In the preface to the third edition of "Frankenstein", Mary Shelley described a villa party in which Lord Byron challenged each of them to begin a ghost story. In attendance also was Percy Shelley, and Byron's physician, John Polidori. 

She described her inability to come up with an idea until one sleepless night in her room, behind closed shutters "with the moonlight struggling to get through.

"I saw with shut eyes, but acute mental vision – I saw the pale student of unhallowed arts kneeling beside the thing he had put together. I saw the hideous phantasm of a man stretched out, and then, on the working of some powerful engine, show signs of life …"


The 1831 edition was revised by Shelley but perhaps not that part. Here is the frontispiece by Theodor von Holst.

Artistic license?  What day did she start writing?  Texas astronomers and historical sleuths say they have the answer. We know she was with her future husband and friends that June of 1816 at Villa Diodati overlooking Lake Geneva and the group read ghost stories to each other, and the poet Byron said each member should write their own tale.

It was a good time to write bleak horror. Revolutionaries had overthrown France and set off chronic war across Europe which had only just ended, and the Tambora volcanic eruption that same year, 1815, had turned 1816 into "the year without a summer."

Historical postmodernists say she fabricated her details but that is what historians do. Science may have other insight. She said they took up the challenge but didn't have an idea 'for days' until they had a philosophical discussion about the nature of life that went past "the witching hour" and after she went to bed she was awakened by a dream in which a scientist attempted to reanimate a corpse. Moonlight was streaming through her window.

Historians suggest she made that up for the 1831 edition but what is undocumented is the June 16th date they accept. They accept it as fact because Dr. Polidori, in attendance as was her stepsister Claire Clairmont, wrote in his diary there was a gathering that evening - but he mentioned nothing about a story and he had arrived on the 10th. It is possible the story challenge had come up prior and she was stumped for ideas for days and then her dream happened. And that it happened after she went to bed that June evening.

A visit to Villa Diodati and some math settled it for the group behind the new calculations for Sky and Telescope.  Topographic measurements of the villa and hillside plus weather records plus lunar calculations led them to declare that a gibbous moon would have been able to shine into Shelley’s bedroom window just before 2 a.m. on June 16th, which made her writing it on June 17th plausible, even if she was stumped for days.