BP (or bp) - How Do Archaeologists Count Backward Into the Past?

What Do Archaeologists Mean by BP, and Why Do They Do That?

First successful Caesium Atomic Clock, 1955 (National Physical Laboratory)
First successful Caesium Atomic Clock, 1955 (National Physical Laboratory). Richard Ash

The initials BP (or bp and rarely B.P.), when placed after a number (as in 2500 BP), means "years Before the Present". Archaeologists generally use this abbreviation to refer to dates that were obtained through the radiocarbon dating technology, although not exclusively; it was certainly made necessary by the quirks of the radiocarbon methodology.

Radiocarbon's Effects

Radiocarbon dating was invented in the late 1940s, and within a few decades, it was discovered that while the dates retrieved from the method have a sound, repeatable progression, they are not a one-to-one match with calendar years.

Most importantly, researchers discovered that radiocarbon dates are affected by the amount of carbon in the atmosphere, which has fluctuated greatly in the past for both natural and human-caused reasons (such as the invention of iron smelting, the Industrial Revolution and the invention of the combustion engine).

Tree rings, which record the amount of atmospheric carbon in their rings, are used to calibrate radiocarbon dates. Scholars use the science of dendrochronology, which measures annular rings and matches them to known carbon fluctuations. That methodology has been refined and improved several times over the last few years. BP was first established as a way to clarify the relationship between calendar years and radiocarbon dates.

Advantages and Disadvantages

One advantage to using BP is it avoids the occasionally ire-filled philosophical debate about whether, in this multicultural world of ours, it is more appropriate to use A.D.

and B.C., with their explicit references to Christianity, or to use the alternatives C.E. (Common Era) and B.C.E. (Before the Common Era). The problem is, of course, that CE and BCE still use the putative date of the birth of Christ as the reference points for its numbering system: the two years 1 BCE, 1 CE are equivalent to 1 BC, 1 AD.

However, a major disadvantage with using BP is--the present year, of course, changes every twelve months. Since the BP designation was originally associated with radiocarbon dating, archaeologists chose the year 1950 as a reference point for 'the present'. The date was chosen because radiocarbon dating was invented in the late 1940s. At the same time, atmospheric nuclear testing, which throws huge amounts of carbon into our atmosphere, was begun in the 1940s. Radiocarbon dates after 1950 are virtually useless unless and until we can figure out a way to calibrate for the excessive amount of carbon still increasing in our atmosphere.

Nonetheless, 1950 is a very long time ago now--should we adjust the starting point to 2050? No, the same problem would have to be addressed in the coming years. To adjust for that, then, scholars now typically cite both raw, uncalibrated radiocarbon dates as years RCYBP (radiocarbon years before the present as 1950), alongside calibrated versions of those dates as cal BP, cal AD and cal BC (calibrated or calendar years BP, AD, and BC). That probably seems excessive, but it will always be useful to have a stable starting point in the past to hook our dates on, despite the outmoded religious underpinnings of our modern, multiculturally shared calendar.

A New Wrinkle: Thermoluminescence Dating

Thermolumiscence dating, on the other hand, has a unique situation. Unlike radiocarbon dates, TL dates are calculated in straight calendar years--and the dates measured range from a few years to hundreds of thousands of years. It might not matter if a 100,000-year-old luminescence date was measured in 1990 or 2010.

But for one that returned a date of only 500 years ago, even 50 years difference would be an important distinction. So, how do you record that? Current practice is to quote the age along with the date it was measured, but other options are being considered. Among those are using 1950 as a reference point; or better still, use 2000, cited in the literature as b2k. See Duller for a discussion of the options being considered.

After the Gregorian calendar was established throughout most of the world, atomic clocks have allowed us to adjust our modern calendars with leap seconds to correct for the slowing spin of our planet and other corrections.

But, perhaps the most interesting outcome of all this investigation, for us geeks, anyway, is the wide variety of modern mathematicians and programmers who have taken a crack at perfecting the matches between ancient calendars using modern technology. See the papers by Edward M. Reingold, Nachum Dershowitz and colleagues as fascinating examples.

Other Common Calendar Designations


  • Dershowitz N, and Reingold EM. 2008. Calendrical Calculations. Cambridge, Massachusetts: Cambridge University Press.
  • Duller GAT. 2011. What date is it? Should there be an agreed datum for luminescence ages? Ancient TL 29(1).
  • Grafton AT. 1975. Joseph Scaliger and Historical Chronology: The Rise and Fall of a Discipline. History and Theory 14(2):156-185.
  • Peters JD. 2009. Calendar, clock, tower. MIT6 Stone and Papyrus: Storage and Transmission . Cambridge: Massachusetts Institute of Technology.
  • Reimer PJ, Bard E, Bayliss A, Beck JW, Blackwell PG, Bronk Ramsey C, Buck CE, Cheng H, Edwards RL, Friedrich M et al. 2013. IntCal13 and Marine13 Radiocarbon Age Calibration Curves 0–50,000 Years cal BP. Radiocarbon 55(4):1869–1887.
  • Reingold EM, Dershowitz N, and Clamen SM. 1993. Calendrical calculations, II: Three historical calendars. Software: Practice and Experience 23(4):383-404.
  • Richards EG. 1999. Mapping Time: The Calendar and its History. Oxford: Oxford University Press.
  • Smith FC, and Abrahamson B. 1999. Calendars and thinking logically. Brookfield, IL: Teachers Press.
  • Taylor T. 2008. Prehistory vs. Archaeology: Terms of Engagement. Journal of World Prehistory 21:1–18.
  • Teres G. 1984. Time computations and Dionysius Exiguus. Journal for the History of Astronomy 15(3):177-188.