[qndreoi]

You can't reflect all the energy of the sun onto the earth with a mirror or a lens. Put a one million mile wide source 93 million miles from the earth. The photons from the left side of the sun will reflect off the mirror about 1/2 degree from those from the right side of the sun. Get more than about 1 million miles from earth, and the size of the reflection of sunlight off the mirror is already larger than the earth, i.e. many of those photons will miss the earth entirely. Optics obeys the 2nd law of thermodynamics like all other heat transfer mechanisms. The target must be cooler than the source.

[mnw21cam]

One of the main reasons why optics obeys the 2nd law of thermodynamics is because as the temperature of the earth approaches that of the sun, the earth starts emitting radiation approaching the same intensity, and the net influx of energy goes down towards zero.

That temperature should still be enough to make a fair dent though.

[s1artibartfast]

I think that helps. You’re saying that because the sun is not a Point Source and the rays aren’t parallel.

Following that logic, couldn’t you still put a dyson mirror around thesis and earth raising the sun and earth temp above 5700? This would raise the temp of both above 5700.

[qndreoi]

Interesting idea. I think if you put perfectly reflective mirrors inside a Dyson sphere larger in radius than Earth's orbit, the photons would bounce around until they hit the Sun, Earth, the Moon, Venus or Mercury. It seems before too long, (a few reflections), the radiation would be uniform from all directions throughout the volume of the sphere. The mirror would get as warm as all the planets, and would fail at some not too extreme temperature. Unless I'm missing something.