Any astronomy geeks? Problem in orbital dynamics

[Kulindahr]

Another perspective here:

There is a real easy solution to unseen moon problem. The moon can be in any orbit that passes through the cone where it can't be seen. All that is required is that it stay in this cone for two weeks. We can make this happen by making the moon move slow enough. This will probably give the moon a huge orbital radius, but the size of the orbit is not a constraint.

I ask a Ph.D. physicist your question. Here is his answer:

The part of the sky you see, at any instant, is the part above a plane, tangent to the earth, at your location. You see more of the sky than this, however, because the earth rotates. The part of the sky that you never see, though, is the volume within a cone whose sides are tangent to the earth at your latitude and whose apex is above the north pole. (From this cone you must exclude the small volume to the north of you.) The moon would have to stay in this volume for two weeks to remain unseen. If you are at 45 degrees north latitude then the cone is a right curcular cone.

Under these circumstances, an orbit that would keep the moon hidden for two weeks is one which carries the moon over both the north and south poles and whose period is eight weeks, about twice that of our moon. During the two weeks the moon is unseen, it would be inside this cone, south of the earth's South Pole. During these two weeks the moon would travel 90 degrees, a quarter of the way around the earth. The orbit would have to be 59% bigger than our moon's orbit, assuming the planet it orbited had the mass of earth. Any orbit larger than this would also work.

I hadn't even thought of a polar orbit!

 

[storm-0]

I love JUB for the variety and kink threads

I really found this thread sexually stimulating!!!!!!!!

 

[seven_sins]

Here is how to hide the moon:

Put it in geosynchronous orbit such that it orbits around earth's equator. Then put yourself on the side of the earth opposite the moon so that you can't see it. Since the orbital period of the moon equals the rotational period of the earth, you won't ever see the moon, even if you wait a million years.

Since the moon orbits the equator, it is always directly above the same point on earth. The huge tidal force it exerts on the earth due to its proximity won't change and the earth will come to equilibrium with it. If this did not happen, the changing tidal stresses would cause earth quakes and would heat the earth.

In geosynchronous orbit the moon would look ten times bigger than it does now. It would subtend 5 degrees.

 

[Kulindahr]

Here is how to hide the moon:

Put it in geosynchronous orbit such that it orbits around earth's equator. Then put yourself on the side of the earth opposite the moon so that you can't see it. Since the orbital period of the moon equals the rotational period of the earth, you won't ever see the moon, even if you wait a million years.

Since the moon orbits the equator, it is always directly above the same point on earth. The huge tidal force it exerts on the earth due to its proximity won't change and the earth will come to equilibrium with it. If this did not happen, the changing tidal stresses would cause earth quakes and would heat the earth.

In geosynchronous orbit the moon would look ten times bigger than it does now. It would subtend 5 degrees.

I don't want a permanently hidden moon, I want one that didn't get seen for two weeks and then showed up. And I don't want one as big as Earth's moon, but one which while it looks bigger than our moon is actually closer, which means much smaller.

I really like the polar orbit bit that professor you know suggested. For kicks, I might use that, too, and have two exceptional moons in their sky!

 

[Harke the Boeotarch]

You are all assuming that the moon isn't made of Dark Matter...

#retires into secret laboratory#

 

[Kulindahr]

You are all assuming that the moon isn't made of Dark Matter...

#retires into secret laboratory#

Well, since it can be seen, it by definition isn't made of dark matter.

Thus, it's not an assumption.

 

[Kulindahr]

A moon orbiting up and over the poles would not stay stationary in the sky, but rise and set as the planet rotates remember.

Yes, but as he said, if it were far enough out, there would be a 'cone of invisibility' that could last two weeks.

The big problem with the polar idea is that trying to keep track of where such a moon is, for story purposes, would drive me crazy.

 

[Harke the Boeotarch]

Well, since it can be seen, it by definition isn't made of dark matter.

Thus, it's not an assumption.

The moon could be made out of something that became visible when exposed to a certain other something (radiation, ion wind etc.) and then its visibility wouldn't be bound to a geostationary orbit.

 

[Kulindahr]

The moon could be made out of something that became visible when exposed to a certain other something (radiation, ion wind etc.) and then its visibility wouldn't be bound to a geostationary orbit.

Whoa.

My story may be science fiction, but that's really stretching things.... Just a moon in not-quite-geosynch is plenty strange.


I like the idea, though -- but maybe another story.

 

[Harke the Boeotarch]

Whoa.

My story may be science fiction, but that's really stretching things.... Just a moon in not-quite-geosynch is plenty strange.


I like the idea, though -- but maybe another story.

You are writing fiction.

One is made up just as much as the other.
What counts is internal consistency and believability.

 

[Kulindahr]

You are writing fiction.

One is made up just as much as the other.
What counts is internal consistency and believability.

And pertinence to the story.

 

[Kulindahr]

I remember a science-fiction story in which the natives on the planet cleared the "street" at odd times of the week, until the Terran was nearly killed by a softball-size moon orbiting four feet off the ground.

It was good for a chuckle.

Ray Bradbury, IIRC.

And it was amusing.

 

[Harke the Boeotarch]

And pertinence to the story.

I was just thinking of a Brown Hole...

 

[Kulindahr]

I was just thinking of a Brown Hole...

I'll bite -- what the heck is a brown hole?

 

[Kulindahr]

There is a possibility of a Sun-synchronous orbit where the moon would appear at the same time of day when it appears.

Don't tempt me....


For story writing, though, there'd have to be a reason for that. I can't even remotely imagine one that would fit with my story.

 

[Harke the Boeotarch]

I'll bite -- what the heck is a brown hole?

Incredibly lame.

 

[Kulindahr]

Incredibly lame.

I looked it up.

It is incredibly lame -- unless you posit a black hole which for some reason gives off an astounding amount of Hawking radiation.


In my imagination, I thought maybe a brown hole was a piece missing from the universe, where a brown dwarf used to be......

 

[Kulindahr]

New question: I've given this moon a not-quite-geosynch orbit of thirty-six days.

What are the phases going to look like?

 

[Kulindahr]

They will look the same



...where's magnetar? Haven't seen him in a while and doesn't respond to IM

The same as what?


I dunno -- he hasn't dropped in on my story in a while. Maybe he's off for the holidays?

 

[Kulindahr]

It will change phase just like the moon. But eclipses, both solar and lunar, will be far more prevalent.

Eclipses should happen monthly if its orbit is in the plane of the ecliptic. And given that I've decided it shows a bigger disk than Luna, solar eclipses will be less spectacular.

I know the phases will look like those of the moon (duh). What I'm trying to picture is whether the phase during one night will be the same (I think so) and how the phases will progress over time.