How to date History

Boris Johnson has given a good account

we call this year 2011 AD, or Anno Domini, is that it is 2011 years after the putative birth of Jesus Christ. We have grown up thinking that the year 2011 BC is so called because it was 2011 years Before Christ.



Left and right, archaeologists are radiocarbon dating objects: fossils, documents, shrouds of Turin. They do it by comparing the ratio of an unstable isotope, carbon-14, to the normal, stable carbon-12. All living things have about the same level of carbon-14, but when they die it begins to decay at uniform rate—the half-life is about 5,700 years, and you can use this knowledge to date objects back about 60,000 years.

However, radiocarbon dating is hardly the only method that creative archaeologists and paleontologists have at their disposal for estimating ages and sorting out the past. Some are plainly obvious, like the clockwork rings of many old trees. But there are plenty of strange and expected ways to learn about the past form the clues it left behind.


It's wasn't so long ago that megafauna ruled the American continent. Sloths and wooly mammoths pushed their weight around; horses and camels had their day. But after the end of the last Ice Age those animals disappeared, so when scientists turn up traces of those animals on archaeological remains, those remains go way back.

Last year, the University of Colorado's Doug Bamforth analyzed a cache of 80-plus tools that a Boulder, Colorado, man accidentally unearthed in his yard. Those tools showed protein residue from camels and horses, so Bamforth dated them to the Clovis people who lived around about 13,000 years ago. (Not all scientists accept the accuracy of these tests, but that's nothing new in archaeology).


Medieval manuscripts have a lot more to say than simply the words on their pages; often they're written on parchment made from animal skins, and organic material keeps its secrets for a long time. Literary historian Timothy Stinson developed a way to extract the DNA from parchment itself, and if you can tell what animal a parchment was derived from, you might be able to tell more about what time and place the document originated.


Moa, the giant flightless birds of New Zealand, may have been extinct for at least 500 years, but their dung is surprisingly resilient. On cave floors and buried in shelters, researchers found dung from the moa, with some of the samples being 15 cm (nearly six inches) in length. The contents of the droppings give more than a window into the giant bird's eating habits—they preserve a record of what the long-gone moa's ecosystem was like.

The arid conditions of New Zealand caves provide the perfect place for poo preservation. Australia should, too, the researchers say, but the droppings of ancient marsupials just haven't turned up. As professor Alan Cooper says, "A key question for us is 'where has all the Australian poo gone?'"



If you think your metal detector has uncovered some treasures, try finding vintage plutonium in the backyard. Jon Schwantes of the Pacific Northwest National Laboratory was called in to analyze a sample of plutonium-239 accidentally discovered in a safe during the cleanup of the Hanford nuclear site in Washington. One clue was the "signature" left by the reactor—every reactor's is different. The fingerprint of this discarded material led him to a reactor not in Hanford, but in Oak Ridge, TN. It also led him to the conclusion that it was created in 1944, meaning it was created during the Manhattan Project, making it one of the world's oldest-known samples of enriched plutonium. [Image courtesy of Popular Mechanics.]


A pile of skeletons probably wouldn't tell us much more than the obvious. But University of Leicester archaeologist Simon James sees evidence that, to him, dates the first known chemical warfare attack back to 256 A.D.

In that year, Persians attacked a Roman garrison at Dura-Europos in Syria; when they tried to mine under the walls, Romans tried to counter by mining under the Persian tunnels. Archaeologists found the pile of Roman bodies in one of the tunnels, but no cause of death. James thinks it was asphyxiation. In the tunnels, he says, there was bitumen and sulfur—materials that, when burned, give off toxic gas. So, he says, the Persians probably used chemical warfare to do in their rivals.


One classical way to date objects is to take note of what strata of rock they occupy—rocks come in layers, with the oldest at the bottom. But those rocks also carry less obvious information—their magnetic signatures. The Earth's magnetic field varies all the time, by both strength and orientation. At the time rocks form, however, their magnetic materials acquire the particular orientation of the planet's magnetism at the time, giving geologists a window into the Earth's magnetic past.



You've probably heard about ice cores, but what are they exactly? Ice sheets are laid down in layers, and the layer corresponding to each year is a little different. The important thing for climate researchers is that the oxygen isotopes present in a layer can help show what the temperature was that year. So by extracting a cylindrical core sample containing layers that go way back, they can build a model of the climate of the past. [Image courtesy of]


Finally, pollen is good for something besides making you sneeze. Deposits of pollen deep in the ground can reveal what the vegetation was like at that time, and ergo, what the area's climate might have been like. Radiocarbon dating has become the standard method to date organic material, making pollen deposits sort of useless in that regard. But pollen can still help scientists interpret the environment of the past.


