[Dragaera] Fwd: Moon(s) and Tides (was Re: Jhegaala etc.)

[Jon Lincicum]

 -------------- Original message ----------------------
From: Ken Koester <kkoester at email.ers.usda.gov>
> Jon Lincicum wrote:
> 
> >The other compelling argument for a moon, however, is the presence of life on 
> Dragaera. It is undeniably the case that without the moon on Earth, life could 
> not have evolved here. Not just due to tides, but also because the moon helps 
> stabilize our orbit, and maintain the planet's rotational speed. Without the 
> moon, the axial tilt of earth would long ago have evened out to be perpendicular 
> to the plane of the Solar system, meaning we would not have seasons. 
> >
>
> Ah--that is hardly undeniably the case, I believe.  

The fact that you are making a counter argument nearly proves this to be the case, I must admit.

> The moon does 
> nothing to stabilize our orbit; note that Mercury & Venus manage just 
> fine without moons, & Mars isn't going to see anything noticeable from 
> Deimos & Phobos.  

I probably meant to say "stabilize our rotation" here, since you are correct about our orbit being perfectly stable on its own. I suspect I should have had my coffee before writing that line. ;-)

>Even worse, a moon would offer very little protection 
> from a wandering Jupiter; maybe, as a sort of 3-body problem, but 
> probably not, considering the disparity of masses.  And far from 
> maintaining our rotational speed, the moon slows us down by several 
> milliseconds a year; a day during the age of dinos was considerably 
> shorter than it is now.  In the distant future, the earth will become 
> tidally locked to the moon & a "day" will be as long as it takes for the 
> moon to complete an orbit around the earth.  

Yes, tidal forces between the moon and Earth slows down our day slightly, and pushes the moon away slightly at the same time, by increasing its orbital velocity. However, by locking us into an axial tilt away from the sun, it slows the Earth from becoming tidally locked with the sun itself, as Merucry nearly is. The net effect here helps keep our days more constant than they would be otherwise. (At least, that is my understanding.)

>And I don't see any reason 
> at all for the axial tilt of the earth to change absent a moon.  It's a 
> gyroscope, after all, and there aren't any forces operating to change 
> it, with or without moon--again, check moonless or effectively moonless 
> planets w/ considerable axial tilts.  

This comes about due to aberrations in gyroscopic precession. Venus is a good example of this. Venus actually rotates counter to its orbit, (and every other rotation and orbit in the solar system). This  indicates that it was struck by a pretty massive object at some point in its development, much as the earth was in the event that caused our own axial tilt. However, Venus, lacking a moon, has since aligned itself almost exactly with the orbital plane of the planets in the likely billions of years since this occurred. (It's orbital plane is 177 degrees, almost exactly 180 from the other planets. Since this is measure due to its retrograde rotation, it means its axial tilt is pretty much zero. As Mercury's is. 

> And if the axial tilt was zero, 
> there would still be seasons due to the eccentricity of the ellipse; 
> they just wouldn't be as pronounced as they are now (& not the 
> same--you'd get "summer" and "winter" at the same time in both 
> hemispheres & it would be reversed from what it is now in the northern 
> hemisphere; our summer currently starts at aphelion).

True enough. And by "not as pronounced" we'd be talking pretty seriously. Like, not really any noticeable seasons--at least, not as we know them here. If Dragaera's orbit was so highly elliptical that this would result in visible seasons, I doubt there would be any atmosphere or oceans left at this point. The sun would boil them away during the aphelion periods. 

> >There are clearly seasons on Dragaera.  And from observations about 
> planting/harvesting times and such in various books, they appear to be quite 
> similar to ours. 
> >
> 
> I don't recall winter, however. . . .
> 
> Snarkhunter

See discussions in "Paths of the Dead" by Piro and Co about the fortuitous timing of their journey compared to the seasons, vs. Morrolan and Co. talking about the poor timing of their journey for the same reasons.

Majikjon