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Iron Age Potters Accidentally Recorded the Strength of Earth's Magnetic Field (npr.org)
Solandri writes: We've only been able to measure the Earth's magnetic field strength for about two centuries. During this time, there has been a gradual decline in the field strength. In recent years, the rate of decline seems to be accelerating, leading to some speculation that the Earth may be losing its magnetic field -- a catastrophic possibility since the magnetic field is what protects life on Earth from dangerous solar radiation. Ferromagnetic particles in rocks provide a long-term history which tells us the poles have flipped numerous times. But uncertainties in dating the rocks prevents their use in understanding decade-scale magnetic field fluctuations.
Now a group of archeologists and geophysicists have come up with a novel way to produce decade-scale temporal measurements of the Earth's magnetic field strength from before the invention of the magnetometer. When iron-age potters fired their pottery in a kiln to harden it, it loosened tiny ferromagnetic particles in the clay. As the pottery cooled and these particles hardened, it captured a snapshot of the Earth's magnetic field. Crucially, the governments of that time required pottery used to collect taxed goods (e.g. a portion of olive oil sold) to be stamped with a royal seal. These seals changed over time as new kings ascended, or governments were completely replaced after invasion. Thus by cross-referencing the magnetic particles in the pottery with the seals, researchers were able to piece together a history of the Earth's magnetic field strength spanning from the 8th century BCE to the 2nd century BCE. Their findings show that large fluctuations in the strength of the magnetic field over a span of decades are normal. The study has been published in the journal PNAS.
Now a group of archeologists and geophysicists have come up with a novel way to produce decade-scale temporal measurements of the Earth's magnetic field strength from before the invention of the magnetometer. When iron-age potters fired their pottery in a kiln to harden it, it loosened tiny ferromagnetic particles in the clay. As the pottery cooled and these particles hardened, it captured a snapshot of the Earth's magnetic field. Crucially, the governments of that time required pottery used to collect taxed goods (e.g. a portion of olive oil sold) to be stamped with a royal seal. These seals changed over time as new kings ascended, or governments were completely replaced after invasion. Thus by cross-referencing the magnetic particles in the pottery with the seals, researchers were able to piece together a history of the Earth's magnetic field strength spanning from the 8th century BCE to the 2nd century BCE. Their findings show that large fluctuations in the strength of the magnetic field over a span of decades are normal. The study has been published in the journal PNAS.
From TFA: ''' "When dealing with such large-scale phenomena, we don't usually think it can occur within a few decades. We usually think it would take thousands or tens of thousands of years," Forman says. The finding, he adds, "opens up a big can of worms" because researchers just don't know how or why that would happen. So there's something missing about scientists' concept of goings on in the Earth's core.''' But hey, at least now we know we don't know :)
+Raider of the lost BBS
So... If the magnetic field protects us from solar radiation, and the magnetic field is in a period of decline, and we're seeing a period of warming....
There are a lot of red brick houses in the world. If only the date of the building of the house is known and the bricks are not repurposed from other buildings, similar information can be derived.
If they can measure the magnetic properties of the mortar you can even get the direction.
BCE/CE has been a standard in discussing historical time periods for a while now. There's nothing impolite about it, just as there's nothing impolite about using Before Christ/Anno Domini, or using a Hebrew calendar, or using a Chinese calendar, or using an Islamic calendar, or a Hindu one, etc.