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Pluto is out from planet dictionary



 
 
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  #71  
Old September 11th 06, 11:42 AM posted to sci.astro.amateur,uk.sci.astronomy,alt.astronomy,sci.astro,alt.astronomy.solar
George Dishman[_1_]
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Default Pluto is out from planet dictionary

Margo Schulter wrote:

snip text

The one thing that might be added -- especially if
you or others can improve on my first quick attempt --
is a set of diagrams to show visually the levels of
definition we're discussing. Here's that crude first
try; note, by the way, that after the diagrams of
5A and 5B, there's a diagram for the kind of compromise
I've proposed, where the term "planet" actually appears
twice, once in a generic meaning (like the defeated 5B)
and once in a specific meaning (like the adopted 5A),
allowing a choice between the two usages.


Margo, having had a longer look myself, I think
your diagrams don't quite reflect the linguistics.
The following shows how I see the intended
outcome of 5B and illustrate how your version
is really the same other than the choice of name.
I have also indicated the priority of the 'satellite'
criterion.

[composed with monospaced font]

-----------------------
Resolution 5A (Adopted)
-----------------------


Solar system bodies
|
---------------------------------------
| |
Is not a Satellite
satellite of another
| body [other
---------------------------------- than Sun]
| | |
Hydrostatic equilibrium, Not enough mass for |
self-gravity produces hydrostatic equilibrium; |
"nearly round" shape; and not a satellite of another |
not a satellite of another body [other than Sun] |
body [other than Sun] | |
| | |
-------------- | |
| | | |
Clears Doesn't | |
orbit clear orbit | |
| | | |
| | | |
"Planet" "Dwarf Planet" "Small Solar System Body" "Satellite"
(e.g. most Solar System (e.g. Moon,
asteroids, TNOs, comets) Titan)

TNO = "Trans-Neptunian Object"


------------------------
Resolution 5B (defeated)
------------------------

Solar system bodies
|
---------------------------------------
| |
Is not a Satellite
satellite of another
| body [other
---------------------------------- than Sun]
| | |
Hydrostatic equilibrium, Not enough mass for |
self-gravity produces hydrostatic equilibrium; |
"nearly round" shape; and not a satellite of another |
not a satellite of another body [other than Sun] |
body [other than Sun] | |
| | |
"Planet" | |
| | |
-------------- | |
| | | |
Clears Doesn't | |
orbit clear orbit | |
| | | |
"Classical" "Dwarf" "Small Solar System Body" "Satellite"
(e.g. most Solar System (e.g. Moon,
asteroids, TNOs, comets) Titan)



--------------------------------
Possible compromise proposal
for future consideration
--------------------------------

Solar system bodies
|
---------------------------------------
| |
Is not a Satellite
satellite of another
| body [other
---------------------------------- than Sun]
| | |
Hydrostatic equilibrium, Not enough mass for |
self-gravity produces hydrostatic equilibrium; |
"nearly round" shape; and not a satellite of another |
not a satellite of another body [other than Sun] |
body [other than Sun] | |
| | |
"Planet" | |
| | |
-------------- | |
| | | |
Clears Doesn't | |
orbit clear orbit | |
| | | |
"Major" "Dwarf" "Small Solar System Body" "Satellite"
(e.g. most Solar System (e.g. Moon,
asteroids, TNOs, comets) Titan)


George

  #72  
Old September 11th 06, 02:15 PM posted to sci.astro.amateur,uk.sci.astronomy,alt.astronomy,sci.astro,alt.astronomy.solar
George Dishman[_1_]
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Default Pluto is out from planet dictionary


Margo Schulter wrote:
In sci.astro George Dishman wrote:

Margo Schulter wrote:


more trimmed

OK, here's my alternative. Consider first Ceres, Pallas
and the other largest main belt obects. If say the top
tem had merged and were collecting the rubble then
they would approach being classed as a planet albeit
of very low mass. We already have a name for objects
which subsequently merge to form planets, that being
"planetesimal". As a result of our discussion, I would
suggest that Ceres etc. should be classed as remnant
planetesimals.


That's certainly one usage of "planetesimal" with
precedent. Personally the term tends to suggest for me
more specifically something maybe around 1-10 km that's
assumed to be one of the bodies serving by accretion to
form some kind of larger planet -- in my terms, it could
be a major planet (the "Big 8" in our Solar System); or
a minor planet (dwarf or smaller). However, I certainly
agree that there's a usage where any kind of minor planet
(notably an asteroid) can be called a planetesimal.


