Directions in space?

On 16 juuni, 10:22, "Mike Dworetsky"
wrote:
"Rebecca Rice" wrote in message

...



A lot of the science fiction I read assumes that the pilot of a spaceship
has carefully learned coordinates of where he wants to go, and inputs them
into the computer so that the ship winds up where it needs to be at the end
of the trip.

Now, I have at least some direction sense, so I surprise people by being
able to find the way back to the car in a strange area or a way around a
traffic slowdown essentially by feel. (Of course, that last part doesn't
always work. I may know that there is another road west of where I am
that I can use, but that doesn't mean that the new road I pick to get to
it actually intersects with it, and doesn't turn south before it gets
there.)

Can you do that in space? I'm not sure what the coordinates are keyed to,
and "north" is not easily determined in 360 degree space. Inside a solar
system you may be able to, since you have a lot of references, but if you
were dumped in deep space and told to get to Vega, could you just look
around and find enough referents to do so?

Rebecca

Depends on how deep "deep" is. Heinlein covered this in the early 50s novel
_Starman Jones_ when a starship got "lost" after a "jump" and none of the
star patterns matched anything familiar or expected at the destination.
Eventually the navigators resorted to spectroscopic classification of stars
in hopes of matching enough bright supergiants to sort out a coordinate grid
of identified stars. No luck, so they were at least a few hundred and
likely a few thousand light years from anywhere known.

Wrong approach. Precisely because individual stars are common and hard
to distinguish dots of light.

A more promising method would be to use galactic halo objects like globular
clusters, which are identifiable a long way off. Though most tend to look
similar to one another.

Yes, but they are sufficiently few.

Finding the Large Magellanic Cloud identifies the hemispheres of Milky
Way. Checking the rough direction to Milky Way centre compared to the
direction to Magellanic clouds (if you see one, you should find the
other) should give an idea of which part of the disc you are.

For several thousands of lightyears around the Sun, there are two
globular clusters which are visible and they are the brightest
globular clusters. Omega Centauri and 47 Tucanae. IIRC they are on
different sides of Milky Way - 47 Tucanae is the one which is on the
same side as Magellanic Clouds - so you could identify 47 Tucanae once
you see it. From Sun, 47 Tucanae is pretty close to Small Magellanic
Cloud, so you could confirm your rough location in respect to Sun just
by checking the parallax between SMC and 47 Tucanae. In any case,
check for nebulae which are far from Milky Way. They are either
nearground objects inside the disc (diffuse nebula? planetary nebula?
supernova remnant? open cluster?) or halo globular clusters which are
very few (as mentioned, for several thousands of ly, there should be
precisely those two)

If you are sure you are somewhere local, say within a few light years of the
sun, then a similar method of using spectra to confirm stellar identities
might work. If you are simply in the outer solar system and told to find
Vega then it isn't a problem. The sky looks much the same out there as it
does from Earth.

In outer solar system? Indistinguishable.

If you are within a few ly, the sky begins to become different. The
constellations consist both of intrinsically dim stars that happen to
be nearby (Toliman, Sirius, Altair...) and of remote, intrinsically
bright stars (Beta Centauri, Orion, Canopus, Deneb...). If you have a
3D star map, you know which are the remote stars (and will have almost
precisely the same brightness, with only slightly distorted pattern)
and which ones are the nearby ones (which will be wildly changed in
location and brightness). But since you know which are the far stars,
you can pick up their pattern and identify which are the intruder/
nearby stars. Then you can go on to figure where the intruders came
from.

E. g. Sirius, Procyon and Orion are near each other. A few ly shift
leaves Orion unchanged but Sirius and Procyon are moved.

By the way, if you are in a different galaxy, such as an irregular one, it
may prove difficult to identify a centre or directions.

Crown-Horned Snorkack wrote in news:9c5761da-dd3c-
:

If you are within a few ly, the sky begins to become different. The
constellations consist both of intrinsically dim stars that happen to
be nearby (Toliman, Sirius, Altair...)

Intrinsically dim???

Gene wrote:

Crown-Horned Snorkack wrote in news:9c5761da-dd3c-
:

If you are within a few ly, the sky begins to become different. The
constellations consist both of intrinsically dim stars that happen to
be nearby (Toliman, Sirius, Altair...)

Intrinsically dim???

He probably means in comparison to giants, supergiants, and hypergiants.
Not quite a fair comparison, since Alpha Centauri (G2 V + K1 V),
Sirius (A1 V), and Altair (A7 V) are all way above average in terms of
luminosity. The majority of stars are red dwarfs (M V), after all.

