But there is a penumbra about the Magna Carta, a shadow cast in which its vision of an ideal nearly eclipses the mundane circumstances of its origins.
-- Edward Rothstein, "It Was a Royal Pain, but It Ended Well", New York Times, May 2010
The foregoing cases suggest that specific guarantees in the Bill of Rights have a penumbra, formed by emanations from those guarantees that help give them life and substance. See Poe v. Ullman, 367 U. S. 497, 516-522 (dissenting opinion). Various guarantees create zones of privacy. The right of association contained in the penumbra of the First Amendment is one, as we have seen.
-- Justice William O. Douglas, Griswold v. Connecticut, U.S. Supreme Court decision
Penumbra was coined as an astronomy term in 1660, but gained widespread modern usage after Supreme Court Justice William O. Douglas wrote about penumbras in the U.S. Constitution, in the landmark 1965 case Griswold v. Connecticut.