Having looked into it a little more, I think I agree with
your criticism. From Wiki, the term "protoplanet"
would be more fitting, though the "proto" prefix is
perhaps not optimum.

Here we get to the question of set or subset definitions
versus the names to give. To me, "planetesimal" could be
both evocative and "cosmogonically correct" for lots of
the smaller minor planets -- "here we're seeing a living
fossil, as it were, of those 1-10 km planetesimals that
were the elementary building blocks of the larger planets,
major or dwarf, etc." It suggests to me something smaller
than a "gravito-spheroidal" planet, or "spheroidal" for
short, which has likely accreted from lots of planetesimals
in the narrower sense -- whether a dwarf planet, a major
planet, or even a larger Small Solar System Body (SSSB)
or "microplanet" as I call it, say Vesta, which isn't
quite massive enough to be (gravito-)spheroidal.

However, your usage seems to have lots of company, and I
realize that from a certain dynamical view, anything other
than a major planet (IAU "planet") could be viewed as
"uncleared rubble."


"Planetesimal" has the advantage of being clearly a
single word. "Protoplanet" could be written "proto-planet"
implying a subtype the definition of planet again leaving
the ambiguity unresolved. That is unfortunate as the term
is more accurate.

If I wanted to propose some term other than "dwarf planet"
or "mesoplanet" for the spheroidal minor planets, maybe
it would be "planetoid" (carrying some science fiction
associations, as has been pointed out) or possibly
"planetino." Then people who wanted could view this
term as referring to a type of planet, and others could
argue the analogy that a neutrino is certainly not a type
of neutron.


On that basis, they could also argue that an asteroid is
a type of star ;-)

Again, it's a matter of taste -- and it seems that
"planetesimal," like "planet," can evoke lots of distinct
semantic preferences.


I doubt there can be any existing term that doesn't
and really this whole exercise is 'damage limitation'.

If Ceres and Pallas at some time came close and
became a binary, that doesn't change their individual
nature so I would further suggest they should then be
classed as a binary planetesimal system.


An interesting question, indeed! If the barycenter (my
provincial spelling, just to let everyone know I'm aware)
is outside the radius of either body, then a binary
planetesimal system -- or in my lingo "binary dwarf planet
or mesoplanetary system" -- would indeed seem correct.


My concerns with the barycentre argument are first
that it sets a size limit that depends on the ratio of
separation to diameter. A simple mass ratio limit,
while arbitrary, could make the distinction more
consistent. Secondly, it requires some knowledge
of the density (to convert mass to radius) which
will be a problem for extra-solar objects where size
will not be directly measurable other than under
fortuitous circumstances (e.g. transit detections).

If the barycenter is within the radius of one of the bodies,
then we have the "not a satellite" question -- does this
apply to satellites of minor as well as major planets?


Ah, well spotted, I had missed that. It's going to
mess up my flowcharts :-( Yes, the test needs to
apply at all levels so you can get a binary asteroid
just as we already have a few asteroids with satellites.
I probably need to include "not a member of a binary"
as well as "not a satellite".

If I were to propose a distinction, maybe based in part on
precedent (catalogued minor planets, it seems to me, should
stay minor planets, although they might also be satellites of
another minor planet), I might argue that maybe for one
minor planet to be a satellite of another in a belt environment
is a bit more subtle of a relationship than the contrast
between major planet and satellite. However, there's a problem
there, too: with minor planet (and more specifically mesoplanet
or dwarf planet) Pluto, Nixie, for example, is in a role much
like that of a satellite of a major planet.

Do we maybe use a mass ratio test, with Ceres-Pallas a "binary"
but Pluto-Nixie a planet plus satellite?


For the reasons given above, I prefer the mass ratio
test but whichever we use I have no doubt that Nix
and Hydra should be seen as satellites (or moons?).

The question then is whether there are two satellites
of the Pluto/Charon binary system or are there three
moons of Pluto?

If the barycenter is between the two bodies, of course, then I'd
say "binary" is the right answer. This is a subtle line of
questions.


I think mass ratio versus barycentre is one question
and moon versus satellite is another.

snip - agreed

Similarly the distinction between a satellite and a
moon is unclear but let me suggest as a minimum
that a planetesimal in orbit around a planet should be
called a moon. The terms satellite and moon relate
to orbital configuration rather than mass and shape
so it would be both a moon and a planetesimal.


