Re: Strength of Materials
- From: "tadchem" <thomas.davidson@xxxxxxx>
- Date: 26 Aug 2006 12:21:24 -0700
pooua@xxxxxxx wrote:
"In short, a 'planet' is now defined as a celestial body that (a) is in
orbit around the Sun, (b) has sufficient mass for its self-gravity to
overcome rigid body forces so that it assumes a hydrostatic equilibrium
(nearly round) shape, and (c) has cleared the neighborhood around its
orbit."
That definition may not stand. Point (c) in particular would seem to
disqualify Neptune because Pluto clutters up its orbit without being
bound to it, and there are entire families of objects that regularly
interact with the orbits of planets without orbiting the planets per se
(Trojan asteroids, Jovian comets, NEOs). By this measure even Jupiter
should lose its planetary status for failure to "clear the
neighborhood" around its orbit.
Yeah, I don't know what they are talking about when they make
statements like that.
Others also feel it s wrong. From:
http://news.bbc.co.uk/1/hi/sci/tech/5283956.stm
"But Dr Stern pointed out that Earth, Mars, Jupiter and Neptune have
also not fully cleared their orbital zones. Earth orbits with 10,000
near-Earth asteroids. Jupiter, meanwhile, is accompanied by 100,000
Trojan asteroids on its orbital path."
....
"If Neptune had cleared its zone, Pluto wouldn't be there," he added.
From:
http://www.gps.caltech.edu/~mbrown/whatsaplanet/revolt.html
"By "cultural definition" what I mean is "what people mean when they
say the word planet." As far as I can tell, most non-astronomers are
content to keep Pluto as a planet, and, when you discover something new
in the outer solar system, the first question they ask is "Is it bigger
than Pluto?" When you admit that, well, no, it is 1/2 the size of Pluto
(2002 discovery of Quaoar) or maybe 3/4 the size of Pluto (2004
discovery of Sedna) they look a little disappointed and say "well, ok,
so I guess it's not a planet, huh?" But when you say "YES! It IS bigger
than Pluto" (2005 discovery of 2003 UB313) they say "Hurrah! The 10th
planet has been found!" Or something like that. To me that clearly
shows that culture believes that Pluto should remain a planet and that
only objects larger than Pluto should be called planets.
My own idea is that a "planet" should meet all 3 of the following
criteria:
(1) average orbital eccentricity e < 0.50 (eccentricity >0.50 suggests
comets)
(2) magnitude of the orbital inclination (relative to the plane of the
angular momentum of the entire system) alpha < 45° (highly inclined or
retrograde orbits imply captured objects or fragments)
(3) mass of the body exceeds one billionth that of the primary star
(smaller objects suggest fragments or poorly consolidated matter)
This definition should be useful to those who observe orbital mechanics
and to the general public (it keeps Pluto as a planet e = 0.2488, alpha
~= 17°, mass = 1 / 158,571,429 sun's mass)
It does not require subtle observations of the shape of an object, but
it does require an estimate of the mass.
Tom Davidson
Richmond, VA
.
- References:
- Strength of Materials
- From: pooua
- Re: Strength of Materials
- From: tadchem
- Re: Strength of Materials
- From: pooua
- Strength of Materials
- Prev by Date: Re: General States Plane Did Not Hit Pentagon
- Next by Date: Re: Do we know if gravity pushes or pulls?
- Previous by thread: Re: Strength of Materials
- Next by thread: Air Ionization using UltraSound
- Index(es):
Relevant Pages
|
