# The Naked Scientists Forum

### Author Topic: Why do moons orbit planets rather than the Sun?  (Read 5992 times)

#### tonyj18

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##### Why do moons orbit planets rather than the Sun?
« on: 27/02/2014 21:33:21 »
I have a question regarding the nature of gravity, if the sun in the centre of our solar system and the planets rotate around it due to its gravitation pull, why is it that the various moons around the planets ,do not rotate around the sun in a similar orbit to the planets ,rather than around the planet in its own orbit.
The answer cannot be because it is solely due to the earth gravitational pull.
We know that the micro gravity outside of earths atmosphere isn't strong enough to prevent a spacecraft travelling to the moon, how could it hold an object the size of the moon in orbit around the earth.
We also know that other planets which are even further away from the sun (i.e. further away from the moon) are kept in orbit by the suns gravitation force, and even an object as small as Pluto and as far away ,is still held in it's orbit.
Could the answer lie in the possibility that gravitation works is two directions and not just one, as scientists current believe.
« Last Edit: 01/03/2014 10:43:22 by chris »

#### syhprum

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##### Re: Gravity
« Reply #1 on: 27/02/2014 22:45:25 »
It is simply a matter of distance gravity varies as to the mass of the attracting bodies but according to the inverse square of the distance between them so that while the mass of the Sun is some million times greater than the mass of the Earth being at a much greater distance and considering the inverse square law its influence is much less.

#### jeffreyH

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##### Re: Gravity
« Reply #2 on: 28/02/2014 02:48:02 »
Think of the timescales of the orbits. It takes years for planets to orbit the sun. It is a very weak but incremental process. The vector deviation is in small increments that decrease over time. As an object is moving away from the sun it is being slowed by a gradually decreasing amount. If escape velocity is achieved this process of slowing down will take an infinite time and the object will never deviate enough to be pulled back in.

#### evan_au

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##### Re: Gravity
« Reply #3 on: 28/02/2014 08:48:48 »
Quote
an object as small as Pluto and as far away ,is still held in it's orbit
Gravity has an infinite range, so the ability of the Sun to attract distant Pluto should not be surprising.

Quote
gravitation works is two directions and not just one, as scientists current believe.
Isaac Newton knew that gravity (and every other force) operates in two directions: "Every action has an opposite and equal reaction". The Earth attracts the Moon just as strongly as the Moon attracts the Earth. This does not mean that they accelerate towards each other equally, since the Earth has a much higher mass, and so moves less.

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why is it that the various moons around the planets ,do not rotate around the sun in a similar orbit to the planets ,rather than around the planet in its own orbit.
The Earth's Moon is an example of what you describe - the Moon is so far from Earth that the Sun's pull on the Moon is almost as strong as the Earth's. If you examined the Moon's orbit from above the Sun's pole, you would see that the Moon really orbits the Sun, not the Earth. It just orbits at the same average distance as the Earth.

Quote from: Wikipedia
Since the orbital velocity of the Moon about the Earth (1 km/s) is small compared to the orbital velocity of the Earth about the Sun (30 km/s), this never occurs. There are no rearward loops in the Moon's solar orbit. Considering the Earth–Moon system as a binary planet...

If you look at the orbits of the inner moons of Jupiter or Saturn, the attraction of the planet is far stronger than the attraction of the Sun, as the planet is much more massive, and the Sun is much more distant than it is from Earth. However, some of the outer moons are thought to be captured asteroids, and are only loosely bound to the planet - they are likely to be temporary moons, eventually taking up an independent orbit around the Sun.

#### CliffordK

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##### Re: Gravity
« Reply #4 on: 28/02/2014 09:43:41 »
There are, in fact, several dwarf planets such as Ceres that orbit around the sun, and are smaller than our moon.  Pluto is even smaller than our moon, with Pluto/Charon better considered as a binary dwarf planet system.

Quote
why is it that the various moons around the planets ,do not rotate around the sun in a similar orbit to the planets ,rather than around the planet in its own orbit.
The Earth's Moon is an example of what you describe - the Moon is so far from Earth that the Sun's pull on the Moon is almost as strong as the Earth's. If you examined the Moon's orbit from above the Sun's pole, you would see that the Moon really orbits the Sun, not the Earth. It just orbits at the same average distance as the Earth.

Yes, it is a complex relationship with both the moon and earth orbiting the sun, as well as both rotating around their common center of gravity.

#### David Cooper

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##### Re: Why do moons orbit planets rather than the Sun?
« Reply #5 on: 01/03/2014 18:12:05 »
A moon is just a planet that's close enough to another, bigger planet to get trapped in orbit around it in addition to orbiting the sun. Despite the fuss made over the categorisation of Pluto, the definition of a planet is still badly thought out, because there are moons out there which are bigger than Mercury and which are manifestly planets in their own right - they just happen to to orbit other planets. If Saturn was orbiting Jupiter, it would by current definitions be relegated to the status of a moon, but determining what is or isn't a planet should not depend on what they happen to be orbiting at any time.

#### tonyj18

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##### Re: Why do moons orbit planets rather than the Sun?
« Reply #6 on: 03/03/2014 03:50:25 »
I don't accept that Gravity has an infinite range, it does however appear to be proportional to the mass of the object , I suspect more closely to the Density of the object. The moon does not orbit the sun, it moves around the earth in its own orbit, if the moons orbit was entirely due to earths gravitation pull, that  would mean anything travelling between the earth and moon would be still subject to the earth gravitational field, we know this is not the case.
It would appear that their is a shared gravitational attraction between the earth and moon and this is directly proportional to their masses, the Sun's gravitational pull and the moon's gravitational pull to the sun are weaker than the earths shared attraction to the sun, therefore the moons attraction to the sun is broken by earth gravitational attraction.