Yes, a "moon" would then be what I recall that Stern/Levison
(2002) call a "planetary-mass satellite" or the like, or what
I might call a "spheroidal satellite."


That is consistent since we now have a definition
of "planetary mass" that is based on "nearly round".
It also mirrors my own view that a "moon" should be
a substantially larger object than the generic term
satellite.

By the way, I'd guess that the major/minor distinction might
not apply for a satellite, since it's "circumstantial" as
Basri would put it -- unless someone wants to estimate
whether the satellite, if a planet in a comparable orbit,
_would_ have sufficient mass to "clear its neighborhood"
(if the major planet it is orbiting weren't there!). That
would get into "what-if" cosmogonic scenarios, I guess.


I think the question is "not applicable", an object too
small to achieve a round shape is almost certainly
incapable of clearing its orbit.

I'm developing a new typology which does address extrasolar
objects, as do your new ones, so maybe I'll have more coherent
views to present soon -- _relatively_ more coherent, anyway
grin.


Excellent, I look forward to seing it.

The criteria used by the IAU would still be applicable,
hence there would be eight planets in the solar system.
The definition could be easily rationalised to allow for
extra-solar planets by replacing "the Sun" by "a star",
even with the proviso that the current definition is
limited to the solar system since AFAIK there is only
one star in it ;-)


True, unless we want to get into the "Nemesis" hypothesis of
the 1980's (as I recall) when the whole mass extinction
connection with asteroid or comet impacts (especially
Cretaceous-Tertiary at around 65 Ma or "Mega-anni ago,"
to use a geological style) led to the hypothesis of an
"invisible companion" to the Sun which every 26 My ("million
years" as duration or interval rather than distance from
present) or so was diverting Oort Cloud objects or the like
toward the inner Solar System, including Earth.

That theory wasn't found persuasive, I guess -- the "Nemesis"
part, as opposed to the impact theory of the Cretaceous-Tertiary
or "K-T" mass extinction, which now seems generally accepted as
at least one main cause of the extinction (with the discovery of
an impact crater that fits the geological timing and the scale
of the hypothesized event).


I believe there was another paper in the last year or so,
it hasn't entirely gone away yet.

I will try to find time to draw this up as a flowchart, but
I have very limited opportunity over the next week, and
perhaps also add a test for fusion to identify stars
(including brown dwarf stars), free-floating 'planemos'
and binary planemo systems.


Those are neat charts!


I am preparing a page of notes to explain the
flowchart but it's not in a state where I can upload
yet. For reference for anyone following previous
posts, my flowchart is here (but needs some work
to incorporate the changes discussed above):

http://www.georgedishman.f2s.com/astronomy/GAD.png

I would appreciate your views on the this proposal, in
particular the criteria for distinguishing binary from
object/satellite and what should qualify a satellite to
be raised to the status of a moon.


For a major planet and a satellite, or for two minor planets
where the barycenter is between the two bodies, we might both
be comfortable with the usual tests, although tending toward
different names for some of the categories. With two minor
planets like Pluto-Nixie, object/satellite seems fine. The
harder situation might be when we have two minor planets
not too different in size with the orbit of one within the
radius of the other. Maybe we say, "Once a minor planet,
always a minor planet," and put a code like the letter "S"
for "satellite" after its minor planet number.

Thus it seems that we both find mass ratio relevant if the
barycenter test doesn't indicate a binary system.

Note that this would mean that many of the moons of
the planets would also be classified as planetesimals
so the precedence of being a satellite and other
definitions might be contentious.


How about planetisimo-satellite or the like? I would say
"spheroid satellite," or "gravitospheroid satellite" if
we want to make it explicit that the "near-roundness"
must result from self-gravity approximatinng hydrostatic
equilibrium.


I think we need to keep the names simple and understandable
for the wider public appreciation while the criterion should be
explicit, unabiguous, scientific and measurable. A press
release saying "New gravitospheroidal satellites discovered!"
just doesn't have the impact of "New moon discovered!"

The more I consider this, the more I lean toward a convention
where minor planets systems with the barycenter within the radius
of one of the bodies get classified as "planet/satellite," but
the satellite still keeps or gets its minor planet number, with
a code like "S," as I described above.


