In almost all archaeology books and articles the authors use dates. Seeing ‘bp’, or ‘BP’, or ‘BC’ is common, and they are often used together. So, what’s the difference between them, and why use both?
AD, ad, CE
‘AD’ means Anno Domini. This is the Christian era in the Gregorian calendar, starting from 1 AD as the year in which Christ was believed to have been born. (The date was calculated about 500 years after the event, so was a broad estimate.)
‘AD’ is generally written in upper case letters. If lower case letters are used, this often means that the date is based on an uncalibrated radiocarbon date (see below for date calibrations). ‘AD’ in upper case can mean the date stands for a historical date (e.g. the Battle of Hastings was in AD 1066), or sometimes it means calibrated radiocarbon age in calendar years.
‘CE’ means Common Era, or Current Era. ‘CE’ is equivalent to ‘AD’ as a date and places the ‘common’ or ‘current’ era as being from the suggested birth of Christ at 1 AD (e.g. Battle of Hastings was in 1066 CE). But, it removes the explicit claim of Anno Domini which means ‘year of our Lord’. First used almost 400 years ago, it has become especially popular from the late twentieth century to emphasise secularism or sensitivity to non-Christians.
BC, bc, BCE
‘BC’ means ‘Before Christ’. This signifies the pre-Christian era in the Gregorian calendar. This runs backwards from 1 BC. As with ‘bc’, the lowercase ‘bc’ often means that the date used is an uncalibrated date.
‘BCE’ means Before Common / Current Era. As with ‘CE’, it removes the explicit reference to Christ, but is still equivalent to ‘BC’ in date.
BP and bp
The initials ‘BP’ stand for ‘years before present’. The use of BP by archaeologists, geologists, and other scientists, refers to radiocarbon ages and results from other radiometric dating techniques. Radiometric dating techniques are those that provide absolute dates based on the decay of radioactive isotopes.
Radiocarbon dating was discovered in the 1940s. All living organisms contain the gas Carbon 14 (C14 or 14C). When an organism dies, the 14C slowly decays at a known rate called its “half-life.”
The half-life of an isotope like 14C is the time it takes for half of it to decay away. In 14C every 5,730 years half of it is gone. Therefore, if you measure the amount of 14C in a dead organism, you can calculate out how long ago it died.
The ‘present’ in BP is set at 1950, as this was close to when radiometric dating began to be used. Therefore a date like 3000 BP means 3000 before 1950 AD: in other words 1050 BC.
As with AD and BC, the use of lower case ‘bp’ usually means the date is uncalibrated, and upper case means calibrated. However, not everyone follows this, and sometimes a ‘BP’ in upper case is in fact an uncalibrated date.
Quite soon after radiocarbon dating was used, scientists realised that even though the dates retrieved from the method have a repeatable progression, they do not have a one-to-one match with calendar years. They discovered that radiocarbon dates are affected by the amount of carbon in the atmosphere that has fluctuated greatly in the past.
In order to be able to calculate how much time has passed since the organism died, you have to be able to know the level of atmospheric carbon level – the radiocarbon ‘reservoir’. One method, is dendrochronology, the dating based on tree rings. A tree grows a new ring each year, and the thickness can be matched to different years. Scientists have compiled a sequence of tree rings spanning thousands of years, and compared the 14C in them. These are then compared to different 14C dates, which allows them to be calibrated. Along with tree rings, other materials are used to calibrate: ice cores, sedimentary layering, cave deposits, and volcanic eruptions.
For calibrated dates, the abbreviation is usually in upper case, or includes the abbreviation ‘cal’.
So, for a date of 5000 bp uncalibrated, which is calibrated to, say, 5640, it would be written as ‘5640 BP’, or ‘5640 cal BP’. This 5640 cal BP can also be written as ‘BC’: 3690 cal BC.
This graph from the Oxford Radiocarbon Accelerator Unit (ORAU) shows how a radiocarbon measurement 3000+-30BP is calibrated. On the left-hand axis is the radiocarbon concentration expressed in BP years and the bottom axis shows calendar years.
“The pair of blue curves show the radiocarbon measurements on the tree rings (plus and minus one standard deviation) and the red curve on the left indicates the radiocarbon concentration in the sample. The grey histogram shows possible ages for the sample (the higher the histogram the more likely that age is). The results of calibration are often given as an age range. In this case, we might say that we could be 95% sure that the sample comes from between 1375 cal BC and 1129 cal BC.” ORAU.
Why use BP and BC together?
As the above example from the ORAU shows, the BP date is what is returned during the dating analysis of a material, say a piece of bone. As this measurement is not precise to one exact year, it is given along with the standard deviation: the above example is 30, so the date is 3000+-30BP. From this a calibrated date is calculated, and given in a range of dates, written as cal BC. So, it is common to write both the radiocarbon date and the calibrated date together: the above example can be written as “3000+-30 BP (1375-1129 cal BC)”.
What about the word order and the punctuation?
For word order the ‘AD’ usually comes before the year number, as that is the correct order in Latin – so it is AD 1066. But, as ‘BC’ is English, it comes after the year number, so it is 3000 BC. Also, as ‘CE” is English, the abbreviation is written after the year numbers; so, AD 1066 is equivalent to 1066 CE. The abbreviation is also commonly used after the number of a century or millenium, as in ‘tenth century AD’ or ‘first millenium AD’.
Above, the abbreviations are all used without punctuations, but it is common to see them with punctuations, e.g. AD 1066.