Astronomy & Celestial Mechanics
the sky, computed
Concept

Earth moon system — where it appears

The Earth and the Moon treated as one dynamical system that shares angular momentum between the Earth's spin and the Moon's orbit. Tides transfer it from one to the other, so the Moon recedes and the day lengthens together, at a rate set by how the oceans dissipate the tide.

Named by 2 essays across 2 fields — each of them below, with the objects they name alongside it.

Where the tide's energy goes, and what the deep ocean needs. Power in terawatts. The Moon and Sun do work on the oceans at about 3.7 TW, measured from the slowing of the Earth's spin and the Moon's recession and, since the 1990s, located by satellite altimetry: about 2.6 TW is lost in shallow seas, about 1 TW in the deep ocean where the tide flows over ridges and seamounts and makes internal waves, and about 0.1 TW in the solid Earth. The open bar is what Munk and Wunsch estimated the deep ocean needs to keep its stratification against the upwelling of cold water, about 2 TW; the winds supply part of it, the tides — on these estimates — about half.

The tide that holds up the deep ocean

Cold water sinks near the poles and fills the deep ocean, and something has to stir heat back down into it or the whole ocean below the surface would become one cold pool. The stirring costs about two terawatts. The winds supply some of it. The rest comes from the Moon: about a terawatt of the tide's energy is lost not on continental shelves but in the deep ocean, where the tide flowing over ridges and seamounts makes waves on the ocean's internal layers that break far below the surface.

gravitation · Tides Two old points, and the history they draw back to the Moon's birth. The Earth–Moon distance against time before the present, with the estimates from tidal rhythmites (620 million years), cyclic sediments (1.4 billion) and banded iron formation (2.46 billion, 321.8 thousand km, ±6.2). Today's tidal efficiency, scaled as the inverse sixth power of the distance, brings the Moon to the Earth 1.54 billion years ago. The efficiency fitted between the two oldest points — 21% of today's — held further back brings it out from the Earth 4.78 billion years ago, close to the Moon's age of about 4.5 billion. The constant law is an assumption; what the points measure is the average between them.

A Moon measured in banded iron

Layers of iron-rich sediment laid down 2.46 billion years ago in what is now Western Australia keep time with the Earth's precession, and the precession's period says how far away the Moon was: about 321,800 kilometres, with a day of seventeen hours. Together with an older-is-closer point from 1.4 billion years ago, it measures how hard the tides worked between them — about a fifth as hard as today. Held further back, that efficiency brings the Moon out from the Earth a few hundred million years too early, which says the young tides worked harder than the middle-aged ones.

sky · Precession

Named alongside it

The objects these essays reach for when they reach for this one.

Tidal dissipationAngular momentumBuoyancy frequencyCritical latitudeCyclostratigraphyInternal wavesLength of dayLunar recessionMilankovitch cyclesOcean mixingPrecessionStratification

All concepts