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A shallow angle from the sun give you that long shadow, but it also gives the light lots of extra air to pass through sideways on it’s way down out of space. The extra air filters out more light, and without an intense enough light your skin won’t make Vitamin D.

I feel famous now.

Black-body radiation is an interesting argument against 100% efficiency, but couldn’t you just extrapolate and argue that the emission will be converted back to heat once it stops reflecting and becomes absorbed?

I’ma second. This is officiallly unofficial permission to begin your dissertation on unforeseen consequences. I’m here for this.