Everything, it seems, has a fingerprint, and volcanoes are no exception—each eruption contains a chemical mix that is all its own. So if you knew the specific signature of say, the 79 A.D. eruption of Mt Vesuvius that buried Pompeii, you could look for that signature elsewhere in Italy and know that it came from the same eruption. Thus, any objects in that "tephra," the name for solids ejected during a single eruption, date to that era of Roman history, and anything below it would be older. This dating system is called tephrochronology.


You probably know that radiation you can't see is flying all around you, but you might not know that not only do objects absorb that radiation, they also let their trapped radiation go when heated up. Knowing this, an archaeologist could heat up an object, watch how much radiation is released and determine how old the thing might be.

It's particularly useful for ceramics. When a potter in Ancient Greece fired his kiln and baked a pot, that released the clay's stored electrons and reset the clock to zero. During all those centuries it sat in the ground, it began storing radiation again at a steady rate. So when a curious 21st century scientist unearths the pot and heats it again, she can measure the radiation released, crunch some numbers and figure out how long ago the pot was first fired.

Astronomical dating

Using astronomy to date historical events

Published on 03/01/2013


I cannot get any clear answers to what should be a simple question. "What percentage accuracy do the ancient astronomers have in fixing dates for the first millennium BCE?"

I am researching the period 1000BCE to 00BCE.
There is a wealth of information about Sumerian and Babylonian astronomical dating.
The "Kings lists" are constantly mentioned.
I simply want to have a reasoned answer to the following:

Allowing for human error, scribal miscopying etc - did the ancients in 1000BCE to 00 accurately observe and plot the planets so that we can retrospectively today recognise that which they recorded and afford it absolutely certain dates (to the nearest five years or so)
Do we know, from the positions that they give a planet in the sky that they called X, that it was that which we call, say, Mars.

Are there examples which clearly establish this e.g.
Carved in stone –
“In the 4th year of King X reign Planet V appeared in the west (whatever) and then moved behind the full moon to disappear in the northeast" - this today our computers tell us is exactly a description of Venus on 4th December 560BCE
Carved in stone –
In the 56th year of the reign of King W planet H did rise in the east and set at midnight one step into the quarter west(whatever) - this is an exact description of Mars on 4th January 500BCE"
Carved in stone –
King X was replaced by King W in his eighth year.

That is - we now know these astronomical events were indeed 60 years apart, they could be seen exactly like that from Babylon and it is clear that this was indeed the 60 years from King X 's 4th year to King W's 56th year.

There is much discussion of accuracy and more of eclipses. The only solid facts must surely come from verifying planet observation and then tracking those dates and comparing them to those given by the ancients.

Or as usual am I being simple minded?

As you have found astronomical observations can indeed be used to help provide support for chronologies of the ancient world, no method is perfect however and there are problems with this.

All astronomical observations one can make related to the Solar system contain many layers of cycles.  Everything of course varies on the cycle of a year as the Earth orbits the Sun, however due to the motion of other bodies and slow periodic changes in Earth's orbit there are also longer period cycles.  For example the times at which Venus rises and sets has a cycle of about 8 years (as well as other longer ones).  In general the shorter cycles are the most prominent while distguishing where an observation falls in a longer term cycle is more difficult.

The end result is that for your example stone carving there might be a match with 560BCE, 552BCE, 544BCE, 536BCE or 528BCE.  Sometimes knowing that it must be one of those dates might be sufficient to combine with other data and pin down an exact match, and sometimes even being able to pin it down within 5 cycles or whatever the case may be might be superior to what was known before, but gaps and inaccuracies in ancient records make it difficult to pin down exact dates from historical astronomical observations.

The most accurate records tend to be those of eclipses (because they are hard to miss and have a short duration), which helps with placing individual eclipses within the longest eclipse cycles and so providing more precise dates.  There are still problems with gaps simply because the further back you go the fewer records have survived whether or not they were originally taken, and scribal errors accumulate over time and are difficult to account for.

An additional problem is that in early history (and even comparatively recently) the calendars were not constant.  Many ancient calendars, such as that used by the Babylonians, were lunisolar with months based on lunar phases plus leap months as required to keep reasonable synchrony with the solar year.  That is fine provided that one knows how the scheme on which the leap months are added, but therein lies the difficulty in that they tend to be adjusted on a more ad-hoc basis or systematically over the medium term but with unknown larger jumps in the long term (think about the break that occurred in England in 1752 with the switch from the Julian to Gregorian calendars).  In fact even today this problem exists in the form of 'leap seconds' added by the international body which supervises global time standards, as they are irregular and unpredictable.

Sorry I can't really give you a simple answer, but I hope this goes some way to at least explaining why it isn't a simple question!

Page last updated: 3 January 2013 at 13:49

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posted Dec 27, 2014, 7:34 AM by Nagendra Mishr   [ updated Dec 27, 2014, 8:08 AM ]

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