A Swiftly Tilting Planet is a science fiction novel by Madeleine L'Engle, the third book in the Time Quintet.

When they receive the news of impending nuclear war caused by the dictator "Mad Dog Branzillo", Mrs. O'Keefe lays a charge on Charles Wallace of Patrick's Rune, a rhyming prayer of protection inherited from her Irish grandmother.

Charles Wallace goes to the star-watching rock, a family haunt, where his recitation summons a winged unicorn named Gaudior, who explains to Charles Wallace that he must prevent nuclear war by traveling through time and telepathically merging with people who lived near the star-watching rock at points in the past.

Gaudior and Charles Wallace's travels bring them to Harcels, a Native American boy at least 1,000 years in the past; Madoc of Wales, a pre-Columbian trans-oceanic traveler; Brandon Llawcae, a Welsh settler in puritan times; Mrs. O'Keefe's brother Chuck Maddox, during their childhood; and Matthew Maddox, a writer during the American Civil War.

Throughout their journey, Meg connects with Charles Wallace from home through "kything", the telepathic communication she learned in A Wind in the Door.

With Ananda** in this fateful hour,I place all Heaven with its power,And the sun with its brightness,And the snow with its whiteness,And the fire with all the strength it hath,And the lightning with its rapid wrath,And the winds with their swiftness along its path,And the sea with its deepness,And the rocks with their steepness,And the Earth with its starkness All these I place by God's almighty help and graceBetween myself and the powers of darkness It is very similar to a portion of James Clarence Mangan's poem "St. Patrick’s Hymn before Tarah,"[2] a poetic rendition of Saint Patrick's Breastplate.

L'Engle explains in Walking on Water that Vespugia is "set in the middle of what used to be called Patagonia, a sizeable area along what are now the boundaries of Chile and Argentina".

At the time of the book's publication, Kirkus Reviews said, "L'Engle's irksomely superior Murry family reassembles here for Thanksgiving dinner...

"[9] In a 2012 essay for Tor.com, American author and critic Mari Ness wrote, "A Swiftly Tilting Planet is simultaneously one of L'Engle's most beautiful and poetic novels, filled with joy and despair, and also one of her most frustrating, a book that both celebrates her earlier books while completely contradicting some of their most important and fiercely argued ethical points.