Thread: A question ...
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Old Apr 21 2012, 02:55 AM   #74

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Join Date: Nov 2004
Location: Oop North

Fan of: Moreta
Default Re: A question ...

*puts on doctoral bonnet*

I'm going to rearrange your post mercilessly to try and build up a clear picture and answer your questions at the same time.

Originally Posted by candamyr View Post
I've found this article done for a school project, which states "As it approaches Pern, the Thread-covered planet is captured likely by the gravity of Rukbat, and is pressed into an erratic orbit nearly parallel to that of Pern. It can keep this up for 50 Turns before then shooting back off towards the Oort Cloud."
An interesting idea, but not plausible. It won't work in practise, and the simplest physical explanation works fine.

Would this mean that the Red Star would, for 50 turns, revolve around Rukbat, alongside Pern, until its expelled again and goes on its 200 turn travel? If so, the orbit is likely to be wider at first, tightening towards the peak of the Pass, then widening again until it escapes from complete gravity again - but this would also mean, that it has to be outside of Pern's orbit for the between-fall times to be longer at first, then shortening (and likely lengthening again), which in turn would also mean that Thread only falls on the dark side of Pern...

The laws of physics mean that the Red Star will orbit Rukbat in an ellipse, with Rukbat itself in one of the two 'foci' of the ellipse. (Google 'ellipse+foci' if you want to look at an appropriate diagram.) Because of how elliptical orbits work, the Red Star (RS for short) will move fastest when it's closest to Rukbat, and slowest when it's far away. This is why the RS grows dramatically in brightness before the Pass starts - it's moved out of the Oort cloud, where it spends most of its time.

And here we have Fandom Misconception #1. Don't worry, you're in good and plentiful company.

Originally Posted by candamyr View Post
My biggest problem with Thread is: even if the Red Star comes close enough to Pern's orbit to throw Thread at it - the part of its path that wraps around Rukbat (no matter if outside or inside Pern's orbit) takes it 50 turns, hence 50 turns of Thread, while Pern obviously takes 1 turn to turn around Rukbat how can the Red Star be seen equally from everywhere on Pern at all times during the turn? For that matter: how can Thread fall equally (even if a longer cycle of on/off days were reported for the beginning of a Pass, shortening at the peak of a Pass) throughout a turn? If Thread is thread-bent to reach Pern, in the period of a turn, it at least should take shorter to get there while Pern is near the Red Star's location (resulting in a shorter period between falls) and longer while Pern is on the other side of the sun from the Red Star's current location on its path (resulting in a longer period between falls).
The RS does not spend 50 turns in the vicinity of Pern.
It doesn't even spend 50 Turns in the inner part of the Rukbat system.

The simple laws of orbital mechanics, and the dimensions of a planetary system, mean that the RS will inevitably be in and out of the visible system within a decade or so.

The RS will also not be seen equally bright at all times of the year. Depending on its placement in the sky, some parts of the planet won't see it at all. We know that its orbit is such that it approaches more or less from the sun-ward direction in the Northern Hemisphere winter, and that its orbit lies pretty much in the same plane as Pern's orbit, rather than diving through it at a steep angle. Before the Pass starts, it'll be a nighttime object in summer and a dawn/dusk/day object in winter -- for the northern hemisphere. Reverse things for the south.

The Pass starts when the RS crosses Pern's orbit and moves into the inner system. At this point, it is dragging a whole shitload of thread spores with it. The RS doesn't stick around close to Pern, but the spores do.

The RS will heat up, produce gaseous eruptions, have its atmosphere disturbed by the sloar wind, and all those spores it carries along with it will get tossed away as it goes, pulled out of the RS's own influence by the gravitational disturbance of other objects. There's enough of it that the whole inner system will become thick with debris. Not ideal physics, but it's the best we've got.

Pern moves through this debris-field of thread spores. Pern rotates. The bunches of Thread that the planet intersects will all take roughly the same time to fall to ground, and this way you can get Thread falling in bunches, landing preferentially in the daytime, and hitting all parts of Pern equally, more or less.

What does the RS look like during a Pass? Well, for the first year it'll behave a lot like a REALLY bright Venus. After that, a REALLY bright Jupiter. And then like a fading comet - it'll slowly take on a more or less fixed position relative to the background stars and fade in brightness by the time the Pass is half-way done. It'll be somewhere similar to where it was first spotted, offset by maybe a few tens of degrees in the sky, but more or less accurately you can say that once again it'd be a summer nighttime object in the Northern hemisphere/winter night in the south, lying close to the equatorial plane - it'll be highest overhead near the equator, but not get high above the horizon at higher latitudes.
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