Miranda asks her father why he created the storm, and Prospero responds with the following words:
By accident most strange, bountiful Fortune, Now my dear lady, hath mine enemies Brought to this shore; and by my prescience I find my zenith doth depend upon
A most auspicious star, whose influence If now I court not, but omit, my fortunes Will ever after droop. (179-85)
Fortune (or Fate) is personified as a lady. During the Renaissance, poets frequently referred to fate as Dame Fortune. But most of the time Dame Fortune was referred to as a fickle mistress or even as a strumpet or whore because, most of the time, fate brought misfortune and displeasure to mankind. Prospero, on the other hand, is referring to Dame Fortune in positive terms. She is a dear lady, a noble aristocratic woman, who is being bountiful or generous to Prospero.
Of course, Shakespeare here is distinguishing between Prospero as a symbol of fate and a greater force that is more powerful than the sorcerer. Fate is mysterious and all-powerful. Prospero is in league with the forces of fate. His will or desire is harmoniously in concert with fate during the main action of the story. However, as his story about the past (that he tells to Miranda) reveals, he was not always so fortunate. Prospero then explains that fate has brought his enemies (meaning his brother Antonio and King Alonso) to the island. One of Prospero’s magical gifts is prescience, the knowledge of actions or events before they happen. And as a knowledgeable magician, Prospero is also well versed in astrology (the belief that the stars and planets have an effect on human affairs). A belief in astrology is essentially a belief in the power of fate. People (who believe in astrology) believe that the planets and stars will control their destinies from the time of their births. They believe that certain events are destined to occur and that they are powerless to alter them. However, they also believe that at certain times the stars or planets are aligned in such a manner that will bring them good luck or good fortune if they act on those occasions.
Prospero informs Miranda that such a fortunate time is occurring for him: an auspicious or lucky star (which could actually mean planet) for Prospero has reached its zenith or highest point in the night sky. Now is the time for Prospero to act. The sorcerer explains that if he fails to act at that time, he will never have such another opportunity. His fortunes will droop and he will encounter misfortune. Dame Fortune will no longer be his dear lady.
Prospero thus needs to take advantage of the opportunity that Fortune is giving him. And so that he can get to his work right away, he casts a magical spell upon Miranda that causes her to sleep. Miranda will have a role to play in his scheme, and she will need to be well rested.