I would say "protoplanet/moon" if both are "nearly round"
or "protoplanet/satellite" if the smaller is a rocky peanut
or "asteroid/satellite" if both are irregular.

Existing small body numbering should be undisturbed by
any of this, though perhaps a few new numbers could be
allocated for consistency.

George

  #73  
Old September 11th 06, 10:34 PM posted to sci.astro.amateur,uk.sci.astronomy,alt.astronomy,sci.astro,alt.astronomy.solar
George Dishman[_1_]
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Default Pluto is out from planet dictionary


"Jeff Root" wrote in message
oups.com...

George Dishman replied to Jeff Root

George Dishman replied to Margo Schulter:


My point on that is that (d) is redundant because an
object cannot be a satellite and also meet criterion
(a) that it be in orbit around the Sun.


All known satellites are in orbit around the Sun.
There is nothing redundant about (d).


You see no difference in the gravitational binding
of Ceres and Moon, both are orbiting the Sun? The
sentence "The Moon orbits the Earth and the Earth-
Moon system orbits the Sun while Ceres orbits the
Sun." appears to recognise a significant difference
regarding the hierarchy of gravitational binding IMO.


Yes, there is a significant difference, but Ceres, the
Earth, the Moon, and the Earth-Moon system all orbit the
Sun.


I think that depends on your understanding of
orbit and perhaps you are raising a significant
point. The IAU needs to define "satellite" to
clear this up.

Saying that a body is in orbit around the Sun does
not determine whether it is a satellite or not. On the
other hand, saying that a body orbits another body which
in turn orbits the Sun means that the first body orbits
the Sun.


Well by the same understanding that says the
Moon orbits the Sun (i.e. its path encompasses
the Sun), Ceres orbits the Earth and Pluto
orbits almost everything! I don't think that is
a helpful definition of the term. Try putting
"define: orbit" into Google and let me know what
you think (you can ignore references to chewing
gum!)

best regards
George


  #74  
Old September 12th 06, 06:35 AM posted to sci.astro.amateur,uk.sci.astronomy,alt.astronomy,sci.astro,alt.astronomy.solar
Cardman
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Default Pluto is out from planet dictionary

On 11 Sep 2006 00:47:18 GMT, Margo Schulter
wrote:

In sci.astro Cardman wrote:
On 05 Sep 2006 21:22:22 GMT, Margo Schulter
wrote:
I should point out though that the IAU have already rejected Dwarf
Planets being classed within a collective planet group, which means
that this idea you will find the hardest to gain general support for.


Hi, and I'd tend to suspect also that the issue was likely not just the
term "Classical" but the "collective planet group" concept. Thus, indeed,
people who supported 5A (and rejected 5B) might not find "major" vs.
"classical" as a distinction making a difference in their votes.


The use of the word "Classical" was clearly unhelpful in 5B, but if
the removal of this word would have changed support for 5B remains to
be seen. I would have assumed that these IAU members would have well
considered the implications for rejecting 5B with or without this
word.

My honest view would be that this was a meeting to kick out Pluto and
leave everything else exactly the same.

More and more, I'd favor "planet" as an even wider umbrella covering
everything customarily called a "major planet" or "minor planet," and
subdivide from there, with the IAU concepts readily applied. I'm
developing some formal definitions and more informal presentations
on how such a scheme might be applied.


I see the major fault in the IAU definition of "planet" is in their
concept that a Dwarf Planet is not a planet. This is not just due to
the contradiction in terms, but that I am sure that the vast majority
of people would have been happy with Pluto being a Dwarf Planet had it
still been a planet.

The mistake that a lot of astronomers are making is in saying that
Pluto should not be a planet when this 2306km diameter object cannot
compare to the likes of Mars or Earth. The mistake in this logic is
that Mars and Earth hardly compare to Jupiter and Saturn either. So if
you are running a singular planet definition then only Jupiter,
Saturn, Uranus and Neptune would be planets, where all the smaller
objects would be given a different suitable name.

And so once you entertain the concept that the "planet" word is
currently defined to include two very different classes of objects can
you see how this could be extended to three classes. And there should
be no comparison between members of these three sub-groups when they
are after all vastly different.

The second excuse I hear is that they would be upset if there was like
47 planets in our solar system. This I find odd when I was under the
assumption that astronomers were scientists. So the number will be the
number there are and not the number that you want there to be.

