In archaeology, the present has been frozen at 1 January 1950
When a scientist dates a bone to 1000 BP, the present they mean is not today. It is 1 January 1950, fixed permanently so that dates never drift. That makes 1000 BP the year 950 AD, and it explains why some people joke that BP stands for Before Physics.
Before Present is a timescale used mainly in archaeology and geology, and it grew up alongside radiocarbon dating, first used in 1949. Because the real present keeps moving, researchers needed a fixed starting point, and from 1954 metrologists settled on 1950. It was a sensible choice: 1950 was the standard astronomical epoch of the day, the first radiocarbon dates had been published in December 1949, and it came just before large-scale atmospheric nuclear testing began pumping extra carbon-14 into the air. Those tests are the origin of the Before Physics joke, since they distort radiocarbon readings for later material.
The benchmark itself is a jar of chemistry. In the late 1950s, the radiocarbon community and the US National Bureau of Standards prepared a large batch of oxalic acid as a reference. Its carbon-14 level turned out about 5 percent above the assumed natural level, so modern carbon was defined as 0.95 times that material's value, referenced to 1950. Radiocarbon ages are then worked out using a half-life of 5568 years, known as the Libby half-life.
There is a catch. Atmospheric carbon-14 has not stayed constant over time, so raw radiocarbon ages differ from calendar years. Researchers correct them with calibration curves built from samples dated in other ways, such as tree rings and sediment layers, and label the results cal BP. Many journals insist on naming the laboratory and confidence levels because methods differ.
Not everyone follows the same convention. Some reserve BP strictly for radiocarbon results, and some archaeologists write lowercase bp for uncalibrated dates. The Centre for Ice and Climate in Copenhagen sidesteps the issue with b2k, counting years before 2000 AD.
Source: Before Present
