What kind of self-respecting planet has a moon 1/2 its diameter?

NONE!

http://www.theweathernetwork.com/new...-so-far/54248/

On Thursday, July 16, 2015 at 1:35:35 AM UTC-4, RichA wrote:
NONE!

http://www.theweathernetwork.com/new...-so-far/54248/

The Moon is ~1/3.7 the diameter of the Earth. Pot meet kettle.

On Thursday, 16 July 2015 08:20:23 UTC-4, wrote:
On Thursday, July 16, 2015 at 1:35:35 AM UTC-4, RichA wrote:
NONE!

http://www.theweathernetwork.com/new...-so-far/54248/

The Moon is ~1/3.7 the diameter of the Earth. Pot meet kettle.

Er, yeah. 3.7 = 2

On Thursday, July 16, 2015 at 6:08:28 PM UTC-4, RichA wrote:
On Thursday, 16 July 2015 08:20:23 UTC-4, wrote:
On Thursday, July 16, 2015 at 1:35:35 AM UTC-4, RichA wrote:
NONE!

http://www.theweathernetwork.com/new...-so-far/54248/

The Moon is ~1/3.7 the diameter of the Earth. Pot meet kettle.

Er, yeah. 3.7 = 2

Where would you like to draw the line?

On Thu, 16 Jul 2015 17:31:34 -0700 (PDT), wrote:

Where would you like to draw the line?

In the professional world, this is a matter of ongoing discussion,
given that the definition of "planet" is still evolving.

Charon is not in orbit around Pluto, but around the Sun. The two
bodies have a barycenter outside of both. By some provisional
definitions, and certainly by some definitions in informal use, both
are planets, and as such would be considered a binary planetary
system.

This is similar to the Earth-Moon system. The Moon isn't in orbit
around the Earth, and it is spherical. So it could be considered to
have planet status. But as long as the Earth-Moon barycenter remains
inside the Earth, the Moon is likely to remain categorized as a moon.
In a few billion years, the barycenter will be above Earth's surface,
and this system might be treated as a binary planet. Not that there's
likely to be any intelligent species around to make that judgment.

On Thursday, July 16, 2015 at 9:07:08 PM UTC-4, Chris L Peterson wrote:
On Thu, 16 Jul 2015 17:31:34 -0700 (PDT), wsnell01 wrote:

Where would you like to draw the line?

In the professional world, this is a matter of ongoing discussion,
given that the definition of "planet" is still evolving.

We were discussing the relative sizes of planets and their satellites, not the definition of "planet."

Try to stay on topic.

On Thu, 16 Jul 2015 19:07:05 -0600, Chris L Peterson
wrote this crap:

On Thu, 16 Jul 2015 17:31:34 -0700 (PDT), wrote:

Where would you like to draw the line?

In the professional world, this is a matter of ongoing discussion,
given that the definition of "planet" is still evolving.

Charon is not in orbit around Pluto, but around the Sun. The two
bodies have a barycenter outside of both. By some provisional
definitions, and certainly by some definitions in informal use, both
are planets, and as such would be considered a binary planetary
system.

This is similar to the Earth-Moon system. The Moon isn't in orbit
around the Earth, and it is spherical. So it could be considered to
have planet status. But as long as the Earth-Moon barycenter remains
inside the Earth, the Moon is likely to remain categorized as a moon.
In a few billion years, the barycenter will be above Earth's surface,
and this system might be treated as a binary planet. Not that there's
likely to be any intelligent species around to make that judgment.

Not from you.


This signature is now the ultimate
power in the universe

On Thursday, July 16, 2015 at 7:07:08 PM UTC-6, Chris L Peterson wrote:

Charon is not in orbit around Pluto, but around the Sun.

The Moon isn't in orbit
around the Earth, and it is spherical.

Charon, Pluto, the Earth, and the Moon are indeed all spherical.