#### RD

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##### Re: Why do moons orbit planets rather than the Sun?
« Reply #7 on: 03/03/2014 07:28:10 »
... the moons attraction to the sun is broken by earth gravitational attraction ..

Nope : the Sun effects moon's orbit of Earth ... http://en.wikipedia.org/wiki/Evection
« Last Edit: 03/03/2014 07:30:10 by RD »

#### evan_au

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##### Re: Why do moons orbit planets rather than the Sun?
« Reply #8 on: 03/03/2014 09:42:59 »
Quote from: tonyj18
I don't accept that Gravity has an infinite range
What range do you propose for gravity?

Astronomers can see that gravity is effective in the solar system, in galaxies and in clusters of galaxies, so this is a pretty fair range.

Some who are looking for a "visible" alternative to the unseen Dark Matter have suggested that gravity may diverge very slightly from an inverse square law at large distances (say, enough to be visible in the rotation of a galaxy) - but they don't debate that gravity is real, and that it works quite well over cosmological distances.

#### yor_on

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##### Re: Why do moons orbit planets rather than the Sun?
« Reply #9 on: 04/03/2014 01:17:46 »
The universe is a balance act, and you might say that gravity is what balances it. Gravity is somewhat like a form factor, defining this universe to us. Everything seems to come out as a balance in my eyes, the final count becoming a fat zero :) meaning that the universe balances itself out. So yea, gravity must be of a 'infinite range' as any speck of dust has to be involved in this final balance act.

#### tonyj18

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##### Re: Why do moons orbit planets rather than the Sun?
« Reply #10 on: 04/03/2014 19:42:24 »
I don't accept that gravity has an infinite range, that would imply it has a constant energy that does not change with distance from its source, but we know that it gets steadily weaker with distance, the balancing act is surely regional.

#### DanielB

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##### Re: Why do moons orbit planets rather than the Sun?
« Reply #11 on: 06/03/2014 18:27:08 »
No gravity does not have a infiniate range,, but it is always Constant with with direction.

Now to answer your question...Why do moons orbit planets rather than the Sun?

It comes down to the formation of the star itself,,

Imagine a Nebula cloud of dust and gas,, and gravity takes a hold of them, it takes the dust and gas,, and compresses and compresses,, and compresses,, over 250 million years,, getting hotter and hotter and hotter,, when it reaches about 15 million degrees inside,, the hydrogen atoms stop missing and start hitting,, and BOOM,,,, the star roars to life,, as a ball and self contained.. What happens after that,, the rest of the dust and clouds of gas,, that are left,, will be held in formation around that particular stars gravity,, and they start forming on their own..   And that's when they start taking their part,, in forming.

And the reason comes down to two things,, Mass,, (being dust and gas,) compressed,, due to the amount of it that gravity attracts.   Its this Large amount,, that collects,, and that sparks the star,, always in the order of formation .   If there isnt enough gas,, or there isnt enough (material) to grow large enough,, You end up with , planets,, asteroids, Meteors, just ,, well I dont want to say the left overs,, but its true, it's what they are.

Its all about the process of star formation, and why whats left over , make up the rest of the whole.

And no gravity does not have a eternal reach,, its why our black hole in the center of our own galaxy,, will not pull us into it.   What you may be thinking about is Dark Matter, and how it is attracted to gravity,, and supports it.

Anyway,, I do hope this has helped your understanding of why planets,, rotate around stars more.

Daniel B

#### Bored chemist

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##### Re: Why do moons orbit planets rather than the Sun?
« Reply #12 on: 09/03/2014 14:44:46 »
Daniel, not only have you got it wrong about gravity, which has an infinite range, whether you like it or not, but you have missed one of the more important aspects of star + planet building- the effect of static electricity.

The mass of a star is pretty much the same just after it lights up as it was just before, so it's gravitational effect doesn't suddenly change.
(there will be a loss of matter "blown off" as it lights up so the gravity will actually fall slightly.)

#### evan_au

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##### Re: Why do moons orbit planets rather than the Sun?
« Reply #13 on: 09/03/2014 20:09:43 »
Quote from: tonyj18
[If] gravity has an infinite range, that would imply it has a constant energy that does not change with distance from its source

I'm not exactly sure what the comment means about "constant energy", but I would describe Gravity as an attractive force field, rather than energy.

However, if you have a massive object like a planet near another massive object like the Sun, there will be an attractive force between them. If the orbit is elliptical, there will be a cyclic exchange between kinetic energy and potential energy as the planet orbits the star. Energy is the product of force and distance.

If you have another planet with the same mass, but at twice the distance from the Sun, it will experience one quarter of the attractive force, due to the inverse square law. Another planet at 1000x distance from the Sun will experience 1 million times less force.

There is a velocity called Escape Velocity which is sufficient for a planet to escape from a star. The force of gravity is always slowing down the planet. But gravity gets weaker the farther away the planet travels, so it slows down less; the result is that if the planet starts off at greater than Escape Velocity, then it will still have a non-zero velocity if you had the patience to allow it to travel to infinity.

Mathematically, it is possible to add up an infinite number of things (even things with the same sign), and come to a finite total. Some examples here:
http://en.wikipedia.org/wiki/Convergent_series#Examples_of_convergent_and_divergent_series

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##### Re: Why do moons orbit planets rather than the Sun?
« Reply #13 on: 09/03/2014 20:09:43 »