Inconstant Moon is a science fiction short story collection by American author Larry Niven that was published in 1973. "Inconstant Moon" is also a 1971 short story that is included in the collection. The title refers to "O, swear not by the moon, th' inconstant moon", a quote from the balcony scene in William Shakespeare's Romeo and Juliet. The collection was assembled from the US collections The Shape of Space and All the Myriad Ways.

The 1974 Sphere paperback version of the collection only contained seven of the twelve stories originally included in the 1973 Gollancz hardback edition.

First appearance: 1971 short story collection All the Myriad Ways. The short story won the 1972 Hugo Award for best short story.

Stan, the narrator, notices that the Moon is glowing much brighter than ever before. The people he meets as the story begins all praise the Moon's increased beauty but lack the scientific background to understand its cause. However the narrator surmises that the Sun has gone nova, the day side of the Earth is already destroyed, and this is the last night of his life. He then calls and visits his girlfriend Leslie, presuming her ignorant of the situation, but she realizes it independently when Jupiter brightens with appropriate delay; they then enjoy their last night on the town, before rain and winds start.

Later, he realizes one other possibility. In case he is right, they find appropriate supplies and seek refuge from the coming natural disasters in Leslie's high-rise apartment. The second possibility turns out to be correct: the Earth has "merely" been struck by an enormous superflare—by far the worst disaster in human history, with most (if not all) people in the Eastern Hemisphere presumed dead, but humans in the Americas have a chance of surviving the cataclysm. The vaporized seawater leads to torrential rains, hurricanes and floods. The story ends at the break of an overcast, gray morning, with Leslie's apartment becoming an island among the raging flood waters, but with the narrator rather optimistically wondering "if our children would colonize Europe, or Asia, or Africa."

In 1996, the story was made into an episode of The Outer Limits television series written by Brad Wright.

Jo Walton in 1997 wrote a short poem, "The End of the World in Duxford", as "an unauthorised version of 'Inconstant Moon', a British equivalent."

First appearance: The Magazine of Fantasy & Science Fiction, April 1966.