Is nimbus a contronym?
On the one hand, it’s a name for a rain cloud, something that obnubilates the sun and reduces the overall luminosity and albedo. On the other hand, it’s a name for a light: a saintly or divine glow – like a halo, but a disk instead of a ring (not holey, just holy), or anyway an inner glow… or a backlit glow bestowing sainthood on some person or thing just by dint of their being between you and the light.
Which, come to think of it, is how a lot of people gain their air of sainthood: not by radiating light but just by blocking light from something (or someone) else and letting just enough around the edges to give the impression of being the source. The cloud has its silver lining, sure, but the silver lining comes from what the cloud is obstructing, and if the cloud weren’t there you’d get much more of it and the cloud wouldn’t be taking the credit.
Which makes me think maybe it’s not that nimbus is a contronym but that in the cloud sense it’s, what shall we call it, a kleptonym – a stolen word. Just as the backlit nimbus is stolen light, and the cloud nimbus is a light thief.
But on the other hand, you can’t stare directly at the sun. It’s too much; you’ll be blinded. The sun’s light is useful to us precisely because things get in its way. When you look at an object, unless it’s a light source itself (like your computer screen) you’re seeing it because light is bouncing off it from some other source (the sun, a lamp, etc.). It gets in the way and scatters the rays back in your direction. Same with people. They may not even be blocking it; they may be just redirecting a bit of it with their hair and clothes. So you can see their nimbus without looking directly towards the sun.
And when you look at something or someone backlit, you see a wonderful glow precisely because they’ve taken advantage of the sun that still shines without concern for them. Or because you’ve taken advantage of them to see the effect of the sun… without seeing their face very well, due to the contrast. You’re taking their shadow, and the diffraction around their edges, and the protection from the direct sun. And they’re giving it to you selflessly, like a saint.