Did an apple really fall on Newton's head?
An apple almost certainly fell. It did not hit his head, and the story is not the invention it is often assumed to be — it comes from Newton himself.
The earliest detailed account was written by William Stukeley, who visited Newton in 1726 and recorded a conversation in his garden. Newton told him that the notion of gravitation had come into his mind decades earlier, prompted by the fall of an apple as he sat in a contemplative mood. Newton told similar versions to his niece's husband John Conduitt and to Voltaire, who repeated it in print. Stukeley's manuscript survives and can be read today.
The episode is usually dated to 1665 or 1666, when plague closed Cambridge and Newton returned to the family home at Woolsthorpe Manor in Lincolnshire. A descendant of the tree still stands there.
What the apple prompted was not the discovery of gravity — that things fall was hardly news. The question Newton asked was why the apple fell straight down toward the centre of the Earth, and whether the same force that pulled it might extend much further out, as far as the Moon. That leap, from a falling object to an orbiting one governed by the same law, is the genuinely radical step.
The popular image of a sudden flash of insight is misleading. Newton worked on the problem intermittently for roughly twenty years, and the Principia, which set out the law of universal gravitation with its mathematical proof, was not published until 1687. The apple was a starting point, not a conclusion.
The detail about it striking his head appears only in later retellings.