wondering how an object's orientation stays
correct as it gets out of earth. I kinda understand this when on earth
but not sure how things work out if it. If two rockets were launched
from two countries, one located towards the top of the sphere and the
other country on the bottom, would "top" be pointing to the same
direction as far as these rockets are concerned, when they reach
space? OK, I am giving up, but hope what I am trying to ask is clear

In article ,
(Voyager) wrote:

The question is relating to gravity and earth's position in space.
Imagine you are traveling towards earth. If you were to travel towards
the bottom part of the sphere, how does one's orientation change so
that gravity is still below? I am thinking it would not be such that
earth would be above you and therefore gravity above you. Or is that
the case? I guess I am wondering how an object's orientation stays
correct as it gets out of earth. I kinda understand this when on earth
but not sure how things work out if it. If two rockets were launched
from two countries, one located towards the top of the sphere and the
other country on the bottom, would "top" be pointing to the same
direction as far as these rockets are concerned, when they reach
space? OK, I am giving up, but hope what I am trying to ask is clear

Hmm, I'm sorry, it didn't come through very clear to me. But let me
take a stab at an answer anyway.

You're talking about the "top" and "bottom" of the sphere (by which I
assume you mean Earth), and also mention a lot of "above" and "below".
I think this is the source of your confusion. With reference to the
Earth, "up" means "away from the center of the Earth" (and more
generally, it might mean "up the gravity gradient"). "Down" means
"towards the center of the Earth" (more generally, "down the gravity
gradient"). Two rockets launched from absolutely anywhere on Earth,
headed towards space, are both going "up" because they're both going
away from the center of the Earth.

But "top" and "bottom" of the Earth does not refer to up and down. It
refers to a particular convention to how we draw maps and make globes.
It has no basis in reality; we could just as easily have settled on a
convention where the north pole is drawn to the right and the south pole
to the left. (Though for a globe that would of course depend on where
you're standing!) The "top" and "bottom" (north and south) parts of the
globe have nothing to do with "up" and "down." I think, from your
question, that you were confused about this point.

Hope this helps,
- Joe

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| Joseph J. Strout Check out the Mac Web Directory: |
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In article ,
Joe Strout wrote:
But "top" and "bottom" of the Earth does not refer to up and down. It
refers to a particular convention to how we draw maps and make globes.
It has no basis in reality; we could just as easily have settled on a
convention where the north pole is drawn to the right and the south pole
to the left...

Indeed, if memory serves, there was a time when European maps normally had
east at the top, because the Garden Of Eden was thought to be in the east
somewhere.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

Hmm, sorry the question was not clear.. Let me try again. Imagine you
are on a space ship traveling towards earth. You would be looking at a
sphere, correct ?, as you approach the planet. If you were to pass by
earth, since you obviously cannot go thru it, you would be passing by
each side of the planet, under it or above it right? I mean there is
no other way. I realize the concept of side, top or bottom is our way
of things here on earth but still, I think even traveling thru space,
they would hold true in some way. I mean you pass by objects, or above
or below them. I would think! So if you are in the space craft and you
are approaching earth, how does your orientation stay correct no
matter what part of the sphere you proceed to and enter thru the
atmosphere? Hope this makes sense!

(Voyager) writes:

Hmm, sorry the question was not clear.. Let me try again. Imagine you
are on a space ship traveling towards earth. You would be looking at a
sphere, correct ?, as you approach the planet. If you were to pass by
earth, since you obviously cannot go thru it, you would be passing by
each side of the planet, under it or above it right? I mean there is
no other way. I realize the concept of side, top or bottom is our way
of things here on earth but still, I think even traveling thru space,
they would hold true in some way. I mean you pass by objects, or above
or below them. I would think!


Then you would be thinking wrongly. There is no "up" in space. There is no
"down" in space. There is no "above" or "below" in space. There is no
"to the left side of" or "to the right side of" in space. Any concept based
on or derived from an up/down dichotomy is totally meaningless in space,
since it is implicitly based on the assumption that you are sitting on a
planetary surface in a noticible gravitational field which you are not,
instead of in free-fall. The only conventional spatial relationship
that makes sense in space is "near by." The only spatial orientation
relationships that make sense are _RELATIVE_ orientations.


So if you are in the space craft and you are approaching earth, how does
your orientation stay correct no matter what part of the sphere you
proceed to and enter thru the atmosphere?

It does =NOT= happen "automatically." You need to make observations of your
approach trajectory and the _RELATIVE_ orientation of your spacecraft with
respect to the planet, and then fire your attitude-control rockets to
spin your spacecraft around until you have _FORCED_ the relative orientation
of your spacecraft with your approach vector to have the correct attitude
for atmospheric entry. If you do not _FORCE_ the spacecraft to have the right
orientation for atmospheric entry, your spacecraft will most likely burn up.


Hope this makes sense!

Sorry, you will need to totally abandon all your planet-centered notions of
spatial orientation such as "up" and "down" to understand what navigating
in space is like, first.


-- Gordon D. Pusch

perl -e '$_ = \n"; s/NO\.//; s/SPAM\.//; print;'

(Voyager) wrote in message om...
[snip]
I realize the concept of side, top or bottom is our way
of things here on earth but still, I think even traveling thru space,
they would hold true in some way. I mean you pass by objects, or above
or below them. I would think! So if you are in the space craft and you
are approaching earth, how does your orientation stay correct no
matter what part of the sphere you proceed to and enter thru the
atmosphere? Hope this makes sense!

I think I understand what you are getting at, barely.
The answer is that it *doesn't*. There is no official
up, down, left, or right in space, nor even with
respect to Earth (other than down being toward the
center and up away from it). You can enter through
the atmosphere upside down or on your side or
what have you. Orientation becomes your responsbility
in space. There is still gravity in space but because
of freefall there is no perception of it in orbit.
You can quite adequately define your local "up" to be
towards the Earth or sideways, or what have you. In
general, astronauts use local cues for orientation
and try to maintain a consistent orientation scheme
(since it's disorienting and disconcerting to have,
say, up, left, and forward be interchangable). But
this is only for convenience.


This is one of the huge faults in the vast majority
of space based science fiction movies and TV shows.
In reality there would be no reason why, for example,
the Enterprise and a Romulan War Bird would have the
same orientation when they approach one another.
Unless one of the ship's were reoriented to match with
the other then they might as well be upside down or
sideways when they meet. Every once in a while they'll
use this in scifi movies or tv shows, but almost always
only for dramatic effect to indicate some sort of
damage to a ship (in the same manner as, say, a boat
laying over on its side indicates it's in trouble).