-Z- (Z@Gundam.Com)
Wed, 31 May 2000 19:26:34 -0700


At 04:39 5/31/2000, you wrote:
>At 20:15 05/30/2000 -0700, -Z- wrote:
>>The Moon and three of the five Lagrange points (L3, L4 and L5) are all
>>moving west-to-east at the same speed.
>
> I forgot part of the astronomy course I took, but shouldn't the
>other two Lagrange points move in the same direction, although not
>neccesarily with the same speed?

The position of the Lagrange points is determined by the positions of the
Earth and Moon, as they are the points where the gravitational pull of
those bodies interact (couple, cancel, or complement) -- as the Moon
moves,ill!do the Lagrange points. L4 and L5, for example, are always 60
degrees ahead of and behind the Moon, equidistant from both Moon and
Earth. L3 is always diametrically opposite the Moon. L1 and L2 are where
the gravitational forces cancel and couple, respectively.

The speed of the halo orbits around the L4 and L5 points is also a function
of the Moon's velocity, such that objects in orbit around the Lagrange
points will have the same period as the Moon around the Earth.

It's a deterministic as Newtonian and Keplerian laws of motion can make it.

> Sigh. I guess I should have put a little bit more thought into
>that.. Would the gyration bring them into contact with each other? I guess
>not. The pairs seems to have been set sufficiently far apart for this to
occur.

Not only a sufficient distance, but a fixed distance. O'Neill originally
specified that the cylinders would be ballistically coupled. This would
set the cylinders in each pair about 70 to 80 km (45 to 50 miles) apart.

>>Again, consider the inertia these things must have. Stopping them from
>>rotating would probably take hours if not days and expend tons of
>>propellent -- and to what end?
>
> To stop the damage caused by the increased rotation? As you said,
>the cylinder appeared to be spinning at 4 times the normal rate. Do we
>actually know whether the structure of the cylinder can withstand this
>increased rate of rotation? The cylinder is likely to have sustained some
>damage during the crash of the 2 Isle, and this rotation can make the damage
>worse.

I think you misunderstood me. The rotation of the colonies is fixed by
Newtonian law -- with a radius of 3.2 km (2 miles), you need exactly
one-half RPM (1RP2M, if you will) to produce an acceleration of one gee at
the inner hull. But one-half RPM is painfully slow, so the animators
speeded it up to an apparent 2RPM or four times the "actual" speed. The
cylinders aren't -- can't -- be rotating that fast, but the animators
needed to make the rotation more apparent. Think of it as "time lapse
photography"....

-Z-

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