Patrick Moore's 1976 gravity hoax told BBC listeners to jump at exactly 9:47 am.
Broadcast on BBC Radio 2 on April 1, 1976, the astronomer claimed that Jupiter and Pluto would briefly align in a way that reduced Earth's gravity. Moore said listeners who jumped at 9:47 would experience a momentary floating sensation. The astronomical explanation was fictitious: the planets' alignment could not produce the claimed effect.
The prank generated hundreds of telephone calls to the BBC from people insisting that they had felt lighter. One caller reportedly said that she and eleven friends had been lifted from their chairs and gently carried around the room, illustrating how expectation can shape what people think they experienced.
The story is also dated by its treatment of Pluto. In 1976 Pluto was widely regarded as the ninth planet; the International Astronomical Union reclassified it as a dwarf planet in 2006. The exact time, rather than noon or another rounded hour, is the key identifying detail.