High over St. Petersburg, Russia, from the top of a construction crane at the Lakhta Center, a huge rainbow can be seen in its full form: a circle. Phil Plait explains the science in his 2014 Bad Astronomy article:
“…to see a rainbow, you face away from the Sun (180°), then look about 42° away from that point (180°–138°). The drops in an arc along that angle will then bend the light back toward you, and you get a rainbow, with the colors spread out a bit because they bend by different amounts.
“Oh, wait. Did I say ‘arc’? Because technically, any raindrop 42° away from the anti-solar point… will bend the light back to you. We see rainbows in the sky because in general the ground is close to you. When we look up toward the sky we see for a long way, and there are lots of raindrops along your eyeline that can add their light together to make the rainbow. When you look down, the ground gets in the way, there aren’t as many drops, and you don’t see a rainbow…
“But if you’re in a place where the ground isn’t getting in the way, like a crane, you can see beyond the arc to the full circle.”
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