Thirty years ago radioastronomy did not exist. Astronomers gathered their information, as they had for three centuries, through optical telescopes. Methods had of course become much more refined and the instruments had grown enormously since Galileo first poked a small brass tube filled with two glass lenses in the direction of Jupiter and the moon to see what he could see; but telescopes remained effective only in penetrating the optical window of the earth’s atmosphere—that part of the spectrum in the visible region between the ultra violet and the infra red to which our eyes are sensitive.

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