This usually trails on into what you would teach school children. That
should be obvious enough, when you teach them about the 4 inner
planets, the 4 gas giants, and then the teacher points out how many
more dwarf planets there are with picking out a few choice examples.

My Very Educated Mother Just Served Us Nachos, and a whole load more!

It is now a sad day to see that our solar system, in the eyes of
school children, has not got 1/9th smaller. I am sure that our solar
system must go nearly half way to the next star and it seems a
worthwhile classroom exorcise for the teacher to point out how BIG
that actually is followed by "and this little section is what we have
explored to date".

The next problem I see with this IAU planet definition is that it is
very warm planet biased, meaning that it only covers the area of our
solar system that was subject to a collapse in the gas cloud. The
evidence for that is obviously in the nice circular motions of the
inner 8 planets. So I am in fact doubtful that Earth has cleared
anything much when this "clearing" seems more a product of our local
star and our closest gas giant neighbour. The trophy in how badly our
planet clears local objects can be clearly seen in that planet-sized
sphere above our heads.

Also I suspect that it is true to say that had even Earth been located
out at about 150 AU, or better yet 120 to 180 AU, then even Earth
would have been unlikely to have cleared the region. The reason for
this is that you no longer have your easy record shaped disk to vacuum
up, when now you work in the 3D. So has anyone actually calculated the
odds of two objects going head to head out in this vastness? And so
this act of clearing could simply be an on-going event due to simply
there not being enough time since our solar system began.

So I am quite looking forwards to them finding something nice and big
out there and seeing them try to label this a "dwarf".

I would in fact say that the whole clearing concept is flawed and
should be removed as such. Or at minimum this is clearly an attribute
of the gas giants, and maybe smaller planets, and this attribute
should be contained to the definition of these two groups.

I can see why they want to strip Pluto of planethood in the belief
that these objects are the left-overs from the creation of the solar
system. I even read a lot of astronomy books saying exactly that, but
time did not stop and there are planets formed out there as well.

So one side of these astronomers needs to accept that the likes of
Pluto and "Xena" are indeed planets, then the other half need to
accept that these are dwarf planets are not as big/important as the
larger planet classes.

I think that is where you will find your agreement.

Curiously, while "planet" for everything larger than a meteroid and
smaller than a brown dwarf might be a bit radical, it could also
in effect "neutralize" or at least play down the differences between
people who have proposed usages where "planet" is defined so as to
imply that either "orbit-clearing" or "hydrostatic equilibrium" is
_the_ proper criterion for "planethood."


Astronomers are mostly split into two groups. Both groups seem rather
biased and hot-headed in defending their position. You can add a third
group into this containing the majority who wisely avoid the entire
debate.

The 300 plus astronomers who next year will hold their own meeting to
redefine "planet", in the way that they are happy with, is a clear
indication that both side desire to remain in complete opposition.

This seems fair proof that the third group was right after all.

If everything traditionally called a "major planet" or "minor planet"
qualifies for the generic sense of planet, then the arguments about
what _kind_ of planet Ceres or Pluto or 2003 UB313 is might become a bit
calmer and less charged.


True. Neither side will easily give ground though.

Also having real planets out there waiting to be found will create
increased interest in the exploration of this region. The IAU already
know that they have the public interest, so why not milk it for all
its worth?

That public can help get them increased budget to explore this region
after all. And we could do with a few more fly-by probes, or better
yet ones that can survive a high speed impact.

Thus "major planet" would be equivalent to "planet" in 5A, and "minor
planet" to what it means now, an object in a belt population ranging
from a 100m asteroid to a spheroid minor planet like Ceres or Pluto,
etc.


The IAU desires anything 800km or above to be ranked for
dwarf-planethood. Anything less is simply a solar system object.

If a minor planet is spheroid (by self-gravitational forces), it
would also be a "dwarf planet" -- in effect, a definition identical
to 5A, but under the umbrella of "planet" along with smaller minor
planets and the major ones too.


The "minor planets" term has been killed off for being obsolete and
has now been replaced with "solar system objects".

The idea of "planet = major planet or minor planet" occurred to me in
the course of a discussion with a planetary scientist who brought this
generic definition to my attention -- easy to document in the OED and
some astronomical reference books also.


That sounds exactly like the old system, which explains why the minor
planet center keeps track of all these lesser objects. They obviously
have had to do some recent renaming as well.

