EARTH × CLIMATE

The Nineteen-Hour Day

~2 to 1 billion years ago

Proterozoic · Paleoproterozoic · Orosirian

The length of the day stayed near nineteen hours between about 2 and 1 billion years ago, a pause in the lengthening that the friction of the tides has driven since the Moon formed.

The nineteen-hour day is a pause of roughly a billion years in the lengthening of the day, during which the day remained near nineteen hours. The Moon raises tides in the oceans, and the friction of those tides brakes the rotation of the Earth. Days have therefore lengthened steadily since the Moon formed. Researchers measure past day lengths in tidal rhythmites, sediments deposited to the rhythm of the tides. The laminae of these sediments, their thin layers, can be counted to give the number of tides within a day and of days within a month. The proposed explanation of the pause is a resonance. The Sun raises tides in the atmosphere, and these atmospheric tides push the rotation forward. During the pause, their push may have matched the braking by the Moon's tides exactly.

Why it matters

The length of the day has a history. It has grown since the Moon formed, except for a billion years in which it stayed the same.

Dating & uncertainty

Researchers infer Precambrian day lengths, those from before the Cambrian period, from tidal rhythms in sediments and from cyclostratigraphy, the study of regular cycles in sediment layers. The measurements scatter. Mitchell and Kirscher (2023) identify a plateau near nineteen hours for about a billion years. Their reconstruction is statistical, and the resonance they propose to explain the plateau is a model.

Earth history as one day

13:25:3810 h 34 min before midnight

if Earth’s 4.54 billion years were compressed into a single day