> Only active cooling can cool anything below the ambient temperature. It's more complex than that. Through thermal radiation objects on the surface of Earth a

> Only active cooling can cool anything below the ambient temperature. It's more complex than that. Through thermal radiation objects on the surface of Earth are also thermodynamically connected to the surface of the Sun (~6000K) and Space (~4K). The way to cool below ambient temperature with passive cooling is to isolate from the Sun, radiation from nearby objects and and conducting heat from surrounding air as much as possible, while making the thermal connection to Space as strong as possible. 1. How to isolate from the Sun: surface reflective in the spectrum where the thermal radiation from the Sun is the strongest, which is not too surprisingly the visible light spectrum (hence this article) 2. How to isolate from surrounding objects: insulation against conduction + surface reflective in the spectrum where the surrounding objects' thermal radiation is strongest (somewhere in infrared). 3. Thermal connection with space: you want to emit in the infrared spectrum as strong as possible. 2 and 3 are seemingly in contradiction, as a surface can't be a good reflector and good emitter at the same wavelength at the same time. But just have a geometry where the surface satisfying point 2 is pointed towards surrounding objects, while the surface satisfying point 3 radiates towards space.