You are overlooking that being round has importance over non-round
objects in the sense that the two should be independent groups.

Also while I am about it I consider all three diagrams flawed when
there is no size split in the satellites section, even though the
satellite grouping is fine. What I mean is that I always hate it when
some astronomer goes that they found another moon around a gas giant
and it turns out to be a oddly shaped pebble.


Maybe this usage is so widespread that it would be hard to change,
especially if we take "moon" as a more informal term -- but there have
been proposals to say things like "a planetary-scale satellite" to indicate
that it approximates a spherical shape brought about by hydrostatic
equilibrium.


Yes, I prefer a three level system, but if the IAU wants to go as low
as 800km, and possibly lower, then I won't argue with them.

Or, as I'd say, a "spheroid satellite" or "gravitospheroid satellite" if we
want to make it explicit that the near-roundness is gravitationally
constrained (I'm not sure what the probability is of finding a
"near-round" satellite of insufficient mass for self-gravity to be the
constraining factor).


You should check out 2003 EL61. This is a dwarf planet sized object
that got deformed by its rapid rotation.

So maybe that is just my personal gripe. Still, once you have finished
playing "What is a planet?" then you can start on "What is a moon?".
As if they don't stop their current method then soon enough your next
moon of Saturn will be the size of a football.


Yes, this satellite question is mentioned in some of the recent literature
on defining a planet, but could well deserve a focus in its own right,
both on its own merits and for a creative change of pace.


I can only hope.

Cardman
http://www.cardman.org
http://www.cardman.com
http://www.cardman.co.uk
  #75  
Old September 12th 06, 10:05 PM posted to sci.astro.amateur,uk.sci.astronomy,alt.astronomy,sci.astro,alt.astronomy.solar
Eric Chomko
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Posts: 2,630
Default Pluto is out from planet dictionary


Margo Schulter wrote:
In sci.astro Paul Schlyter wrote:
In article . com,
Jeff Root wrote:

http://www.bautforum.com/showthread.php?t=34542

The questions I asked:

Were the Sun and Moon commonly referred to as "planets" in
ancient times?

When was Earth first called a "planet", or described as a body
comparable to a planet, even if it wasn't thought to wander?

When were the stars first recognized as being like the Sun?


Probably around Copernicus' time. One prevailing argument against
a heliocentric solar system was that the Earth's orbital motion
around the Sun ought to cause a quite visible yearly parallax
among the stars --- unless of course the stars were extremely
distant and not just a little farther away than Saturn, as was
commonly believed in that time.


"Show us the stellar parallax" was also one of the lines of
rebuttal to Galileo -- since if one followed the view expressed
by St. Roberto Bellarmino (1615) that the traditional geocentric
interpretation of certain Bible passages should be altered only
on the basis of indubitable proof, the "missing" parallax could
still be a reason for doubt. As I recall, the Church gradually
grew more and more reconciled to Galileo's perspective as the
18th century progressed, but didn't make it fully "official"
until the early 19th century, when parallax was confirmed for
one or more of the nearest stars to our Sun.

When the stars did turn out to be that vastly distant, they
must also be very bright --- like the Sun.

Herschel was one of the first trying to find the actual distance
to some stars, and he thought Sirius was some 3 light years away.


Interesting! Maybe that's about the same degree of accuracy as
Roemer's estimate of the speed of light (1676) using the moons of
Jupiter. It was an awesome leap -- I'm trying to find a worthy
adjective -- from the state of things earlier in the century,
when Galileo had tried the experiment of having two people a few
miles away show lamps to each other and try to estimate any delay
in seeing them at that distance.


I found two speeds for Roemer's expeirment depending on the distance of
an AU. Initially the time for light to travel an AU was thought to be
11 minutes, which is 11/8 or 1.375 of its actual time. Which is clearly
beyond 8.6 LYs.

However, by using Roemer's estimate of an AU (See:
http://en.wikipedia.org/wiki/Ole_R%C3%B8mer), the speed is 135,000 km
per second compared to the roughly 300,000 km per second it is. So,
Sirius at 8.6 LY, we get 135/300 = .45 * 8.6 = 3.87 LY, so Roemer and
Herschel (using 3 LY) were not that far off.

