EARTH × CLIMATE

The Nineteen-Hour Day

~2 to 1 billion years ago

Proterozoic · Paleoproterozoic · Orosirian

The Moon pulls on the oceans and brakes the Earth's spin, so days have been growing longer ever since the Moon formed. We know because sediments laid down to the rhythm of the tides record, layer by layer, how long the old days were. Yet for roughly a billion years the lengthening stopped: in the atmosphere, tides raised by the Sun pushed as hard as the tides the Moon raises in the sea, and the day stayed stuck near nineteen hours.

Why it matters

The length of the day is not a constant of nature; it has a history, and for a billion years that history stood still.

Dating & uncertainty

Precambrian day lengths are read from tidal rhythms in sediments and from cyclostratigraphy, and the measurements scatter. The reconstruction that finds a stall near 19 hours for about a billion years is statistical, and the resonance proposed to explain it is a model.

Sources

1. Mitchell & Kirscher (2023), Nature Geoscience · Mid-Proterozoic day length stalled by tidal resonance
2. Wu et al. (2024), Science Advances · A 650-Myr history of Earth's axial precession frequency and the evolution of the Earth-Moon system
3. Williams (2000), Reviews of Geophysics · Geological constraints on the Precambrian history of Earth's rotation and the Moon's orbit

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