But his point is correct -- if you've been "teleported" somewhere
randomly in the Galaxy, even if it's only a few hundred or thousand
light-years from home, you're not going to be looking for the
above-average but once-nearby stars, you're going to be looking for the
powerhouses to try to zero in on where you are -- the gaints and
supergiants. Now you'd just better hope that you haven't been
teleported too far in the future so that the stars have moved ... or
evolved ...

--
Erik Max Francis && && http://www.alcyone.com/max/
San Jose, CA, USA && 37 18 N 121 57 W && AIM, Y!M erikmaxfrancis
Make it come down / Like molasses rain
-- Sandra St. Victor

Erik Max Francis wrote in
t:

But his point is correct -- if you've been "teleported" somewhere
randomly in the Galaxy, even if it's only a few hundred or thousand
light-years from home, you're not going to be looking for the
above-average but once-nearby stars, you're going to be looking for the
powerhouses to try to zero in on where you are -- the gaints and
supergiants.

Not me. I'm starting with the Magellanic Clouds.

On Tue, 17 Jun 2008 20:46:25 GMT, Gene wrote:

Crown-Horned Snorkack wrote in news:9c5761da-dd3c-
:

If you are within a few ly, the sky begins to become different. The
constellations consist both of intrinsically dim stars that happen to
be nearby (Toliman, Sirius, Altair...)

Intrinsically dim???

They return to that same dimness when you turn off the Bergenholm (duh)
--
I must not fear. Fear is the mind-killer.
Fear is the little-death that brings total obliteration.
I will face my fear. I will permit it to pass over me and through me.
And when it has gone past I will turn the inner eye to see its path.
Where the fear has gone there will be nothing. Only I will remain.
(Bene Gesserit)

William George Ferguson wrote:
On Tue, 17 Jun 2008 20:46:25 GMT, Gene wrote:
Crown-Horned Snorkack wrote in news:9c5761da-dd3c-
If you are within a few ly, the sky begins to become different. The
constellations consist both of intrinsically dim stars that happen to
be nearby (Toliman, Sirius, Altair...)

Intrinsically dim???

They return to that same dimness when you turn off the Bergenholm (duh)

ObSF: Arcot, Wade, & Morey having to figure out how to navigate inside the
pocket universe created when they turned on a new version of their drive,
with the green and violet stars...

Dave
--
\/David DeLaney posting from "It's not the pot that grows the flower
It's not the clock that slows the hour The definition's plain for anyone to see
Love is all it takes to make a family" - R&P. VISUALIZE HAPPYNET VRbeableBLINK
http://www.vic.com/~dbd/ - net.legends FAQ & Magic / I WUV you in all CAPS! --K.

Crown-Horned Snorkack wrote:
On 16 juuni, 10:22, "Mike Dworetsky"
wrote:
"Rebecca Rice" wrote in message

...

A lot of the science fiction I read assumes that the pilot of a spaceship
has carefully learned coordinates of where he wants to go, and inputs them
into the computer so that the ship winds up where it needs to be at the end
of the trip.

Can you do that in space? I'm not sure what the coordinates are keyed to,
and "north" is not easily determined in 360 degree space. Inside a solar
system you may be able to, since you have a lot of references, but if you
were dumped in deep space and told to get to Vega, could you just look
around and find enough referents to do so?

Depends on how deep "deep" is. Heinlein covered this in the early 50s novel
_Starman Jones_ when a starship got "lost" after a "jump" and none of the
star patterns matched anything familiar or expected at the destination.
Eventually the navigators resorted to spectroscopic classification of stars
in hopes of matching enough bright supergiants to sort out a coordinate grid
of identified stars. No luck, so they were at least a few hundred and
likely a few thousand light years from anywhere known.

Wrong approach. Precisely because individual stars are common and hard
to distinguish dots of light.

A more promising method would be to use galactic halo objects like globular
clusters, which are identifiable a long way off. Though most tend to look
similar to one another.

Yes, but they are sufficiently few.

Finding the Large Magellanic Cloud identifies the hemispheres of Milky
Way. Checking the rough direction to Milky Way centre compared to the
direction to Magellanic clouds (if you see one, you should find the
other) should give an idea of which part of the disc you are.

As well as the galactic centre and milky way (a bit woolly but still a
useful clue). You can use the local group bright galaxies like Andromeda
would still give you a pretty good approximation to fixed stars. And the
M81/M82 pairing is pretty distinctive too, also the Virgo cluster with
M87, and the M83 group with Centaurus A. These have the handy property
of being very bright radio sources too so easy to find.

