You know the old Hughes Mearns poem, "As I was going up the stair, I met a man who wasn't there ..."Astronomers study objects we assume are really there. But sometimes they're not. A few months ago we mentioned rainbows, which stirred some controversy. Are they there? Sort of. A rainbow is nothing but a group of light rays converging on your eyes. It has no solidity. It cannot cast a shadow. But i...
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