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Moons of the Solar System

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Street23
1316284.  Mon Mar 11, 2019 9:41 pm Reply with quote

In Series L, episode 15 Stephen Fry said that the Earth has no moon or moons, but the Earth and the celestial body we call the Moon are a binary planetary system. By this logic does this mean that every planet from Mercury to Neptune that has a moon or moons orbiting that planet is actually a binary planetary system for the planets that have one moon and Duosexagesimal planetary system for the planet that has 62 moons. What I'm getting at is with their definition of a planetary system does that mean all the celestial objects in the solar system that we call moons are actually a planet and not really moons at all. Please let me know what you think. I would like to know if our Solar System has more than 8 planets in it.

 
crissdee
1316302.  Tue Mar 12, 2019 4:51 am Reply with quote

I think the difference lies in where the centre of the orbits are found. If the orbit of one is centred on the centre of the other*, then they are "moon" and "planet". If they both orbit the same point, then they are binary planets.

NB. I am not an astronomer and may be talking horse feathers!.






*Clumsy sentence but you know what I mean!

 
PDR
1316318.  Tue Mar 12, 2019 8:09 am Reply with quote

When one body is in orbit around another the two actually orbit their common centre of mass. So two homogeneous objects of equal mass would orbit a point halfway between them.

The mass of the international space station is around 0.42x10^6kg. It orbits the Earth at a radius of around 6,700km (athough this varies). The mass of the Earth is around 6x10^24kg. So by orbiting the Earth the ISS causes the Earth to "wobble" by:

((0.42x10^6) / (6x10^24) x 6,700 = 469x10^-18km = 469x10^-15m

or roughly 0.0005nanometres

Which isn't a lot...

PDR

 
Alfred E Neuman
1316339.  Tue Mar 12, 2019 10:06 am Reply with quote

PDR wrote:
Which isn't a lot...

Another thing it isnít is an answer to the OPís question. :)

 
suze
1316353.  Tue Mar 12, 2019 12:15 pm Reply with quote

We really need dr.bob for this, since he actually is an astronomer!

But I thought that the key point here is where the centre of mass of the planet+moons system lies. PDR shows above that the International Space Station doesn't move the centre of mass of the Earth system by very much at all. The Moon moves it by somewhat more, but the centre of mass is still within the solid body that is the Earth.

For that reason, we do not normally think of Earth+Moon as a binary system, but as a planet and its moon. On the other hand, Pluto's largest moon Charon is a big enough proportion of the bigness of Pluto that the centre of mass of the Pluto+moons system lies outside the solid body that is Pluto. Therefore, we do consider Pluto+Charon as a binary object rather than (dwarf) planet+moon.

Or something like that. Paging the Astronomy Elf ...

 
PDR
1316365.  Tue Mar 12, 2019 1:09 pm Reply with quote

Yes, that's kinda what I was getting at in answer to the OP, but I seem to have become sidetracked.

Wrist-slap accepted.

PDR

PS - I though DrBob was an astrophysicist rather than an astronomer?

 
AlmondFacialBar
1316369.  Tue Mar 12, 2019 1:15 pm Reply with quote

I think so, too, but wouldn't that make him even more capable to answering a question relating to gravitational centres?

:-)

AlmondFacialBar

 
PDR
1316370.  Tue Mar 12, 2019 1:44 pm Reply with quote

AlmondFacialBar wrote:
I think so, too, but wouldn't that make him even more capable to answering a question relating to gravitational centres?

:-)

AlmondFacialBar


Absolutely - my point was rather the reverse. Bob is (AFAIK) a proper astrophysicist (someone with expertise, experience and qualifications in physics and cosmology) rather than just an astronomer (someone with experience of looking at stars potentially using anything from naked eye to radio-telescope arrays).

ie I feared Suze was understating his suitability to answer the question!

PDR

 
suze
1316391.  Tue Mar 12, 2019 6:27 pm Reply with quote

I asked Mr Google to tell me what actually is the difference between astronomy and astrophysics, and it found me an answer from an American academic in the field:

"Technically speaking, astronomy is the science of measuring the positions and characteristics of heavenly bodies, and astrophysics is the application of physics to understand astronomy. However, nowadays, the two terms are more or less interchangeable since all astronomers use physics to understand their findings." source.

But either way, dr.bob seems admirably placed to answer the question. Am I allowed to call out an engineer for expressing numbers in engineering format when an astronomer or astrophysicist would use normal form scientific format? (ie 4.2x10^5 kg, and so on).

 
PDR
1316394.  Tue Mar 12, 2019 7:05 pm Reply with quote

Second slap accepted.

PDR

 
tetsabb
1316414.  Wed Mar 13, 2019 5:31 am Reply with quote

PDR wrote:
Second slap accepted.

PDR


Starting to enjoy it?
😉

 
PDR
1316421.  Wed Mar 13, 2019 6:14 am Reply with quote

Shhh!! - don't say that or she'll stop!

PDR

 

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