Galaxies with a sharp central nucleus in the optical would be preferable
to avoid having too much slop in the coordinate system.

The way it was done on the Voyager probes was to encode the arrangement
of and frequency of pulsars as viewed from the Earth as an indicator of
our location. Whether or not another galactic civilisation would
recognise it as such and decode it correctly is another matter. More so
since they will spin down and some have significant proper motions.

For several thousands of lightyears around the Sun, there are two
globular clusters which are visible and they are the brightest
globular clusters. Omega Centauri and 47 Tucanae. IIRC they are on
different sides of Milky Way - 47 Tucanae is the one which is on the
same side as Magellanic Clouds - so you could identify 47 Tucanae once
you see it. From Sun, 47 Tucanae is pretty close to Small Magellanic
Cloud, so you could confirm your rough location in respect to Sun just
by checking the parallax between SMC and 47 Tucanae. In any case,
check for nebulae which are far from Milky Way. They are either
nearground objects inside the disc (diffuse nebula? planetary nebula?
supernova remnant? open cluster?) or halo globular clusters which are
very few (as mentioned, for several thousands of ly, there should be
precisely those two)

I think a nominally "fixed star" based navigation net is much easier to
deal with even if you have to look progressively deeper and with a
bigger scope to find one.

But if you knew the night sky you would get some idea of oerientation
and from that where you were in this galaxy with just a pair of
binoculars (and a terrestrial star atlas). Although a nice 3D model of
the stars and their brightness would be much better.

Incidentally it is quite funny just being in the wrong hemisphere of the
Earth - some very familiar constellations look quite different.
Orion doing a handstand for instance.

Regards,
Martin Brown
** Posted from http://www.teranews.com **

On 18 juuni, 00:37, Erik Max Francis wrote:
Gene wrote:
Crown-Horned Snorkack wrote in news:9c5761da-dd3c-
:

If you are within a few ly, the sky begins to become different. The
constellations consist both of intrinsically dim stars that happen to
be nearby (Toliman, Sirius, Altair...)

Intrinsically dim???

He probably means in comparison to giants, supergiants, and hypergiants.
Not quite a fair comparison, since Alpha Centauri (G2 V + K1 V),
Sirius (A1 V), and Altair (A7 V) are all way above average in terms of
luminosity. The majority of stars are red dwarfs (M V), after all.

Way below average.

For reference see:
http://www.cosmobrain.com/cosmobrain...rightstar.html

(Several such lists exist - but this one has the nice data on distance
uncertainty)

15 brightest stars, from Sirius to Spica, have magnitude less than
1,00.

The average star turns out to be Aldebaran, absolute magnitude -0,65.
7 of the 15 are dimmer and 7 of the 15 are brighter than Aldebaran.
Toliman is the dimmest, Altair third dimmest and Sirius fourth dimmest
out of those 15. (The second dimmest is Procyon, and fifth dimmest is
Vega.)

Going to dimmer stars, between magnitudes 1,00 and 1,50 there are
seven stars (from Pollux to Adhara) out of which 3 are dimmer than
Aldebaran and 4 are brighter. Between magnitudes 1,50 and 2,00 there
are 25 stars (from Gacrux to Hamal) out of which 8 are dimmer than
Aldebaran and 17 are brighter. So maybe Aldebaran is below average as
well. You can also note that the absolutely dimmest star in those 32,
Fomalhaut, is brighter than 3 out of the first 15. Absolutely
brightest star in the last 25, Castor, is brighter than Sirius.

But his point is correct -- if you've been "teleported" somewhere
randomly in the Galaxy, even if it's only a few hundred or thousand
light-years from home, you're not going to be looking for the
above-average but once-nearby stars, you're going to be looking for the
powerhouses to try to zero in on where you are -- the gaints and
supergiants. Now you'd just better hope that you haven't been
teleported too far in the future so that the stars have moved ... or
evolved ...


They are still dots of light. You can guess that most of the bright
dots are giants and supergiants, but you do not know which are which,
and which are not giants.

If you know your general location, a logical thing to look out for
might be open clusters. Taurean Pleiades is visible enough. And you
have also the Southern Pleiades of Carina to look out for.

Those two Pleiades should usually be possible to tell apart. Not
because of their brightness (this changes with distance - duh!) and
not because of their shape (for one thing it changes with direction,
and for another, you do not know how it may change because their 3D
structure is unknown), but the relative brightnesses of constituent
stars are independent of distance and direction. Theta Carinae
dominates Southern Pleiades more than Alcyone dominates Taurean
Pleiades.