The Earth-Moon system orbits the Sun, and the Charon-Pluto system orbits the
Sun as well. To determine the position of the Moon (and this applies to Charon
as well, in respect of Pluto) it is simplest to treat the Moon as a body
orbiting the Earth, with the effects of the Sun's gravity left over, after
those which are neglected because they result in the Earth's orbit around the
Sun, which is ignored, as a perturbation to the Moon's orbit.

So it still makes sense to treat the Moon as orbiting the Earth, and Charon as
orbiting Pluto. That the Moon's path around the Sun doesn't have loops in it,
but is convex, if lumpy, seems to not be really important, even if people can
be startled by statements to the effect that the Moon "really" orbits the Sun
based on it.

John Savard

On Fri, 17 Jul 2015 07:45:49 -0700 (PDT), Quadibloc
wrote:

The Earth-Moon system orbits the Sun, and the Charon-Pluto system orbits the
Sun as well. To determine the position of the Moon (and this applies to Charon
as well, in respect of Pluto) it is simplest to treat the Moon as a body
orbiting the Earth, with the effects of the Sun's gravity left over, after
those which are neglected because they result in the Earth's orbit around the
Sun, which is ignored, as a perturbation to the Moon's orbit.

So it still makes sense to treat the Moon as orbiting the Earth, and Charon as
orbiting Pluto. That the Moon's path around the Sun doesn't have loops in it,
but is convex, if lumpy, seems to not be really important, even if people can
be startled by statements to the effect that the Moon "really" orbits the Sun
based on it.

Yes, certainly, there are many reasons to use a frame of reference
that has the Earth and Moon orbiting each other (or the Moon orbiting
the Earth), or to have Pluto and Charon orbiting each other. But from
a dynamical standpoint, looking at their solar orbits, these bodies
are best understood as independently orbiting the Sun. When it comes
to how they are categorized- as planets or moons, as binary or not,
these are the dynamics that are considered.

On Thursday, July 16, 2015 at 7:07:08 PM UTC-6, Chris L Peterson wrote:

In the professional world, this is a matter of ongoing discussion,
given that the definition of "planet" is still evolving.

Given that in some sense the "real" definition of "planet" is "one of the big,
important objects in the Solar System to which we pay attention because there
aren't very many of them", and it is _because_ that's the real definition that
it was OK to have Ceres, Vesta, Pallas, and Juno as four additional planets in
the Solar System... but when a large bunch of other asteroids started to be
discovered (including many larger than Vesta) then the asteroids had to be
dropped, then that Pluto got dropped as a planet because the alternative was
Eris and Sedna and dozens of other Kuiper Belt objects being acknowledged as
planets too.

So the idea is that one draws the line at including objects as planets wherever
the line needs to be drawn not to have hundreds or thousands of them.

This is quite simple; I am saddened for sentimental reasons at the fact that Pluto, which I was used to recognizing as a planet, is no longer recognized as one, but the only two alternatives a

recognize Pluto as a planet just because we used to do so, even though other bodies just about as big are rejected, or

have an immense number of planets in the Solar System.

Of course, there is a *third* alternative: recognize Eris, and Eris alone, among the KBOs as a new planet... and wait until such time as a whole bunch of bodies larger than Eris and Pluto - or at least 90% as big as either of them - get discovered. Absent that, adding one or two new planets beyond Eris would be OK as well.

That third alternative *would* have made more people happy (Wow! A new planet has been discovered! ... exciting for us, even better for the guy who discovered it) but it would have meant that instead of "clearing the orbit", the threshhold would have been a very ill-defined one based on size.

Anything that is spherical, and bring Ceres back into the fold? Well, the Dwarf Planet definition *almost* did that... and, of course, an asteroid is really a "minor planet"... so there _are_ lots of planets, just few "major planets".

So we could accept the IAU's definitions, and say that we think that all dwarf planets *as well as* all major planets - but still excluding the minor planets, are "planets" for our purposes. And thus tables of the planets in textbooks would include Ceres and Eris both as well as Pluto!!!

Ah, yes. Pluto _is_ a planet, it's just "special".

John Savard