reply to post by Acidtastic
That is not only correct, it also gave me a badly-needed chuckle.
It is mathematically possible to have two objects orbiting the same central
body, 180-degrees apart in a circular orbit. Of course, in the real world, perfect things get mucked-up almost instantly (don't we all know it); but
in this case, reality turns out to be much more interesting.
The above setup only stays stable if the orbits are precisely aligned, and no outside force acts on them. In our solar system, the other planets
(particularly Jupiter, which is more massive than all the rest put together) are just that outside force. Even small gravitational interactions,
after hundreds of millions of years (and millions of trips around the Sun), add up to significant changes. In our example, Earth and "CD" get
pulled at different times and by different amounts, so their perfect alignment falls apart as the two worlds settle into different orbits.
It sounds like things should get more and more chaotic, and for a while, it does. The early solar system was cluttered, and interactions common.
Each planetesimal had one of three fates: Either it got ejected completely from the Solar System, or it collided with other objects, forming larger
asteroids & planets, or it settled into an orbit that was less sussceptible to interactions (This last one needs some more thought. An orbit that is
more elliptical is more likely to cross other orbits and therefore interact. Conversely, a more circular orbit is less likely to interact, and is
therefore more stable). As the eons pass, everything that isn't ejected or "collected" by a larger planet (at high speed
) settles into an
orbit that is almost circular (the process is still going on - every time you see a "shooting star" you witness the Solar System evolving a little
As a side note: Generally speaking, the more trips around the Sun an object makes, the more interactions it has and the more likely it is to find one
of the three "fates". Thus, objects in the outer solar system move more slowly, complete fewer orbits per unit time, take longer to settle and are
more numerous. This is why Gerard Kuiper predicted a large number of small frozen objects in the outer solar system 40 years before the first
"Kuiper Belt" object was identified.
Anyway, back to our example. We last left Earth and it nemesis in orbits that were close, but not quite identical. Inevitably, one of them (let's
say "CD") will orbit slightly closer to the Sun, and therefore orbit faster (I'll say that again - objects in lower orbits move faster. Now, think
about how hard you have to throw a rock to make it go higher. You have to add
energy (i.e. accellerate) to raise an orbit, where it will move
more slowly. It sounds paradoxical, and even the early astronauts didn't get it, but it works).
"CD", moving faster, slowly catches up with Earth. You'd think they'd simply collide, but because they are almost
in the same orbit,
something cooler happens: As they get closer, they start to gravitationally interact. Earth, in the lead, pulls "CD" towards it, accellerating it
so that "CD" moves into a higher orbit, where it moves more slowly. Meanwhile, "CD" tugs on Earth from behind, robbing it of energy. Earth then
drops to a very slightly lower orbit, where it moves faster. Thus the two worlds draw together, interact, switch places and Earth ends up in the
inner, faster orbit and pulls ahead of "CD". Several hundred laps around the Sun later, it is Earth that comes up from behind "Certain Death"
(whose name, we realize, is now inaccurate). They interact with the roles reversed, switch orbits again, and pull apart.
Two pairs of Saturn's moons have "reversing orbits" like these. Though she didn't know it when she wrote it, Anne McCafferey's Pern and the
"Red Star" do this too.
[edit on 7-4-2010 by Saint Exupery]