And then there are the other nearby open clusters. Big Dipper, Hyades,
Hair of Berenice. No matter where you go, they are going to be
somewhere in your sky - and the way they distort could be informative.

"Martin Brown" wrote in message
...
Crown-Horned Snorkack wrote:
On 16 juuni, 10:22, "Mike Dworetsky"
wrote:
"Rebecca Rice" wrote in message

...

A lot of the science fiction I read assumes that the pilot of a
spaceship
has carefully learned coordinates of where he wants to go, and inputs
them
into the computer so that the ship winds up where it needs to be at the
end
of the trip.

Can you do that in space? I'm not sure what the coordinates are keyed
to,
and "north" is not easily determined in 360 degree space. Inside a
solar
system you may be able to, since you have a lot of references, but if
you
were dumped in deep space and told to get to Vega, could you just look
around and find enough referents to do so?

Depends on how deep "deep" is. Heinlein covered this in the early 50s
novel
_Starman Jones_ when a starship got "lost" after a "jump" and none of
the
star patterns matched anything familiar or expected at the destination.
Eventually the navigators resorted to spectroscopic classification of
stars
in hopes of matching enough bright supergiants to sort out a coordinate
grid
of identified stars. No luck, so they were at least a few hundred and
likely a few thousand light years from anywhere known.

Wrong approach. Precisely because individual stars are common and hard
to distinguish dots of light.

A more promising method would be to use galactic halo objects like
globular
clusters, which are identifiable a long way off. Though most tend to
look
similar to one another.

Yes, but they are sufficiently few.

Finding the Large Magellanic Cloud identifies the hemispheres of Milky
Way. Checking the rough direction to Milky Way centre compared to the
direction to Magellanic clouds (if you see one, you should find the
other) should give an idea of which part of the disc you are.

As well as the galactic centre and milky way (a bit woolly but still a
useful clue). You can use the local group bright galaxies like Andromeda
would still give you a pretty good approximation to fixed stars. And the
M81/M82 pairing is pretty distinctive too, also the Virgo cluster with
M87, and the M83 group with Centaurus A. These have the handy property of
being very bright radio sources too so easy to find.

Galaxies with a sharp central nucleus in the optical would be preferable
to avoid having too much slop in the coordinate system.

The way it was done on the Voyager probes was to encode the arrangement of
and frequency of pulsars as viewed from the Earth as an indicator of our
location. Whether or not another galactic civilisation would recognise it
as such and decode it correctly is another matter. More so since they will
spin down and some have significant proper motions.

As I recall Carl Sagan's explanation, the plaque was designed so that
numbers were in binary, with lengths in terms of the 21-cm wavelength of
hydrogen and times in terms of the oscillation period. The pulsars all had
periods of the order of 0.5-2.0 sec, which it was hoped would be recognised
as pulsar periods (written in binary with units of the H frequency).
Chances are that any space-faring civilization would be able to work this
out. And they would have the spacecraft itself with extra hints about
scale.

Variations in period would, hopefully, be used (along with evidence of
ablation of the spacecraft) to give a date for the launch, since it is
likely that discovery would be after millions of years.


For several thousands of lightyears around the Sun, there are two
globular clusters which are visible and they are the brightest
globular clusters. Omega Centauri and 47 Tucanae. IIRC they are on
different sides of Milky Way - 47 Tucanae is the one which is on the
same side as Magellanic Clouds - so you could identify 47 Tucanae once
you see it. From Sun, 47 Tucanae is pretty close to Small Magellanic
Cloud, so you could confirm your rough location in respect to Sun just
by checking the parallax between SMC and 47 Tucanae. In any case,
check for nebulae which are far from Milky Way. They are either
nearground objects inside the disc (diffuse nebula? planetary nebula?
supernova remnant? open cluster?) or halo globular clusters which are
very few (as mentioned, for several thousands of ly, there should be
precisely those two)

I think a nominally "fixed star" based navigation net is much easier to
deal with even if you have to look progressively deeper and with a bigger
scope to find one.

But if you knew the night sky you would get some idea of oerientation and
from that where you were in this galaxy with just a pair of binoculars
(and a terrestrial star atlas). Although a nice 3D model of the stars and
their brightness would be much better.

Incidentally it is quite funny just being in the wrong hemisphere of the
Earth - some very familiar constellations look quite different.
Orion doing a handstand for instance.

