Level 1 — Absolute Beginner
We say that one day is 24 hours. This is not exactly true. A day can be a little longer or a little shorter.
The difference is very small. It is only a few milliseconds. A millisecond is one thousandth of a second.
Deep inside Earth there is a ball of hot metal. Scientists call it the inner core.
Scientists in Canada say this ball pulls on the rock above it. The pull changes how fast Earth turns. That is why a day is not always the same.
- day
- The time it takes Earth to turn around once.
- hour
- A period of sixty minutes.
- second
- A very short period of time; sixty make one minute.
- metal
- A hard material such as iron.
- core
- The centre part of something.
- pull
- To move something toward you.
- rock
- The hard material that mountains are made of.
- turn
- To move around in a circle.
Level 2 — Elementary
Most people think a day lasts exactly twenty four hours, but that is only an average. Very accurate clocks show that some days are a few milliseconds longer and some are a few milliseconds shorter, and the pattern changes slowly over decades.
Scientists have argued about the cause for a long time. A new study in the journal Nature by Huifeng Zhang, a physics student, and Professor Mathieu Dumberry at the University of Alberta offers an answer that comes from far below the surface.
Earth has a solid inner core made mostly of iron, surrounded by a liquid outer core, and above that a layer of rock about three thousand kilometres thick called the mantle. The inner core is not a perfect sphere, and the researchers calculate that it slowly changes shape over about ten years.
Because it is not perfectly round, the inner core pulls unevenly on the mantle. That gravitational pull, working against friction at the boundary between core and mantle, speeds the outer part of the planet up and slows it down again. The result is the small wobble in the length of the day that clocks have been recording for decades.
- average
- The usual amount, found by adding and dividing.
- accurate
- Exactly correct.
- decade
- A period of ten years.
- journal
- A magazine where scientists publish research.
- iron
- A common heavy metal.
- mantle
- The thick layer of rock between Earth's crust and core.
- sphere
- A perfectly round three dimensional shape.
- friction
- The force that slows things down when they rub together.
Level 3 — Intermediate
The twenty four hour day is a convenient fiction. Atomic clocks, which are accurate to a fraction of a billionth of a second, have shown for decades that the planet's rotation drifts: the length of the day creeps up by a millisecond or two, then creeps back down, in cycles that play out over roughly a human generation. Tides and the atmosphere explain part of the variation, but not the slow decadal swing, and that residue has been an open problem in geophysics for most of a century.
Writing in Nature, Huifeng Zhang, a doctoral student in physics, and his supervisor Mathieu Dumberry at the University of Alberta argue that the missing mechanism lies at the very centre of the planet. Their model treats the solid inner core not as a rigid ball but as a body that deforms viscously, slowly flowing into a slightly different shape over a characteristic timescale of about ten years.
That matters because the inner core is not spherical, and an irregular mass exerts an uneven gravitational tug on the mantle above it, the roughly three thousand kilometre shell of rock that carries the continents and the clocks. As the shape shifts, so does the torque. Working against it are electromagnetic and frictional forces at the boundary between the mantle and the liquid outer core, and the competition between the two produces an exchange of angular momentum.
Angular momentum has to be conserved, so when the deep interior gains a little rotation the outer planet loses a little, and the day lengthens by an amount too small to notice but easily large enough to measure. The appeal of the explanation is that it ties three separate observations, the decadal length of day signal, the wandering of the geomagnetic field, and seismic hints that the inner core is not rigid, to a single physical process.
- atomic clock
- An extremely precise clock based on atomic vibrations.
- drift
- To change slowly and gradually over time.
- decadal
- Happening over a period of about ten years.
- geophysics
- The study of the physics of Earth.
- viscously
- Flowing slowly, in the manner of a thick liquid.
- torque
- A twisting force that makes something rotate.
- angular momentum
- The quantity of rotation a spinning body has.
- seismic
- Relating to earthquakes and the waves they send through Earth.
Level 4 — Advanced
That the day is not exactly eighty six thousand four hundred seconds long is among the oldest embarrassments of precision metrology. Tidal braking accounts for the secular lengthening, seasonal atmospheric angular momentum for the annual ripple, but a stubborn decadal oscillation of one to two milliseconds has resisted tidy attribution since Harold Jeffreys first drew attention to it. Its period is conspicuously similar to that of geomagnetic secular variation, which has long invited the suspicion that the core is implicated, without anyone specifying the coupling convincingly.
In Nature, Huifeng Zhang and Mathieu Dumberry of the University of Alberta supply a candidate mechanism, and its novelty lies in a single relaxed assumption. Most treatments idealise the inner core as a rigid oblate body coupled to the mantle by gravity. Zhang and Dumberry instead allow it to relax viscously, adjusting its figure toward hydrostatic equilibrium on a characteristic timescale near a decade, which is precisely the band where the unexplained signal sits.
The consequences follow from geometry. A non hydrostatic inner core exerts gravitational torque on the heterogeneous mantle; as the core's figure deforms, the magnitude and phase of that torque evolve, and the resulting angular momentum is traded between the solid inner core, the fluid outer core and the mantle, mediated by electromagnetic and topographic coupling at the core mantle boundary. Conservation does the rest. What surfaces at the top of the system, registered by atomic clocks and very long baseline interferometry, is a millisecond scale modulation of the rotation rate.
The model's attraction is parsimony. A single viscoelastic parameter, the effective viscosity of inner core material, simultaneously sets the decadal band of the length of day variation, constrains the gravitational coupling implicated in geomagnetic jerks, and sits comfortably alongside seismological evidence that the inner core is anisotropic and imperfectly rigid. Whether the inferred viscosity is physically plausible for iron alloy at those pressures is now the question the field will press, and it is an answerable one: mineral physics experiments and improved seismic normal mode observations both bear on it directly.
- metrology
- The science of measurement.
- secular
- In science, occurring steadily over a very long period.
- oscillation
- A regular movement back and forth between two states.
- oblate
- Flattened at the poles, like a slightly squashed sphere.
- hydrostatic equilibrium
- The shape a body settles into under its own gravity.
- heterogeneous
- Made of parts that are not the same throughout.
- parsimony
- Explaining much with as few assumptions as possible.
- anisotropic
- Having properties that differ depending on direction.
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