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View Full Version : Another one of those (sorta belongs here) threads......

Drakheim
2004-Mar-16, 08:40 PM
Back in the day before the pile o' goo started forming stuff, there was a big boom from the Moon impacting with the Earth. After everything settled down, and the moon reformed / Earth stopped being a seething hot lump o' molten rock, we were spinning pretty fast. If I remember correctly our day was about 8 hours long. As the moon pulled further and further away from the Earth, we slowed down.

Now am I correct in applying that logic to the passing of Nancy's PX? Would it increase our rotational speed, or would a rogue (insert NL's current theory of what PX is here) not have that same effect?

tofu
2004-Mar-16, 10:17 PM
If I remember correctly our day was about 8 hours long.

You're older than you look! :wink:

I'm no physics major, so I'm sure someone will correct me if I'm wrong.

The rotation of the Earth imparts a little bit of force on the Moon. Of course, for every action there is an equal and opposite reaction. So when Earth pushes, the Moon pushes back. The Moon picks up a little orbital velocity which causes it to move further away. Earth loses a little bit of angular momentum which cause it to slow down.

The Earth and the Moon are a closed system pretty much. They orbit each other around a common center of mass and pass energy back and forth – but the total energy is conserved within that system (except what's lost through internal friction).

If there was something like Planet X, it would be in orbit around the Sun. Right? So, it's not going to have the same effect on the Earth as the Earth has on the Moon.

One thing it could do is if Planet X's orbit was highly elliptical it could maybe change our orbit around the sun. If it passed in front of Earth it could accelerate us so that for part of the year we'd go a little farther from the Sun. I bet we would survive that without too much of a problem though.

Imagine if apoapsis of our orbit was halfway between where it is now and Mars' orbit – and periapsis remained unchanged. Would that kill us all? I doubt it.

On the other hand, it could slow us down so that we'd go closer to the sun for part of the year. That might be a little worse.

But anyway the answer to your question is, I don't see any way it could change how fast Earth is revolving. If anything, it would just affect the eccentricity of our orbit. And that's only if it came into the inner solar system. How close to each other are most double stars? I don't know, but I bet it's hundreds of AU.

You want to know what the real danger of such an object would be? As it passes through the Oort cloud, it would change the orbits of a lot of comets and stuff in just the same way that I said it would change the orbit of the Earth. That would cause some of those comets to come into the inner solar system and that could maybe be dangerous. But guess what. We are in orbit around the galactic center and every few tens of millions of years we pass a hair closer to another star and this happens anyway.

JohnOwens
2004-Mar-16, 11:09 PM
Back in the day before the pile o' goo started forming stuff, there was a big boom from the Moon impacting with the Earth. After everything settled down, and the moon reformed / Earth stopped being a seething hot lump o' molten rock, we were spinning pretty fast. If I remember correctly our day was about 8 hours long. As the moon pulled further and further away from the Earth, we slowed down.

Now am I correct in applying that logic to the passing of Nancy's PX? Would it increase our rotational speed, or would a rogue (insert NL's current theory of what PX is here) not have that same effect?

The technical but unrealistic answer is that if it achieved an orbit closer to Earth than geosynchronous, i.e. with a period &lt; 24 hr (23hr 56m 4s, if you want to be precise), it would indeed accelerate the Earth's rotation. If it approaches close and fast enough in a hyperbolic orbit that its angular velocity measured from the center of the Earth is greater than about 15''/second at perigee, then it will accelerate the rotation while it's going that fast or faster (oh, in the same direction, of course; if it's not lined up nicely with the equator, you'll need greater angular velocity), and after that, slow down the rotation again, for a net loss eventually. Any other case will result in nothing but steady loss the whole time.
Even in the orbit around the Earth cases, whether faster or slower than a 24 hr period, it'll be centuries at least before it becomes a noteworthy effect. If it's closer than geosynchronous orbit, though, those are going to be some pretty mean tides! :o