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Earth Rotation Studies

Earth Science in Space 4: What a difference a day's length makes

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LOD earthquakes

Cumulative change in LOD due to large earthquakes.

Intl. Earth Rotation Service image

The more information we gather from space, the subtler the questions we can ask about the Earth. For instance, take what might seem a simple question—how fast is the Earth turning?

Einstein is supposed to have said that a theory should be as simple as possible, but no simpler. In the case of Earth's rotation, describing its motion as "once every 24 hours" is too simple. Even the answer "23 hours 56 minutes 4.091 seconds" is just a starting point. That number is an average, and every day is a slightly different length by a few microseconds.

How the Earth varies from the average on any given day is not random. There is information—what scientists call a signal—in that variation. And length-of-day, or LOD, has been carefully measured for many decades as one small part of the field of geodesy (the International Association of Geodesy or IAG is the mother ship of this field of study).

The International Earth Rotation Service (IERS) keeps close tabs on this quantity as well as the actual position of the Earth as the world turns. Laser ranging—lidar—and long-baseline interferometry allow the positions of satellites and spots on the Earth's surface to be determined to within about a centimeter. More recently the amazing GPS or Global Positioning System has made these studies almost effortless. With high-quality data, compiled over decades, variations in LOD can usually be traced to particular causes. The simplest possible theory of LOD is complicated.

The length of the day varies when any mass on or in the Earth moves, affecting the state of its angular momentum. Take weather in the atmosphere, for instance. The seasonal changes in the trade winds and monsoons have a well-known effect on the length-of-day over the course of the year. The IERS calculates the angular momentum of the whole atmosphere every six hours, allowing the signal of large-scale weather systems to be detected.

The tides of the ocean have the long-term effect of slowing the Earth down and speeding up the Moon (which thus moves away from Earth a few centimeters per year). They also have short-term effects that are being modeled more accurately all the time. Changes in ocean currents change the length-of-day. Our computer models of ocean circulation are getting good enough, thanks to centimeter-precise measurements of the sea surface, that we can analyze this signal too. The National Earth Orientation Service has a page explaining this stuff in clear detail. (These are also the people who announce leap seconds.)

Other factors affecting the LOD data include rises and subsidences of the land surface, the buildup of glaciers, large earthquakes, large-scale pumping of groundwater and construction of reservoirs, and the shape of the ocean's surface in response to air masses above it.

Each of these can be estimated and their signals extracted from the raw data, untangling the many mixed threads of information in the LOD record. One by one, the sources of variation can be determined and subtracted out, leaving another level to be analyzed.

The last level of variation, a slow drift on the decade scale, seems to be related to the motion of liquid iron in the Earth's core. This layer allows the solid inner core to rotate freely with respect to the outer mantle and crust. Thus every twist and torque exerted by the atmosphere, oceans, Moon, Sun, other planets and the rest of the universe stirs that inner iron ocean, affecting the great dynamo that drives the Earth's magnetic field.

Length-of-day data, then, carries profound information. And without the space program we'd be almost blind to it. Not bad for asking one simple question.

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