They've both got their glasses on, eyes to the sky, waiting for the Sun to wink out as it's engulfed by the Moon.
With only a small sliver of the Sun left showing, Marty can already feel the temperature start to drop. He huddles a little closer to Doc in anticipation of how cold it'll get; ten to twelve degrees is nothing to sneeze at. It's a little funny how intimate this all feels, like they're lying together in their bedroom instead of a heavily populated observation field in Oregon, ready to witness what may as well be a once-in-a-lifetime event, it happens so infrequently.
Suddenly, they're plunged into darkness. Doc starts in on a mini-lecture, explaining the various parts of a solar eclipse: the corona, Baily's beads, all the tiny solar flares erupting around the edge, so on and so forth. Marty's already familiar with a lot of what Doc's talking about, but he makes no move to interrupt him. Doc's voice is soothing to him, low and steady and filled with barely subdued excitement. He's been exposed to celestial phenomena like this countless times over the years and learned many of the nitty-gritty details about them, when he explains it all to Marty, he sounds as though it's his first time encountering such incredible knowledge and he can't wait to share it with someone. It's one of many things that endears Marty to him.
"If you think that's something," Doc says, "just wait until you see the finale!" Marty thinks Doc's talking about the "diamond ring", since that's about the only trick the eclipse has left up its sleeve.
Sure enough, the Moon slides out of the way just a bit, and it's like a flash bulb goes off. When everything clears, right near the top corner of the eclipse is a bright point of light, gently pulsating against the dark backdrop of the sky. Taken with the corona, the whole effect is that of a giant, cosmic diamond ring. When Doc speaks again, it's with a sort of wistful fondness.
"It's almost a shame I can't steal it out of the sky for you," he says. "It would make one hell of a wedding ring, don't you think?"
Marty looks down to where his left hand is clasped in Doc's right. The plain sterling silver band, the one Doc got him years ago (and whose mate has near-permanent residence on Doc's left ring finger) manages to shine in what little light is cast by the gradually re-emerging Sun. "I don't know, Doc, I think I like the one I've got just fine."