B.P. originally did mean "before present" but eventually was changed to mean
"before physics" after A.D. 1950 was standardized as the fixed point from
which age determinations are (and I hope always will be!) calculated.
Laboratory results are best considered age determinations and written as an
age in radiocarbon years bp (rcybp) because it now is known that the pool of
the radioactive isotope of carbon - c-14 - is not and has not been a steady
proportion of the total carbon reservoir in the atmosphere.  Conversion of an
age determination to a calendar date is done by calibrating the date using
printed calibration curves or computer programs that take the pain out of
calibration.  The raw date (age determination) should be written as xxxx rcybp
and given as calendar age, whether we call it AD/BC, ADC/BCC, ACE/BCE,or what
only if it has been calibrated, in which case the notation cal, as in 4250 cal
BC, might be applied to make it absolutely clear that it is a calibrated date.

Donna Roper

>===== Original Message From Museum discussion list
<[log in to unmask]> =====
>Back in my archaeology days, I was taught to read B.P. as "Before
>Physics," meaning, of course, the physics of radiocarbon dating,
>developed in 1950.  I wouldn't doubt that B.P. start off as "Before
>Present," but that it didn't take too long for someone to figure out
>that in order to maintain the accuracy of "Before Present" dates, they
>would have to be adjusted yearly.
>
>As far I as know, B.P. has always referred to "before 1950," and always
>will--it'd really throw off the last five decades of archaeological
>writing if the reference date for "B.P." were changed now!
>
>
>
>------------------------------------------------------------
>Tim McShane, Assistant--Cultural History
>Medicine Hat Museum and Art Gallery
>1302 Bomford Crescent S.W.
>Medicine Hat, AB   T1A 5E6
>(403) 502-8587
>
>>> [log in to unmask] 08/08/02 02:46PM >>>
>Actually, BP was a product of C14 dating which used a standard of "# of
>years before 1950".  So it's about 52 years out of date and I'm not sure
>if it's been adjusted yet.
>
>"B.P. was added to the mix in the 1950s by some researchers as a
>reference related to early carbon dating of fossils and artifacts. B.P.
>stands for Before Present, where the present is, logically enough, 1950.
>Huh? Yeah, 1950. That's no typo. So 50 BP is actually 1900 AD. Are
>things getting weird enough for you yet?" from
>"An Illustrated Speculative Timeline of Future Technology and Social
>Change" by J.R. Mooneyham < http://www.jrmooneyham.com/spdat.html>
>
>
>
>Donna Stubbs, RPA                  HDR Engineering, Inc.
>Archaeologist                               6190 Golden Hills Dr.
>(763)591-5442                             Minneapolis, MN  55416
>mailto: [log in to unmask]
>
>=========================================================
>Important Subscriber Information:
>
>The Museum-L FAQ file is located at http://www.finalchapter.com/museum-l-faq/
. You may obtain detailed information about the listserv commands by sending a
one line e-mail message to [log in to unmask] . The body of the
message should
>read "help" (without the quotes).
>
>If you decide to leave Museum-L, please send a one line e-mail message to
[log in to unmask] . The body of the message should read "Signoff
Museum-L" (without the quotes).

=========================================================
Important Subscriber Information:

The Museum-L FAQ file is located at http://www.finalchapter.com/museum-l-faq/ . You may obtain detailed information about the listserv commands by sending a one line e-mail message to [log in to unmask] . The body of the message should read "help" (without the quotes).

If you decide to leave Museum-L, please send a one line e-mail message to [log in to unmask] . The body of the message should read "Signoff Museum-L" (without the quotes).