Re: OT: Celestial maps
|From:||Mark J. Reed <markjreed@...>|
|Date:||Tuesday, January 15, 2008, 15:33|
On Jan 15, 2008 9:58 AM, Benct Philip Jonsson <bpj@...> wrote:
> On 15.1.2008 Herman Miller wrote:
> > Going back millions of years, even many of these
> > relatively distant stars will have moved quite a bit
> So closer objects appear to move more quickly than distant
Yes, assuming they're not moving directly toward or away from you.
The principle is called "parallax" and is the principal way we know
how far away a given star is.
> Of course even down here on Earth a car which is
> further down the road appears to move slower than one nearby
> even if their actual speed is much the same.
Well, yes. What we see as linear motion across our line of vision is
actually angular motion in an arc centered on our point of view. When
real-world linear motion is translated into angular motion, the radius
can overwhelm the arc, so objects that are far away appear to follow
us (the moon, clouds), while objects that are closer to us zip quickly
past our eyes - even though we're moving at the same speed relative to
all of them.
> It would be cool if there was a program where you could
> view the celestial map from any choosen coordinate in
> the galaxy...
There are such programs. Our map of the galaxy is incomplete, but
there are programs that will let you zip around and see those stars
for which we have accurate positional data.
> I've heard that the Moon used to be much closer to Earth,
> and that its time of orbit was different, but I guess that
> was *very* long ago.
Depends on what you mean by "different". The Moon is receding from
the Earth at a rate of about 38mm/year. So, for instance, back at the
start of the Pleistocene it was about 40,000 miles closer to us than
it is now.
> Tolkien had some misgivings that he hadn't bothered to find
> out how the lunar cycles and constellations were ten to
> twelve thousand years ago, but given the liberties he took
> with geography it may be excused.
Ten to twelve thousand years is not a very significant amount of time
for such things, anyway. The cycles in question would have been
measurably different, but not greatly so.
Mark J. Reed <markjreed@...>