The year the day stood still or The continuing story of the resonance hypothesis and the evolution of the semi-diurnal atmospheric thermal tide on Earth by Kevin Zahnle NASA Ames Research Center, Moffett Field, CA, USA

LaPlace (1825) understood that the largest atmospheric tides were excited not by gravity but rather by solar heating. As he had few data, and those only recorded in Paris, he could say no more. In 1882, Sir William Thomson (a.k.a. Lord Kelvin), with access to a much longer record of tropical data, showed that unlike ordinary sea tides, gravitational torques on the semi-diurnal atmospheric tide act to accelerate Earth’s rotation. He estimated that the accelerating torque was 10% as large as the decelerating torque. Based on the large amplitude of the semi-diurnal tide, Thomson also conjectured that the 12 hour forcing must be near a natural period of Earth’s atmosphere implicit in LaPlace’s tidal theory. Thus began the resonance hypothesis, which has gone in and out of favor several times since. It  was established in the 1960s that the true resonance is at 10.5 hours; hence it was passed in the past. Zahnle and Walker (1987) conjectured that rotational acceleration and deceleration could have been balanced with a constant 21 hour day for much of the Precambrian. After a brief interval, two years ago several groups independently revived the resonance hypothesis, but now for a 19-20 hour day ca 1-2.5 billion years ago. This new version of the resonance hypothesis---if correct---makes extreme demands of Earth’s Proterozoic climate. Here I will tell my story, and why it should be true, and why it probably isn’t.