Regards,
Martin Brown
** Posted from http://www.teranews.com **


--
Mike Dworetsky

(Remove pants sp*mbl*ck to reply)

Crown-Horned Snorkack wrote:

On 18 juuni, 00:37, Erik Max Francis wrote:
Gene wrote:
Crown-Horned Snorkack wrote in news:9c5761da-dd3c-
:
If you are within a few ly, the sky begins to become different. The
constellations consist both of intrinsically dim stars that happen to
be nearby (Toliman, Sirius, Altair...)
Intrinsically dim???
He probably means in comparison to giants, supergiants, and hypergiants.
Not quite a fair comparison, since Alpha Centauri (G2 V + K1 V),
Sirius (A1 V), and Altair (A7 V) are all way above average in terms of
luminosity. The majority of stars are red dwarfs (M V), after all.

Way below average.

For reference see:
http://www.cosmobrain.com/cosmobrain...rightstar.html

(Several such lists exist - but this one has the nice data on distance
uncertainty)

15 brightest stars, from Sirius to Spica, have magnitude less than
1,00.

Assuming the list is correct, yes, I have no doubt that among the 1 or 5
or 10 or 20 or 50 or 100 visually brightest stars in our sky, the Sun's
_intrinsic luminosity_ isn't above average for the _intrinsic
luminosity_ of the other stars in those samples. But that's comparing
apples and oranges and is the statistical equivalent of cooking the
books. You're ranking things by visual magnitude, and then comparing
them by absolute magnitude. That doesn't make a stitch of sense, really.

The average star turns out to be Aldebaran, absolute magnitude -0,65.
7 of the 15 are dimmer and 7 of the 15 are brighter than Aldebaran.
Toliman is the dimmest, Altair third dimmest and Sirius fourth dimmest
out of those 15. (The second dimmest is Procyon, and fifth dimmest is
Vega.)

Going to dimmer stars, between magnitudes 1,00 and 1,50 there are
seven stars (from Pollux to Adhara) out of which 3 are dimmer than
Aldebaran and 4 are brighter. Between magnitudes 1,50 and 2,00 there
are 25 stars (from Gacrux to Hamal) out of which 8 are dimmer than
Aldebaran and 17 are brighter. So maybe Aldebaran is below average as
well. You can also note that the absolutely dimmest star in those 32,
Fomalhaut, is brighter than 3 out of the first 15. Absolutely
brightest star in the last 25, Castor, is brighter than Sirius.

And if you only consider the brightest star in the sky, then the Sun is
_exactly_ average, since it _is_ the brightest star in the sky. O can
bull**** with statistics, too.

But in terms of the number of _actual stars_ out there, the Sun is way
above average. Three-quarters of all stars are red dwarfs, and giants
and supergiants are very rare and exceptionally rare, respectively. We
see intrinsically bright stars overrepresented in our sky -- for reasons
which should be obvious -- but that doesn't mean they're common.

The problem here posed in this thread is finding yourself in a strange
part of space and trying to identify which stars are signposts to
getting yourself home. In that context, it is quite important to
consider that the Sun is _not_ a below-average star, otherwise you're
going to really confuse yourself. It's way above average, as I pointed
out. The signposts you're looking for -- if you try to take that route
-- will be way, way, _way_ above average.

But his point is correct -- if you've been "teleported" somewhere
randomly in the Galaxy, even if it's only a few hundred or thousand
light-years from home, you're not going to be looking for the
above-average but once-nearby stars, you're going to be looking for the
powerhouses to try to zero in on where you are -- the gaints and
supergiants. Now you'd just better hope that you haven't been
teleported too far in the future so that the stars have moved ... or
evolved ...

They are still dots of light. You can guess that most of the bright
dots are giants and supergiants, but you do not know which are which,
and which are not giants.

I guess you're not big on spectroscopy, then? Dwarfs, giants, and
supergiants were distinguished by spectroscopy before we ever understood
the physical mechanism involved. They are clearly differentiated if you
do your homework.

They don't look different from the other, more mundane, nearby stars to
the naked eye, but then, if you're only using your naked eye, you'll
_never_ find your way home, and that's pretty freakin' obvious.

If you know your general location, a logical thing to look out for
might be open clusters. Taurean Pleiades is visible enough. And you
have also the Southern Pleiades of Carina to look out for.

The latter isn't really its name, by the way.

--
Erik Max Francis && && http://www.alcyone.com/max/
San Jose, CA, USA && 37 18 N 121 57 W && AIM, Y!M erikmaxfrancis
Melancholy men, of all others, are the most witty.
-- Aristotle