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Orbit Question


mincam
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I am a newbie, and really uninformed (read stupid.)

Here is my question: Would it be possible for the Earth (or any planet,) to have a moon whose orbit was such that it never came between the sun and the Earth?

In other words, could we have a moon that never cast a shadow on the  Earth? Or is it inevitable that sometime during a year a moon must cast at least a partial shadow on its planet?

 

 

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46 minutes ago, mincam said:

I am a newbie, and really uninformed (read stupid.)

Here is my question: Would it be possible for the Earth (or any planet,) to have a moon whose orbit was such that it never came between the sun and the Earth?

In other words, could we have a moon that never cast a shadow on the  Earth? Or is it inevitable that sometime during a year a moon must cast at least a partial shadow on its planet?

 

 

Uranus exhibits this phenomenon. Its orbit is tilted over and its moons would probably not cast a shadow.

Uranus - Wikipedia

hubbleuranus-580x398.jpg.4a7253163783506a803c693f1388656f.jpg

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24 minutes ago, Cornelius Varley said:

Uranus exhibits this phenomenon. Its orbit is tilted over and its moons would probably not cast a shadow.

Uranus - Wikipedia

 

I was going to suggest Uranus too, but then I thought that twice during its 84 year orbit, the planet would be ‘side-on’ (can’t remember the terminology) to the sun and its moons could potentially cast a shadow.

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If the moon orbits quickly compared to the planet's orbit around the sun, all orbits integrate to a spheroid over time so a shadow can't be avoided. I think to make it possible would require a polar moon orbit and resonance to make the moon pass over a pole when the orbit is side-on to the sun's. Not impossible but improbable.

I think...but I could be wrong!

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Thanks for the replies I have some comments.

1. It would seem to me that if the planet never sees the moon's shadow, the moon will also never see the planet's shadow. Correct, or no?

2. I am probably misunderstanding the orbital physics, but from what has been said about the moons of Uranus, please comment on the following observations, based on the doctored image of  its orbits that I have uploaded.

Case A:  If we rotate the orbit around the red axis in the directions of the red arrows, we can go a small distance with the rotation before we will ever cast a shadow on Uranus. Further, the greater the radius of the orbit, the further we will be able to rotate without causing a shadow.

Case B:  If we rotate the red axis of rotation in the direction of the green arrows,  we can go a small distance with the rotation before we will ever cast a shadow on Uranus. Further, the greater the radius of the orbit, the further we will be able to rotate without causing a shadow.

If either Case A or Case B, or both, are true, there is an almost infinite number of orbits that will not cast a shadow on the planet.

Again, I appreciate the responses, and hope for more.

 

Orbit.jpg

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3 hours ago, mincam said:

if the planet never sees the moon's shadow

I think that you are considering the planet as "stationary", with the same pole always facing the sun rather than considering the change in direction of sunlight over the "year". At one time, one of the poles will be facing the sun, as in the edited picture, and the moons will not cast a shadow. A quarter of a year later, the equator is facing the sun, so all the moons pass between planet and sun and both cast shadows on the planet and see the dark side of the planet. Another quarter of a year, and the other pole is facing the sun, so you are back to no shadows.

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11 hours ago, Ricochet said:

I think that you are considering the planet as "stationary", 

Thank you.  From the comments about Uranus, I considered that it was meant that its moons' orbits were "stationary,"  with the orbital plane (if that is what it is called,) always facing the sun. 

It would seem t me that if the moons of Uranus act as you describe (which I guess is at the core of my original question,) then they do indeed cast a shadow on Uranus., and it would be impossible for the moons of any planet not to cast a shadow on their planet. So is that correct, and the moons of Uranus do indeed cast a shadow? I don't see how they could be an exception, but I will be the first to admit that I do not understand planetary physics.

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I am now guessing that the answer to my question is obvious and that a moon always casts a shadow on its planet sometime during the year, because it cannot have an "orbital plane" that is stationary to the sun. For it if it did have a stationary orbital plane it could/would not travel across the sky in the manner that our moon does. DUH. This seems intuitive to me, but I can't explain why, and I understand that I am probably confused and likely have it wrong. Maybe it's not as obvious as I now think. Maybe Uranus' moons don't cast shadows, although I think they have to.

HELP!

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If a moon's orbit is not in the plane of the planet's ecliptic AND there's just the right orbital resonance, it's possible for the moon's shadow always to pass above or below the planet. Likewise, if a moon orbits far enough out, its shadow falls short so there's an annular eclipse. Our own moon is not in resonance but it's far enough from the ecliptic for solar eclipses to be (relatively) rare.

Simplify your thinking, pretend all orbits are circular and imagine a moon whizzing around so it blurs and looks like a hula-hoop. Now try and find a place to fit the hula-hoop so it never casts a shadow from the sun as the planet moves through an entire orbit. Don't forget the hula-hoop's Centre MUST be located at the planet's centre.

Can't be done unless the moon always passes from above to below the ecliptic and vice versa when it's in the planet's shadow, which means the right resonance between the orbits.

Note that I've only thought the above through, logically I hope,  in my head...so I may have made errors!

Edited by wulfrun
grammar
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I can easily imagine the hula hoop being the Epsilon Ring shown in the image of Uranus and never casting a shadow, IF that ring is "stationary" with regard to the sun.

As I understand it, the claim claim Uranus' moons do not cast a shadow on Uranus depends upon their orbital planes being stationary and always facing the sun. In that case the "moon's orbit is not in the plane of the planet's ecliptic " but intersects it. However that intersection is always "off to the side" and never casts a shadow on Uranus.

I am pretty sure I can feel my question is answered in a way I can understand if I can be assured that the moon's orbital plane can or cannot stay "stationary" with regard to the sun.

Here is what I am now thinking: "If the moon's orbital plane rotates along with the planet, it will always cast a shadow on the planet some time or other; however, if the moon's orbital plane is stationary and does not rotate along with the planet, it is quite possible that the moon will never cast a shadow on the planet. But darned if I know which case is true, although my Earthly experience should make the answer obvious - so I'm goin' with always a shadow. But I'd like to be wrong."

So does Uranus get shadowed or not?

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How about a planet like Uranus where the axis of rotation has been tilted ~90 degrees with respect to orbital plane, but where the planet is close enough to it's parent star that it's tidally locked. With the billions of potential exoplanets out there I'd be surprised if this hasn't happened at least once.

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