Eric

  #76  
Old September 13th 06, 08:33 PM posted to sci.astro.amateur,uk.sci.astronomy,alt.astronomy,sci.astro,alt.astronomy.solar
Jeff Root
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Posts: 242
Default Pluto is out from planet dictionary


George Dishman replied to Jeff Root:

All known satellites are in orbit around the Sun.
There is nothing redundant about (d).

You see no difference in the gravitational binding
of Ceres and Moon, both are orbiting the Sun? The
sentence "The Moon orbits the Earth and the Earth-
Moon system orbits the Sun while Ceres orbits the
Sun." appears to recognise a significant difference
regarding the hierarchy of gravitational binding IMO.


Yes, there is a significant difference, but Ceres, the
Earth, the Moon, and the Earth-Moon system all orbit the
Sun.

I think that depends on your understanding of
orbit and perhaps you are raising a significant
point. The IAU needs to define "satellite" to
clear this up.


No it doesn't. We don't need definitions. We can talk
about these things just as well without definitions as
with. Descriptions usually work better than definitions.

Saying that a body is in orbit around the Sun does
not determine whether it is a satellite or not. On the
other hand, saying that a body orbits another body which
in turn orbits the Sun means that the first body orbits
the Sun.


Well by the same understanding that says the Moon
orbits the Sun (i.e. its path encompasses the Sun),
Ceres orbits the Earth and Pluto orbits almost
everything! I don't think that is a helpful definition
of the term. Try putting "define: orbit" into Google
and let me know what you think


I haven't done that yet, but just a couple of hours
before I saw this reply from you yesterday morning, I
wrote something on the same subject in another forum,
trying to explain to someone that "revolving" and
"orbiting" are different things in astronomy:

Orbiting is in general a synonym for "revolving", but in
astronomy it means being in a trajectory which is primarily
determined by a single gravitational source. It is possible
for a body to be in several different orbits simultaneously,
with those orbits determined by different gravity sources.
An Apollo spacecraft orbited the Moon; the spacecraft and the
Moon orbit the Earth; the spacecraft, the Moon, and the Earth
orbit the Sun; the spacecraft, the Moon, the Earth, and the
Sun orbit the center of mass of the Milky Way galaxy.

I didn't put a lot of thought into that paragraph before
posting it on the other forum, because I wasn't attempting
to *define* the term "orbit", but it seems pretty good.

The "primary" of the Apollo spacecraft as it orbited the
Moon was the Moon. The Moon was the body at the center of
mass of the Moon-Apollo system. Likewise, Earth is the
primary of the Earth-Moon system, and the primary of the
Earth-Moon-Apollo system. The Sun is the primary of the
Sun-Earth system, the Sun-Earth-Moon system, and the Sun-
Earth-Moon-Apollo system. And the Milky Way galaxy as a
whole is the primary of the Galaxy-Sun system, the Galaxy-
Sun-Earth system, the Galaxy-Sun-Earth-Moon system, and
the Galaxy-Sun-Earth-Moon-Apollo system.

You can say that Ceres orbits the entire inner Solar
System, which comprises the Sun, Mercury, Venus, Earth,
Mars, and lots of asteroids. That isn't much different
from saying that it orbits the Sun. The Sun, being the
most massive body in the system, and the body closest to
the center of mass of the system, is the primary body of
the system. The Earth is not the primary body of any
system that Ceres belongs to.

-- Jeff, in Minneapolis

  #77  
Old September 13th 06, 10:56 PM posted to sci.astro.amateur,uk.sci.astronomy,alt.astronomy,sci.astro,alt.astronomy.solar
George Dishman[_1_]
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Posts: 2,509
Default Pluto is out from planet dictionary


"Jeff Root" wrote in message
oups.com...

George Dishman replied to Jeff Root:

All known satellites are in orbit around the Sun.
There is nothing redundant about (d).

You see no difference in the gravitational binding
of Ceres and Moon, both are orbiting the Sun? The
sentence "The Moon orbits the Earth and the Earth-
Moon system orbits the Sun while Ceres orbits the
Sun." appears to recognise a significant difference
regarding the hierarchy of gravitational binding IMO.

Yes, there is a significant difference, but Ceres, the
Earth, the Moon, and the Earth-Moon system all orbit the
Sun.

I think that depends on your understanding of
orbit and perhaps you are raising a significant
point. The IAU needs to define "satellite" to
clear this up.


No it doesn't. We don't need definitions. We can talk
about these things just as well without definitions as
with. Descriptions usually work better than definitions.


