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

"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