# Leap second dating

Archaeologists have long been used to being dependent on physicists for radiometric dating, but gravimetric dating? A new paper deposited last week to arXiv suggests so:

The physical origin of the leap second is discussed in terms of the new gravity model. The calculated time shift of the earth rotation around the sun for one year amounts to $\displaystyle{\Delta T \simeq 0.621 s/ year}$. According to the data, the leap second correction for one year corresponds to $\Delta T \simeq 0.63 \pm 0.03 s/ year$, which is in perfect agreement with the prediction. This shows that the leap second is not originated from the rotation of the earth in its own axis. Instead, it is the same physics as the Mercury perihelion shift. We propose a novel dating method (Leap Second Dating) which enables to determine the construction date of some archaeological objects such as Stonehenge.

So how do we get from leap seconds to Stonehenge? The authors are claiming that the predictions of general relativity allow us to estimate the time shift of the earth’s rotation around the sun at ~ 10.3 minutes / 1000 years. The same process that leads to us adding ‘leap seconds’ to the calendar allows us to measure the difference in sunrise / sunset over long time periods. Now, I’m not a physicist so I can’t follow all that other stuff, other than understanding that the shift in Mercury’s perihelion is one of the demonstrations of general relativity used by Einstein. So let’s grant it.

The authors claim that “some of the archaeological objects may well possess a special part of the building which can be pointed to the sun at the equinox.” And if you expect the alignment to occur at sunrise but you’re off by 10 minutes, well, it must be because it was built 1000 years ago, right? But with a shift of 10 minutes per millennium, you’ve got a new problem, namely that you’re going to get a whole bunch of false positive solar alignments. The authors’ assumption that we know in advance which objects are aligned to particular solar events is incorrect.

Moreover, the authors note correctly that “It should be noted that the new dating method has an important assumption that there should be no major earthquake in the region of the archaeological objects.” Indeed, one would need to ensure that there had been virtually no movement of the celestially-aligned features – post-glacial rebound, for instance, can cause massive shifts in elevation over the time scale we’re considering, not to mention garden-variety post-depositional processes. And bear in mind that an alignment requires at least two archaeological features that can be demonstrated to be associated with one another. The error bars would be HUGE.

Finally, the idea that new dating techniques allows physical scientists to ‘tell’ archaeologists the date of their stuff is incorrect. When radiocarbon dating was developed in the late 40s, it required evidentiary confirmation, confirmation which could only come from dating archaeological materials of known age – in this case, Egyptian materials dated non-radiometrically (e.g. papyri containing dates), which could confirm that the rate of C-14 formation was (more or less) constant (Trigger 2006: 382). We don’t have anything like that here.

I’m not saying that this idea is so ridiculous that no one should try it – though it might be. But my advice to astrophysicists is to take a deep breath and consult an archaeologist before claiming to have developed a new dating technique. In other words: look before you leap.

Fujita, Takehisa. and Naohira Kanda. 2009. Physics of leap second. arXiv:0911.2087v1.
Trigger, Bruce. 2006. History of archaeological thought, 2nd ed. New York: Cambridge University Press.

(Hat tip to the weird and wacky folks at Improbable Research)