OK, let's see how we get on.

Saying that a body is in orbit around the Sun does
not determine whether it is a satellite or not. On the
other hand, saying that a body orbits another body which
in turn orbits the Sun means that the first body orbits
the Sun.


Well by the same understanding that says the Moon
orbits the Sun (i.e. its path encompasses the Sun),
Ceres orbits the Earth and Pluto orbits almost
everything! I don't think that is a helpful definition
of the term. Try putting "define: orbit" into Google
and let me know what you think


I haven't done that yet,


It would help.

but just a couple of hours
before I saw this reply from you yesterday morning, I
wrote something on the same subject in another forum,
trying to explain to someone that "revolving" and
"orbiting" are different things in astronomy:


I think "revolving around" and "orbiting".

Orbiting is in general a synonym for "revolving", but in
astronomy it means being in a trajectory which is primarily
determined by a single gravitational source. It is possible
for a body to be in several different orbits simultaneously,
with those orbits determined by different gravity sources.


However, those are generally split into the primary
influence you describe below and perturbations of
that orbit. An exception might be the phrase "chaotic
orbit" which is almost self-contradictory but in a
sense that merely emphasises the normal meaning.

An Apollo spacecraft orbited the Moon;


Yes, the craft was revolving around the Moon.

the spacecraft and the
Moon orbit the Earth;


The craft/Moon system as a whole revolved around
the Earth but I think it would be inaccurate or
at least misleading to say the craft was revolving
around the Earth while the LEM was on the surface.

the spacecraft, the Moon, and the Earth
orbit the Sun; the spacecraft, the Moon, the Earth, and the
Sun orbit the center of mass of the Milky Way galaxy.

I didn't put a lot of thought into that paragraph before
posting it on the other forum, because I wasn't attempting
to *define* the term "orbit", but it seems pretty good.


I don't think it helps. If I draw the locus of the
Moon over a year, the path encompases the Sun,
however the Moon moves in a nearly Keplerian orbit
around the Earth with Solar gravity producing only
a perturbation of that orbit. On the other hand
Ceres orbits the Sun with the Earth/Moon system
being one perturbing influence.

The "primary" of the Apollo spacecraft as it orbited the
Moon was the Moon.


And to me that is precisely what it means to say
that the Moon orbits the Earth, not the Sun.

The only exception I can think of to that rule
would be possibly where a planet orbits a binary
system at much larger radius than the separation
of the binary components.

The Moon was the body at the center of
mass of the Moon-Apollo system. Likewise, Earth is the
primary of the Earth-Moon system, and the primary of the
Earth-Moon-Apollo system.


But it is not the primary of the Earth-Apollo
system since over a single orbit the Earth is not
enclosed by the path of the craft.

The Sun is the primary of the
Sun-Earth system, the Sun-Earth-Moon system, and the Sun-
Earth-Moon-Apollo system. And the Milky Way galaxy as a
whole is the primary of the Galaxy-Sun system, the Galaxy-
Sun-Earth system, the Galaxy-Sun-Earth-Moon system, and
the Galaxy-Sun-Earth-Moon-Apollo system.

You can say that Ceres orbits the entire inner Solar
System, which comprises the Sun, Mercury, Venus, Earth,
Mars, and lots of asteroids.


You can if you define 'orbit' as meaning that the
path of Ceres encloses that paths of those bodies
but not if you define it as indicating which is
the primary gravitational influence as I do.

That isn't much different
from saying that it orbits the Sun. The Sun, being the
most massive body in the system, and the body closest to
the center of mass of the system, is the primary body of
the system. The Earth is not the primary body of any
system that Ceres belongs to.


No, but Earth is the primary influence on the path
of the Moon, not the Sun, hence "The Moon orbits the
Earth, not the Sun." is an accurate statement by my
definiton of "orbit". The reason I mentioned Google
is that I think my version better mirrors common
usage based on the numerous dictionaries that the
"define: " prefix searches.

If I were to get technical I would suggest something
along the lines of saying that the volume swept by
the satellite is bounded by a surface on which the
satellite would have zero kinetic energy relative
to the primary. Simply put, the Moon cannot get
too far from the Earth even though, if it were to
be displaced nearer to Venus and in solar orbit,
it might have the same total energy, because there
is a peak of gravitational potential separating the
volumes. That's difficult to explain but do you see
what I am trying to convey?

George


 




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