Interesting idea! Also one with lots of problems. I last remember hearing something of this sort proposed as a temporary solution to global warming. You probably could do it, given enough resources, but here are some of the downsides:
1) Itís expensive
2) Itíd have to be awfully big (planet-sized) The global-warming solution may only need to occlude out a small part of the sunís disk for any given spot on the planet. Iím not sure if thatís good enough for this application, though.
3) Iím pretty sure thereís not a stable orbit of anything between the Earth and the Sun with the right period and which always stays in line. Thereís a Lagrange point between the Earth and the sun, but the orbitís unstable. Youíd have to constantly adjust its position with thrusters. The stable Lagrange points are in the orbit path of the Earth and wonít work for this application.
4) Not everyone on the planet may appreciate their sunshine blotted out (for example, farmers, beach resorts far away from the poles, and owners of solar power arrays).
5) Reducing photosynthesis, even away from farms, is not a great idea either. Mike W.
(published on 10/22/2007)