What is the rainbow in Thomas Pynchon's Gravity's Rainbow (1973)?

The rainbow in Thomas Pynchon’s Gravity’s Rainbow is the arc a rocket makes from launch to target.

The novel is set in World War II Europe at the time German V-2 rockets were falling on London.

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How did Thomas Chatterton (1752-70) die? Thomas Chatterton was the author of several pseudo fifteenth-century poems…
Who were the parents of the family on the TV series "An American Family" (PBS, 1973)? William (Bill) Loud and Pat Loud were the parents of…
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What is the rainbow in Thomas Pynchon’s Gravity’s Rainbow (1973)?